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Which phylum appeared most recently

Which phylum appeared most recently


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I'm aware that our earliest records of many major animal and plant phyla come from the Cambrian or Precambrian periods, and I'm also vaguely aware of some of the objections raised with general concept of phyla. With this in mind, I'm curious which of widely accepted biological phyla appeared most recently, and what evidence do we have of their relatively recent appearance?

I'm most interested in animals, but I'd also welcome any information about other organisms.


In my view, we simply don't have good enough data to answer this question. The fossil evidence is too sparse prior to the Cambrian and the evidence that we do have suggests that the phyla were already too separated. Meanwhile, the depth of time and the different lifecycles and circumstances of the species involved mean that any "genetic clocks" we might use are likely to be too poor at keeping time and, indeed, different attempts have delivered different estimates of the time of divergence.


As far as I know there is no phylum which appeared after the Cambrian. Every discussion beyond that is close to speculation, as the divergence estimates of different studies vary significantly. You might want to look into one of the resources mentioned below:


List of 11 Important Phylum | Animal Kingdom

Here is a list of eleven important phylum:- 1. Phylum Protozoa 2. Phyllum-Porifera 3. Phylum Cnidaria 4. Phylum Ctenophora 5. Phylum Platyhelminthes 6. Phylum Nemathelmlnthes 7. Phylum Annelida 8. Phylum Arthropoda 9. Phylum Mollusca 10. Phylum Echinodermata 11. Phylum Chordata.

1. Phylum Protozoa (Approximately 30,000 Known Species):

Unicellular Animals like Amoeba, Paramoecium, Monogystis and Malaria parasite. Protozoa are microscopic in size. Each individual consists of only one cell which has to carry on all the vital activities. They are abundantly found in water containing decaying organic matter. Some, such as the dysentery amoeba and the malaria parasite, live within other animals. Still others live in damp soil, or in fresh water, or in the sea.

The single-celled condition is an important feature which sets the protozoa apart from all other animals. These unicellular crea­tures have therefore been placed in the subkingdom protozoa, which includes only one phylum, the protozoa. The remaining phyla of animals, all of which are many-celled, comprise the sub- kingdom metazoa.

2. Phyllum-Porifera (Approximately 5000 Known Species):

These are pore-bearing sedentary animals found mostly in the sea. A few species occur in the fresh water but none on the land. The sponges, like plants, are attached to a substratum. The outer surface of the sponge is perforated by numerous pores and the body wall is supported by a framework which is composed of lime, or of silica or of an organic substance called spongin.

3. Phylum Cnidaria (Approximately 10,000 Known Species):

Hydra, Jelly-Fishes, Sea-Anemones and Corals.

Most of the cnidaria are marine but Hydra is found in fresh water. Some, such as the corals and sea-anemones, are attached to a substratum others are slow moving or adapted for drifting in the water. All are radially symmetrical. This means that the animal is the same all round, and has no right or left side. It is symmetrical around a median vertical axis, and can be divided into similar halves by a number of vertical planes.

Body wall is composed of two layers it encloses a central digestive cavity which communicates with the exterior by only one opening, the mouth. Thus, the cnidarian body is essentially a two-layered hollow sac opening by the month the sac may be tubular, as in hydra, or saucer-shaped, as in jelly fish. There are movable arm­ like structures near the mouth, called tentacles, which carry pecu­liar stinging cells for stunning the prey.

4. Phylum Ctenophora (Approximately 80 Species):

Beroe, Hormiphora, Pleurobrachia.

The phylum derives its name from two Greek words—Ktenos= comb, phoros= bearing. Ctenophores are all marine. They have bi-radially symmetrical bodies. They possess eight meridionally placed ciliated plates. They resemble the cnidarians on many counts but differ from them in not having the nematocysts. Their ectomesoderm is gelatinous and bear mesenchymal muscle cells. They possess a specialised aboral sense organ and the tentacles bear adhe­sive cells. All are planktonic.

5. Phylum Platyhelminthes (Approximately 6500 Known Species):

Flat-worms, Flukes and Tape-worms.

These are flat, un-segmented, worm-like creatures with soft and bilaterally symmetrical body. In a bilaterally symmetrical animal there is a right side and a left side, a fore end and a hind end, a dorsal or back surface and a ventral or front surface. There is only one plane of symmetry by which the body can be divided into two equal halves.

Leaf-like liver-flukes and ribbon-like tape­worms are parasites but there are several free-living species, marine as well as fresh-water. Digestive canal is incomplete, with only one opening, the mouth there is no anus. Excretion of waste products is effected by peculiar flame cells.

6. Phylum Nemathelmlnthes (Approximately 10,000 known Species):

These are cylindrical, un-segmented, worm-like animals with soft, bilaterally symmetrical body, tapering at both the ends. Diges­tive canal is complete, with two openings, a mouth in front and an anus behind it is a straight tube running through the body from end to end. Most of the group are aquatic. A few inhabits damp soil. Others, such as hook-worms, thread-worms and filaria worms are parasites of man and cattle.

7. Phylum Annelida (Approximately 7500 Know Species):

Earth-worms, Leeches and Sand-worms.

These are true worms with soft, elongated, bilaterally sym­metrical body, divided into a series of ring-like segments or meta- meres. The annelids are, therefore, known as the segmented worms. The annelidan body is built on the tube-within-a-tube plan.

The outer tube represents the body wall and the inner tube represents the digestive canal. The two tubes are separated from one another by a space called body cavity or coelom. Most of the annelids, such as the sand-worms, are marine others, like the leeches, are fresh-water but the earth-worm is sub-terrestrial.

8. Phylum Arthropoda (Approximately 750,000 Known Species):

Prawns, Crabs, Cockroaches, Centipedes, Millipedes, Scorpions, and Spiders.

Arthropods are bilaterally symmetrical, segmented animals with soft parts of the body protected by a hard chitinous external skeleton. Each segment of the body bears paired legs or appen­dages which are jointed. This phylum is the largest of the animal phyla and includes nearly three-fourths of all the known species of animals.

9. Phylum Mollusca (Approximately 90,000 Known Species):

Clams, Oysters, Snails, Cuttle-fishes and Octopus.

Molluscs are un-segmented and without appendages. The soft parts of the body are enclosed in a Hard calcareous shell, as in snails and oysters. A fleshy muscular foot for locomotion is often present. Many of the molluscs are marine, some are fresh-water, and a few like the garden snails are terrestrial.

10. Phylum Echinodermata (Approximately 6,000 Known Species):

Starfishes, Sea-urchins, Sea-cucumbers and Sea-lilies.

Echinoderms are characterised by spiny skin. All are marine, inhabiting the shore and bottom of the sea. A few such as the sea-lilies are attached but the majority are free to move about. Locomotion is very sluggish and effected by peculiar structures called tube-feet. This is the only phylum possessing a water- vascular system. The body is radially symmetrical and star-like as in starfishes, brittle-stars and basket-stars.

11. Phylum Chordata (Approximately 100,000 Known Species):

Balanoglossus, Ascidians, Amphioxus and Vertebrates.

The chordates possess a stiff supporting rod, called notochord. Leaving aside a few lower forms, such as balanoglossus, ascidians and amphioxus, all chordates are vertebrates. Vertebrates possess the backbone which forms the supporting skeleton for the long axis of the body.

Vertebrate body is bilaterally symmetrical and is typically composed of head, trunk and tail. There are two pairs of appendages, either in the form of paired fins or limbs, or wings. They comprise the highest animals and include man.

Vertebrates are divided into the following classes:

(1) The cyclostomata including lampreys and hag fishes which are round- mouthed and without a lower jaw

(2) The chondrichthyes or cartilaginous fishes such as sharks and electric rays

(3) The osteicthyes or body fishes like Bhetki and Rohu

(4) The amphibians such as toads, frogs and salamanders with moist, naked skin

(5) The reptiles including snakes, lizards, tortoises and crocodiles with scales on their outer surface

(6) The aves or birds with feathers and wings for flight

(7) The mammals including duck-billed mole, kangaroo, guinea-pig and man, with hairy skin and with young ones fed by the mother with her own breast-milk.


Cambrian explosion

Cambrian fossils

Panarthropoda is a superphylum that comprises phyla Arthropoda, Onychophora, and Tardigrada, with the latest group's position still uncertain. Panarthropoda are widely represented in the Cambrian fauna. A lobopod may have been the ancestor of tardigrades and a lobopod with appendage articulations like the Lower Cambrian, Fuxianhuia protensa isp. Hou, 1987 ( Fig. 4.8 ) may have been ancestor of euarthropods and onychophorans ( Xianguang and Bergstrom, 1997 ).

Figure 4.8 . Fossil of the arthropod Fuxianhuia protensa Hou, 1987. (A) Fossil specimen with articulated limbs from Maotianshan Hill, Chengjiang County, Yunnan Province, China. (B) Reconstruction of the fossil specimen of F. protensa Hou, 1987.

From Xianguang, H., Bergstrom, J., 1997. Arthropods of the lower Cambrian Chengjiang fauna, southwest China. Foss. Strata 45, 1–116.

There is a substantial fossil record of arthropods and the total number of the extant arthropod species, most of them insects, amounts to several millions. The earliest euarthropod trace fossil records are dated around 537 Ma (an earlier origin of euarthropods seems to be disproven by the lack of such trace fossils in Ediacaran Lagerstätten), but many believe they cannot be older than 550 Ma ( Daley et al., 2018 ). Crown group arthropods appear at the base of the Atdabanian, ∼ 530–524 Ma, and the earliest fossil of the group may be Rusophycus, commonly associated with trilobites, which appears before trilobites, earlier at the Tommotian ( Budd and Jensen, 2017 ).

The fossil record has shown that early Cambrian radiodontan arthropods possessed large compound eyes ( Cong et al., 2016 ) ( Fig. 4.8 ).

Panarthropoda are characterized by appearance of the ventral nerve cord and the central nervous system in Euarthropoda, Tardigrada, and Chengjiangocaris kunmingensis. They evolved segmental ganglia, whereas the latter developed regularly spaced nerve roots similarly to the VNC of Onychophora, whose postcephalic CNS is lateralized and lacks segmental ganglia ( Yang et al. (2016) ( Fig. 4.9 ).

Figure 4.9 . Simplified cladogram showing the evolution of the postcephalic CNS in Panarthropoda. The topology supports a single origin for the condensed ganglia (ga) in the VNC in a clade including Tardigrada and Euarthropoda note that the presence of multiple intersegmental peripheral nerves (ipn) in Chengjiangocaris kunmingensis represents an ancestral condition. Given the morphological similarity between peripheral and leg nerve roots, the presence of a single pair of leg nerves (lgn) in C. kunmingensis is hypothetical (dashed lines) and based on the condition observed in crown-group Euarthropoda.

From Yang, J., Ortega-Hernández, J., Butterfield, N.J., Liu, Y., Boyan, G.S., Hou, J.-B. et al., 2016. Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda. Proc. Natl. Acad. Sci. U.S.A. 113: 2988–2993.

†, fossil taxa ?, uncertain character polarity within total-group Euarthropoda asn, anterior segmental nerve cn, longitudinal connectives co, commissure dln, dorsolateral longitudinal nerve ico, interpedal median commissure irc, incomplete ring commissure pn, peripheral nerve psn, posterior segmental nerve rc, ring commissure. Reconstruction of VNC in Onychophora adapted from Dewel et al. (1993) . Distribution of serotonin in the trunk of Metaperipatus blainvillei (Onychophora, Peripatopsidae): implications for the evolution of the nervous system in Arthropoda. J. Comp. Neurol. 507(2), 1196–1208.

About 540 Ma, Cambrian panarthropods evolved four main brain types, and a great diversity of complex behaviors ( Fig. 4.10 ), but they evolved no novel ground patterns ever since ( Strausfeld et al., 2016 ). The central nervous system of the Cambrian arthropods is largely conserved in extant arthropoda.

Figure 4.10 . Trace fossils associated with nonbiomineralized carapaces in Burgess Shale–type deposits. (A) The arthropod Arthroaspis bergstroemi showing regular polygonal networks displaying true branching (black arrows) and secondary successive branching (white arrow), Sirius Passet, Greenland. (B) Close-up of (A) showing annulated structures (arrow) and delicate, narrow-caliber, filament-like structures mostly confined to areas among network branches).

From Mángano, M.G., Buatois, L.A., 2016. The Cambrian explosion ( Chapter 3 ). In: M.G. Mángano, L.A. Buatois (Eds.), The Trace-Fossil Record of Major Evolutionary Events. Topics in Geobiology, vol. 39, Springer, Dordrecht, pp. 77 (73–126).

Trilobites are derived arthropods and they may have appeared first at the Ediacaran-Cambrian boundary, but the earliest trilobite body fossils are dated 521 Ma (earlier soft-bodied trilobites left no traces), from Siberia (Profallotaspis jakutensis and Profallotaspis tyusserica), Morocco (Hupetina antiqua), Spain (Lunagraulos tamamensis), and Laurentia (Fritzaspis generalis) followed, within a few million years, by finds into other areas of the earth ( Daley et al., 2018 ). Another trilobite-like arthropod, Arthroaspis bergstroemi, is discovered in Sirius Passet, Groenland ( Stein et al., 2013 ). The first trilobite radiation event occurred between 520 and 513 Ma ( Lin et al., 2006 ) ( Fig. 4.11 ) and the group of trilobites existed 20–70 million years before they were extinct ( Lieberman, 2008 ). Trilobite-like fossils like genus Parvancorina ( Lin et al., 2006 ), are found toward the end of Ediacaran in a few Lagerstätten in Burgess Shale, Australia, and South China.

Figure 4.11 . Kootenia sp, a typical trilobite from the Middle Cambrian of Greenland, a product of the enormous radiation of trilobites that took place around 520 Ma. The body is approximately 87 mm long.

From Budd, G.E., 2013. At the origin of animals: the revolutionary cambrian fossil record. Curr. Genomics 14, 344–354.

The head of trilobites evolved of various combinations of body segments. Recently fossil evidence shows that the extinct trilobite Schmidtiellus reetae Bergström, 1973, possessed sophisticated eyes of apposition type, like those of extant bees or dragonflies ( Schoenemann et al., 2017 ) ( Fig. 4.12 ).

Figure 4.12 . Schematic drawing of the visual unit of S. reetae. b, basket cc, crystalline cone L, lens om, ommatidium p, pigment screen r, rhabdom sc, sensory (receptor) cells. (Scale bar: 200 μm).

From Schoenemann, B., Pärnaste, H., Clarkson, E.N.K., 2017. Structure and function of a compound eye, more than half a billion years old. Proc. Natl. Acad. Sci. U.S.A. 114, 13489–13494.

Molluscs . The fossil record of the lower Cambrian includes clades of the phylum Mollusca, comprising seven extant classes. The crown group Mollusca had already evolved in the beginning of the Tommotian, more than 530 Ma, but no consensus exists about their stem group lineage ( Budd and Jensen, 2017 ). Besides the controversial Ediacaran Kimberella, among the clades that left fossils in the early Cambrian are gastropods, bivalves, and rostroconchs, as well as helcionellid molluscs, which appear as the earliest gastropods ( Landing et al., 2002 ). Many believe stem group molluscs evolved in the terminal Ediacaran (∼542 Ma) but fossils of mollusc species, along the brachiopods, appear first in the Fortunian ∼537 Ma ( Zhuravlev and Wood, 2018 ). Helcionellids, the small mollusc fossils, commonly seen as molluscs, appear first at the very beginning of the Cambrian over 540 Ma ( Steiner et al., 2007 ) until 530 Ma.

Another group of sclerite-bearing metazoans of the lower Cambrian are halkieriids, a crown group molluscs, among which the better studied, Halkieria evangelista, from the Sirius Passet fauna of North Greenland ( Conway Morris, 1998 ), together with Wiwaxia and Odontogriphus belong to the stem group of the superphylum Lophotrochozoa ( Butterfield, 2006 ).

Fossils of phylum Brachiopoda appear in the terminal Ediacaran (∼542 Ma). This phylum is widely represented in the Cambrian fossil record. Its representatives are among the first skeletal organisms of the Lower Cambrian. Brachiopod fossils are found around the world and in a large number of centers of diversification ( Ushatinskaya, 2008 ). The evolutionary relationship of this group with the extinct lower Cambrian chancelloriids and the extant Lophotrochozoan phyla - molluscs, annelids and brachiopods is not resolved.

Annelids. Annelids are another large phylum of the superphylum Lophotrochozoans comprising more than 9000 species ( Zhang, 2011 ). The oldest fossils of the phylum Annelida belong to the class Polychaeta found in early Cambrian mudstones (520 million years old) from the Sirius Passet deposit of North Greenland ( Parry, 2014 ) ( Fig. 4.13 ). Phragmochaeta canicularis gen. et sp. nov. is an early Atdabanian (∼530 Ma) fossil, composed of about 20 segments, from the Lower Cambrian Sirius Passet (Greenland) that is interpreted as a polychaete annelid ( Conway Morris and Peel, 2008 ). Another likely late Early Cambrian annelid fossil is that of Myoscolex ateles Glaessner, 1979, found in Australia ( Dzik, 2004 ).

Figure 4.13 . Cambrian fossil polychaetes and their relationships. (A) Phragmochaeta canicularis, Geological Museum of Copenhagen MGUH 30888, scale bar 1.5 mm (B) Burgessochaeta setigera, Smithsonian Museum of Natural History 198705, scale bar 2 mm.

From Parry, L., 2014. Fossil focus: annelids, Palaeontol. 4, 1–8.

Hemichordates: Hemichordate fossils abound beginning with the lower Cambrian ( Bengtson, 2004 ). In hemichordates, the branchial slits that earlier were used for filter feeding evolved into pharyngeal slits and into gill slits in fish. A middle Cambrian Burgess Shale fossil, Oesia disjuncta, previously compared to annelids and tunicates, now is considered a suspension feeder hemichordate (enteropneust) that dwelled inside tubes of the green algae Margarita ( Nanglu et al., 2016 ).

The Yunnanozoan fossils found in the Chengjiang fossil Lagerstätte, South China, are interpreted as the earliest hemichordate fossils and are considered as a link between the invertebrates and vertebrates Shu et al., 1996a,b Chen et al. (1999) .

Phylum Chordata comprises three subphyla: subphylum Cephalochordata (Acrania or lancelets), subphylum Tunicata (earlier Urochordata) and subphylum Vertebrata ( Fig. 4.14 ). Urochordata and Vertebrata are sister groups, but urochordates have lost segmentation ( Nielsen, 2012 , p. 348). Their common Bauplan is characterized by

Figure 4.14 . Phylogenic relationships of deuterostomes and evolution of chordates. (A) Schematic representation of deuterostome groups and the evolution of chordates. Representative developmental events associated with the evolution of chordates are included. (B) A traditional view. FT, fish-like or tadpole-like.

From Satoh, N., Rokhsar, D., Nishikawa, T., 2014. Chordate evolution and the three-phylum system. Proc. Biol. Sci. 281, 20141729.

segmented body with paired articulated legs,

the presence of the notochord, formed from the roof of the archenteron,

the neural tube, formed from the ectoderm in contact with the notochord,

longitudinal muscles running along the notochord ( Nielsen, 2012b , p. 349).

A crucial event in the evolution of the subphylum Vertebrata compared to its sister groups, cephalochordates and tunicates, is the emergence of the neural crest, which was essential for the exceptional increase in the structural and functional complexity of vertebrates ( Fig. 4.15 ).

Figure 4.15 . Neurulation in amphioxus and vertebrates. Top: at the late gastrula stage both amphioxus (Cephalochordata) and vertebrates have a neural plate with a neural plate border region. Second from top: at the early neurula stage, in amphioxus, the neural plate border region detaches from the edges of the neural plate and moves over it by lamellipodia. By contrast, in vertebrates, the neural plate border region remains attached to the neural plate as it rounds up. Third from top: at the late neurula stage, in amphioxus, the free edges of the neural plate border region fuse in the dorsal midline, and the neural plate begins to round up underneath the dorsal ectoderm. In vertebrates at a comparable stage, the neural tube has completed rounding up. Bottom: In amphioxus, the neural plate rounds up completely and detaches from the ectoderm. In vertebrates, the neural tube detaches from the ectoderm, and the neural plate border region gives rise to neural crest cells that migrate below the ectoderm and give rise to such structures as pigment cells, cells of the adrenal medulla, parts of cranial ganglia.

From Holland, L.Z., 2015. The origin and evolution of chordate nervous systems. Phil. Trans. R. Soc. B 370, 20150048.

The evolution of chordates is an integral part of the Cambrian explosion although Cambrian chordate fossils are scanty ( Chen and Li, 1997 ). The earliest Cambrian fossil identified and widely accepted as a basal chordate is genus Pikaia represented by a single species Pikaia gracilens ( Conway Morris and Whittington, 1979 ) dated to the Middle Cambrian ( Fig. 4.16 ).

Figure 4.16 . Pikaia gracilens, as reconstructed by Conway Morris and Caron [1]. The head bears a pair of tentacles, probably sensory in nature, and paired rows of ventrolateral projections that may be gills. Not shown: the expanded anterior (pharyngeal) region of the digestive tract, and the dorsal shield-like structure, the anterior dorsal unit, that lies above it. The boxed detail shows the main axial features: the dorsal organ (do), and the putative notochord (not) and digestive tract (dt). The size range among specimens is 1.5–6 cm, which makes this animal very close in size to the adult stage of modern lancelets (amphioxus).

From Lacalli, T., 2012. The Middle Cambrian fossil Pikaia and the evolution of chordate swimming. EvoDevo 3, 12.

Cambrian fossils of P. gracilens are described as basal to chordates. It had a notochord and about 100 myomeres, resembling chordate myomeres, and notochord. Pikaia was not a fast swimmer because its monomers contained only slow twitch muscle fibers. It is considered as “the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans” ( Conway Morris and Caron, 2012 ).

Later, another chordate fossil, Cathaymyrus diadexus, about 10 million years older than Pikaia, was discovered in South China Lagerstätte ( Shu et al., 1996a,b ). Chordate fossils and even agnathan fishes are part of the Chengjiang Cambrian fauna in South China found in deposits of 545–490 Ma, but chordate fossils evolved as early as ∼555, if not earlier ( Shu et al., 1999 ).

Commonly tunicates are considered to be the closest relatives to vertebrates. They conserve the notochord only during larval development but lose it as adults, after metamorphosis. All the tunicate fossils described as such so far are controversial. The only one that is widely accepted and bears a striking resemblance to modern ascidians is a tunicate species, identified as Shankouclava shankouense from the Lower Cambrian Maotianshan Shale, South China, dated abut 520 million years ago ( Chen et al., 2003 ) ( Fig. 4.17 ).

Figure 4.17 . Lower Cambrian tunicate Shankouclava reconstructed. Abbreviations: An, anus Ap, possible atrial pore At, atrium B, branchial bars Bs, branchial slits En, endostyle Es, esophagus In, intestine M, mantle Os, oral siphon Ot, oral tentacle Se, body segments Sm, stomach St, stalk.

From Chen, J.-Y., Huang, D.-Y., Peng, Q.-Q., Chi, H.-M., Wang, X.-Q., Feng, M., 2003. The first tunicate from the early cambrian of south China. Proc. Natl. Acad. Sci. U.S.A. 100, 8314–8318.

Lower Cambrian fossils of Haikouella lanceolata and a few other similar fossils of Chengjiang Lagerstätte of Yunnan province (South China) are identified as jawless chordate fish (agnathans) related to the extant hagfish and lampreys. Of the same Chengjiang origin are fossils of two other chordate taxa Haikouichthys and Zhangjianichthys ( Conway Morris, 2008 ).

Based on the similarities between the ciliary bands of the echinoderm and enteropneust larvae and the neural tube of amphioxus as well as in the patterns of gene expression in the anterior protostomian CNS and in the brain of vertebrate embryos, biologists ( Arendt et al., 2008 Nielsen, 2013d ) hypothesize that an inversion of the nerve cord and dorso-ventral organization occurred in the chordate CNS, which evolved via the circumoral ciliary band of a dipleurula, a hypothetical ancestral form of bilaterian echinoderms and chordates. Hence, the dorsal side of chordates is thought to be homologous to the ventral side of protostomes and enteropneusts. Similarly, the vertebrate eyes are considered to be homologous to the ciliary frontal eye of amphioxus ( Nielsen, 2012c ).


Contents

Evolution Edit

The order Carnivora belongs to a group of mammals known as Laurasiatheria, which also includes other groups such as bats and ungulates. [6] [7] Within this group the carnivorans are placed in the clade Ferae. Ferae includes the closest extant relative of carnivorans, the pangolins, as well as several extinct groups of mostly Paleogene carnivorous placentals such as the creodonts, the arctocyonians, and mesonychians. [8] The creodonts were originally thought of as the sister taxon to the carnivorans, perhaps even ancestral to, based on the presence of the carnassial teeth. [9] but the nature of the carnassial teeth is different between the two groups. In carnivorans the carnassials are positioned near the front of the molar row, while in the creodonts they are positioned near the back of the molar row. [10] and this suggests a separate evolutionary history and an order-level distinction. [11] In addition recent phylogenetic analysis suggests that creodonts are more closely related to pangolins while mesonychians might be the sister group to carnivorans and their stem-relatives. [8]

The closest stem-carnivorans are the miacoids. The miacoids include the families Viverravidae and Miacidae, and together the Carnivora and Miacoidea form the stem-clade Carnivoramorpha. The miacoids were small, genet-like carnivoramorphs that occupy a variety of niches such as terrestrial and arboreal habitats. Recent studies have shown a supporting amount of evidence that Miacoidea is an evolutionary grade of carnivoramorphs that, while viverravids are monophyletic basal group, the miacids are paraphyletic in respect to Carnivora (as shown in the phylogeny below). [12] [13] [14] [15] [16] [17] [18] [19]

carnivoramorph sp. (UALVP 50993 & UALVP 50994)

carnivoramorph sp. (UALVP 31176)

carnivoramorph sp. (WW-84: USNM 538395)

carnivoraform undet. Genus A (UCMP 110072)

carnivoraform undet. Genus B (SDSNH 56335)

carnivoraform sp. (PM 3868)

Carnivoramorpha as a whole first appeared in the Paleocene of North America about 60 million years ago. [4] Crown carnivorans first appeared around 42 million years ago in the Middle Eocene. [1] Their molecular phylogeny shows the extant Carnivora are a monophyletic group, the crown group of the Carnivoramorpha. [20] From there carnivorans have split into two clades based on the composition of the bony structures that surround the middle ear of the skull, the cat-like feliforms and the dog-like caniforms. [21] In feliforms, the auditory bullae are double-chambered, composed of two bones joined by a septum. Caniforms have single-chambered or partially divided auditory bullae, composed of a single bone. [22] Initially the early representatives of carnivorans were small as the creodonts (specifically, the oxyaenids) and mesonychians dominated the apex predator niches during the Eocene, but in the Oligocene carnivorans became a dominant group of apex predators with the nimravids, and by the Miocene most of the extant carnivoran families have diversified and become the primary terrestrial predators in the Northern Hemisphere.

The phylogenetic relationships of the carnivorans are shown in the following cladogram: [23] [24] [25] [26] [27]

Nimravidae (false saber-toothed cats)

Classification of the extant carnivorans Edit

In 1758 the Swedish botanist Carl Linnaeus placed all carnivorans known at the time into the group Ferae (not to be confused with the modern concept of Ferae which also includes pangolins) in the tenth edition of his book Systema Naturae. He recognized six genera: Canis (canids and hyaenids), Phoca (pinnipeds), Felis (felids), Viverra (viverrids, herpestids, and mephitids), Mustela (non-badger mustelids), Ursus (ursids, large species of mustelids, and procyonids). [28] It wasn't until 1821 that the English writer and traveler Thomas Edward Bowdich gave the group its modern and accepted name. [2]

Initially the modern concept of Carnivora was divided into two suborders: the terrestrial Fissipedia and the marine Pinnipedia. [29] Below is the classification of how the extant families were related to each other after American paleontologist George Gaylord Simpson in 1945: [29]

  • Order Carnivora Bowdich, 1821
    • Suborder Fissipedia Blumenbach, 1791
      • Superfamily CanoideaG. Fischer de Waldheim, 1817
        • Family CanidaeG. Fischer de Waldheim, 1817 – dogs
        • Family UrsidaeG. Fischer de Waldheim, 1817 – bears
        • Family ProcyonidaeBonaparte, 1850 – raccoons and pandas
        • Family MustelidaeG. Fischer de Waldheim, 1817 – skunks, badgers, otters and weasels
        • Family ViverridaeJ. E. Gray, 1821 – civets and mongooses
        • Family HyaenidaeJ. E. Gray, 1821 – hyenas
        • Family FelidaeG. Fischer de Waldheim, 1817 – cats
        • Family OtariidaeJ. E. Gray, 1825 – eared seals
        • Family OdobenidaeJ. A. Allen, 1880 – walrus
        • Family PhocidaeJ. E. Gray, 1821 – earless seals

        Since then, however, the methods in which mammalogists use to assess the phylogenetic relationships among the carnivoran families has been improved with using more complicated and intensive incorporation of genetics, morphology and the fossil record. Research into Carnivora phylogeny since 1945 has found Fisspedia to be paraphlyetic in respect to Pinnipedia, [30] with pinnipeds being either more closely related to bears or to weasels. [31] [32] [33] [34] [35] The small carnivoran families Viverridae, [36] Procyonidae, and Mustelidae have been found to be polyphyletic:

        • Mongooses and a handful of Malagasy endemic species are found to be in a clade with hyenas, with the Malagasy species being in their own family Eupleridae. [37][38][39]
        • The African palm civet is a basal cat-like carnivoran. [40]
        • The linsang is more closely related to cats. [41]
        • Pandas are not procyonids nor are they a natural grouping. [42] The giant panda is a true bear [43][44] while the red panda is a distinct family. [45]
        • Skunks and stink badgers are placed in their own family, and are the sister group to a clade pertaining Ailuridae, Procyonidae and Mustelidae sensu stricto. [46][45]

        Below is a table chart of the extant carnivoran families and number of extant species recognized by various authors of the first and fourth volumes of Handbook of the Mammals of the World published in 2009 [47] and 2014 [48] respectively:

        Carnivora Bowdich, 1821
        Feliformia Kretzoi, 1945
        Nandinioidea Pocock, 1929
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Nandiniidae Pocock, 1929 African Palm Civet Sub-Saharan Africa 1 Nandinia binotata (J. E. Gray, 1830)
        Feloidea G. Fischer de Waldheim, 1817
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Felidae G. Fischer de Waldheim, 1817 Cats Americas, Africa, and Eurasia (introduced to Madagascar, Australasia and several islands) 37 Felis catus Linnaeus, 1758
        Prionodontidae Horsfield, 1822 Linsangs Indomalayan realm 2 Prionodon linsang (Hardwicke, 1821)
        Viverroidea J. E. Gray, 1821
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Viverridae J. E. Gray, 1821 Civets, genets, and oyans Southern Europe, Indomalayan realm, and Africa (introduced to Madagascar) 34 Viverra zibetha Linnaeus, 1758
        Herpestoidea Bonaparte, 1845
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Hyaenidae J. E. Gray, 1821 Hyenas Africa, the Middle East, the Caucasus, Central Asia, and the Indian subcontinent 4 Hyaena hyaena (Linnaeus, 1758)
        Herpestidae Bonaparte, 1845 Mongooses Iberian Peninsula, Africa, the Middle East, the Caucasus, Central Asia, and the Indomalayan realm 34 Herpestes ichneumon (Linnaeus, 1758)
        Eupleridae Chenu, 1850 Malagasy mongooses and civets Madagascar 8 Eupleres goudotii Doyère, 1835
        Caniformia Kretzoi, 1945
        Canoidea G. Fischer de Waldheim, 1817
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Canidae G. Fischer de Waldheim, 1817 Dogs Americas, Africa, and Eurasia (introduced to Madagascar, Australasia and several islands) 35 Canis familiaris Linnaeus, 1758
        Ursoidea G. Fischer de Waldheim, 1817
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Ursidae G. Fischer de Waldheim, 1817 Bears Americas and Eurasia 8 Ursus arctos Linnaeus, 1758
        Phocoidea J. E. Gray, 1821
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Odobenidae J. A. Allen, 1880 Walrus The North Pole in the Arctic Ocean and subarctic seas of the Northern Hemisphere 1 Odobenus rosmarus (Linnaeus, 1758)
        Otariidae J. E. Gray, 1825 Eared Seals Subpolar, temperate, and equatorial waters throughout the Pacific and Southern Oceans and the southern Indian and Atlantic Oceans 15 Otaria flavescens (Linnaeus, 1758)
        Phocidae J. E. Gray, 1821 Earless Seals The sea and Lake Baikal 18 Phoca vitulina Linnaeus, 1758
        Musteloidea G. Fischer de Waldheim, 1817
        Family English Name Distribution Number of Extant Species Type Taxon Image Figure
        Mephitidae Bonaparte, 1845 Skunks and stink badgers Americas, western Philippines, and Indonesia and Malaysia 12 Mephitis mephitis (Schreber, 1776)
        Ailuridae J. E. Gray, 1843 Red Panda Eastern Himalayas and southwestern China 1 Ailurus fulgens F. Cuvier, 1825
        Procyonidae J. E. Gray, 1825 Raccoons Americas (introduced to Europe, the Caucasus, and Japan) 12 Procyon lotor (Linnaeus, 1758)
        Mustelidae G. Fischer de Waldheim, 1817 Weasels, otters, and badgers Americas, Africa, and Eurasia (introduced to Australasia and several islands) 57 Mustela erminea Linnaeus, 1758

        Craniodental region Edit

        The canine teeth are usually large and conical. The canines are thick and incredibly stress resistant. All of the terrestrial species of carnivorans have three incisors on the top and bottom row of the dentition (the exception being is the sea otter (Enhydra lutris) which only has two lower incisor teeth). [49] [50] The third molar has been lost. The carnassial pair is made up by the fourth upper premolar and the first lower molar teeth. Like most mammals the dentition is heterodont in nature, though in some species like the aardwolf (Proteles cristata) the teeth have been greatly reduced and the cheek teeth are specialised for eating insects. In pinnipeds the teeth are homodont as they have evolved to grasp or to catch fish, and the cheek teeth are often lost. [50] In bears and raccoons the carnassial pair is secondarily reduced. [50] The skulls are heavily built with a strong zygomatic arch. [50] Often a sagittal crest is present, sometimes more evident in sexual dimorphic species like sea lions and fur seals, though it has also been greatly reduced seen in some small carnivorans. [50] The braincase is enlarged and the frontoparietal is position at the front of it. In most species the eyes are position at the front of the face. In caniforms the rostrum is usually longer with many teeth, where in comparison with felifoms the rostrum is shorter and have fewer teeth. The carnassial teeth in feliforms, however is more sectional. [50] The turbinates are large and complex in comparison to other mammals, providing a large surface area for olfactory receptors. [50]

        Postcranial region Edit

        Aside from an accumulation of characteristics in the dental and cranial features, not much of their overall anatomy unites them as a group. [49] All species of carnivorans have quadrupedal limbs with usually five digits at the front feet and four digits at the back feet. In terrestrial carnivorans the feet have soft pads. The feet can either be digitigrade seen in cats, hyenas and dogs or plantigrade seen in bears, skunks, raccoons, weasels, civets and mongooses. In pinnipeds the limbs have been modified into flippers. Unlike other marine mammals, such as cetaceans and sirenians which have fully functional tails to help them swim, pinnipeds use their limbs underwater for locomotion.

        In earless seals they use their back flippers sea lions and fur seals use their front flippers, and the walrus use all of their limbs. This resulted in pinnipeds having significantly shorter tails. Aside from the pinnipeds, dogs, bears, hyenas, and cats have distinct and recognizable appearances. Dogs are usually cursorial mammals and are gracile in appearance, often relying on their teeth to hold to prey bears are much larger and rely on their physical strength to forage for food. Cats in comparison to dogs and bears have much longer and stronger frontlimbs armed with retractable claws to hold on to prey. Hyenas are dog-like feliforms that have sloping backs due to their front legs being longer than their hindlegs. The raccoon family as well as the red panda are small, bear-like carnivorans with long tails. The other small carnivoran families Nandiniidae, Prionodontidae, Viverridae, Herpestidae, Eupleridae, Mephitidae and Mustelidae have through convergent evolution maintained the small, ancestral appearance of the miacoids, though there is some variation seen such as the robust and stout physicality of badgers and the wolverine (Gulo gulo). [49] Male carnivorans usually have bacula, though they are absent in hyenas and binturongs. [51]

        Depending on the environment the species is, the length and density of their fur varies. In warm climate species the fur is often short in length and lighter. In comparison to cold climate species the fur is either dense or long, often with an oily substance to keep them warm. The pelage coloration comes in many colors, often including black, white, orange, yellow, red, and many shades of gray and brown. There can be colored patterns too, such striped, spotted, blotched, banded, or otherwise boldly patterned. There seems to be a correlation between habitat and color pattern as for example spotted or banded species tend to be found in heavily forested environments. [49] Some species like the grey wolf is a polymorphic species with different individual variation in colors. The arctic fox (Vulpes lagopus) and the stoat (Mustela erminea) the fur goes from white and dense in the winter to brown and sparse in the summer. In pinnipeds, polar bears, and sea otters have a thick insulating layer of blubber to help maintain their body temperature.


        Contents

        Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: [4]

        • A notochord, a fairly stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, and in wholly aquatic species this helps the animal to swim by flexing its tail.
        • A dorsalneural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. . The pharynx is the part of the throat immediately behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live.
        • Post-anal tail. A muscular tail that extends backwards behind the anus.
        • An endostyle. This is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus. [5] It also stores iodine, and may be a precursor of the vertebrate thyroid gland. [4]

        There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition:

        • All chordates are deuterostomes. This means that, during the embryo development stage, the anus forms before the mouth.
        • All chordates are based on a bilateralbody plan. [6]
        • All chordates are coelomates, and have a fluid-filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm(see Brusca and Brusca). [7]

        The following schema is from the 2014 edition of Vertebrate Palaeontology. [8] [9] The invertebrate chordate classes are from Fishes of the World. [10] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

        • Phylum Chordata
          • †Vetulicolia?
          • Subphylum Cephalochordata (Acraniata) – (lancelets 30 species)
            • Class Leptocardii (lancelets)
            • Subphylum Tunicata (Urochordata) – (tunicates 3,000 species)
              • Class Ascidiacea (sea squirts)
              • Class Thaliacea (salps)
              • Class Appendicularia (larvaceans)
              • Class Sorberacea
              • Superclass 'Agnatha'paraphyletic (jawless vertebrates 100+ species)
                • Class Cyclostomata
                  • Infraclass Myxinoidea or Myxini (hagfish 65 species)
                  • Infraclass Petromyzontida or Hyperoartia (lampreys)
                  • Class †Placodermi (Paleozoic armoured forms paraphyletic in relation to all other gnathostomes)
                  • Class Chondrichthyes (cartilaginous fish 900+ species)
                  • Class †Acanthodii (Paleozoic "spiny sharks" paraphyletic in relation to Chondrichthyes)
                  • Class Osteichthyes (bony fish 30,000+ species)
                    • Subclass Actinopterygii (ray-finned fish about 30,000 species)
                    • Subclass Sarcopterygii (lobe-finned fish: 8 species)
                    • Class Amphibia (amphibians 8,100+ species) [12]
                    • Class Sauropsida (reptiles (including birds) 21,300+ species – 10,000+ species of birds and 11,300+ species of reptiles) [13][14][15]
                    • Class Synapsida (mammals 5,700+ species)

                    Cephalochordata: Lancelets Edit

                    Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. [16] These burrowing filter-feeders compose the earliest-branching chordate sub-phylum. [17] [18]

                    Tunicata (Urochordata) Edit

                    Most tunicates appear as adults in two major forms, known as "sea squirts" and salps, both of which are soft-bodied filter-feeders that lack the standard features of chordates. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus [19] salps float in mid-water, feeding on plankton, and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies. [20] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails they also have rudimentary brains, light sensors and tilt sensors. [19] The third main group of tunicates, Appendicularia (also known as Larvacea), retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps. [21] The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail. [22] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used. [19]

                    1. Notochord, 2. Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill slit, 6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic

                    Craniata (Vertebrata) Edit

                    Craniates all have distinct skulls. They include the hagfish, which have no vertebrae. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails". [23]

                    Most craniates are vertebrates, in which the notochord is replaced by the vertebral column. [24] These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates, [25] but as members of the craniates, the group from which vertebrates are thought to have evolved. [26] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns. [27]

                    The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish. [28] However, molecular phylogenetics, which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish. [29] If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a clade, which has been named the Cyclostomata. [30]

                    Overview Edit

                    There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures. [32]

                    Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic, meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that craniates' nearest relatives are tunicates.

                    All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that they are vertebrates. Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial.

                    It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" shows that many of the traditional classes are paraphyletic.

                    While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux. [33]

                    The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates. [34] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes. [35] [36] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years ago , and hence well before the start of the Cambrian 541 million years ago . [34] The Ediacaran fossil Ernietta, from about 549 to 543 million years ago , may represent a deuterostome animal. [37]

                    Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, 542 million years ago . [38] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. [39] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. [40] [41] Another fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth. [41] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish. [31] [42] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate. [43] On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian. [44]

                    The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected. [5] Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved. [45] Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives. [5] [46]

                    Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago . [46] However, molecular estimates of dates often disagree with each other and with the fossil record, [46] and their assumption that the molecular clock runs at a known constant rate has been challenged. [47] [48]

                    Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.

                    Diagram Edit

                    Phylogenetic tree of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported [49] [50] [51] [52]


                    Modern Advances in Phylogenetic Understanding Come from Molecular Analyses

                    The phylogenetic groupings are continually being debated and refined by evolutionary biologists. Each year, new evidence emerges that further alters the relationships described by a phylogenetic tree diagram.


                    Watch the following video to learn how biologists use genetic data to determine relationships among organisms.

                    Nucleic acid and protein analyses have greatly informed the modern phylogenetic animal tree. These data come from a variety of molecular sources, such as mitochondrial DNA, nuclear DNA, ribosomal RNA (rRNA), and certain cellular proteins. Many evolutionary relationships in the modern tree have only recently been determined due to molecular evidence. For example, a previously classified group of animals called lophophorates, which included brachiopods and bryozoans, were long-thought to be primitive deuterostomes. Extensive molecular analysis using rRNA data found these animals to be protostomes, more closely related to annelids and mollusks. This discovery allowed for the distinction of the protostome clade, the lophotrochozoans. Molecular data have also shed light on some differences within the lophotrochozoan group, and some scientists believe that the phyla Platyhelminthes and Rotifera within this group should actually belong to their own group of protostomes termed Platyzoa.

                    Molecular research similar to the discoveries that brought about the distinction of the lophotrochozoan clade has also revealed a dramatic rearrangement of the relationships between mollusks, annelids, arthropods, and nematodes, and a new ecdysozoan clade was formed. Due to morphological similarities in their segmented body types, annelids and arthropods were once thought to be closely related. However, molecular evidence has revealed that arthropods are actually more closely related to nematodes, now comprising the ecdysozoan clade, and annelids are more closely related to mollusks, brachiopods, and other phyla in the lophotrochozoan clade. These two clades now make up the protostomes.

                    Another change to former phylogenetic groupings because of molecular analyses includes the emergence of an entirely new phylum of worm called Acoelomorpha. These acoel flatworms were long thought to belong to the phylum Platyhelminthes because of their similar “flatworm” morphology. However, molecular analyses revealed this to be a false relationship and originally suggested that acoels represented living species of some of the earliest divergent bilaterians. More recent research into the acoelomorphs has called this hypothesis into question and suggested a closer relationship with deuterostomes. The placement of this new phylum remains disputed, but scientists agree that with sufficient molecular data, their true phylogeny will be determined.


                    Phylum Platyhelminthes : General Characteristics and Its Classification

                    The representatives of the phylum Platyhelminthes are commonly known as the flatworms or tapeworms. The word ‘Platyhelminthes’ is derived from the Greek word, ‘platy’ meaning flat and ‘helminth’ meaning worm. They are simple soft-bodied, bilaterian, unsegmented invertebrate animals. The Phylum Platyhelminthes makes up the 4th largest phylum among the animal kingdom. But among the acoelomate organisms, the phylum Platyhelminthes constitutes the largest phylum with more than about 20,000 known species throughout the world. Among them, around 80% live as parasitic life on humans and other animals and few are free-living.

                    The parasitic forms cause some trouble to the host animals, feed on host`s tissues and make certain diseases such as Schistosomiasis, or snail fever, Taeniasis, etc. while the free-living flatworms are scavengers or predators. Generally, free-living species live in water and some in shaded, humid terrestrial ecosystems, such as leaf litter. The members of this phylum have diverse sizes which range from microscope to 3 feet long.


                    Giant Squid

                    The giant squid is a cephalopod species native to the deep sea. They are elusive and rarely observed alive, but are famous for their immense size, growing up to 43 ft (13 m) in length and weighing up to 606 lb (275 kg). Indeed, the giant squid is one of the largest known invertebrate species living today, second only to the colossal squid. There is some debate about the number of different species of giant squid. However, the most recent genetic evidence suggests that there is only one known species, Architeuthis dux.

                    Description

                    The Giant squid appears similar to many other more commonly observed squid species. They have eight arms and two longer tentacles surrounding their hard beaks and radula, a unique structure used for breaking food into pieces small enough to ingest. At the base of their tentacles the mantle begins, which is flanked by two fin-like structures that aid in mobility. The mantle terminates in a spade-like shape. Only the colossal squid grows larger than the giant squid, with the most recent estimates putting females at about 43 ft (13 m) long and males at about 33 ft (10 m). Giant squid can weigh more than 600 lb (272 kg), a staggering mass by any standard.

                    Distribution

                    Giant squids are deep-ocean dwellers, making them difficult study subjects. In fact, the first image of a giant squid in the wild wasn’t collected until 2004. Normally they are found in fishing nets that are deployed in deep waters. This occurs in all of the world’s oceans, suggesting they widespread. The North Atlantic ocean is particularly deep, with squid often caught near Newfoundland, Norway, the British Isles.

                    Other squid species are known to complete significant diurnal vertical migrations, visiting shallow waters in darkness in order to hunt and then retreating to deeper waters in the daylight hours. It is unclear how deep giant squid live or how near the surface they may normally venture, but based on knowledge of sperm whales – their primary prey species – giant squid likely inhabit waters between 980-3 280 ft (300-1 000 m) deep.

                    Behavior and Ecology

                    Despite being one of the largest two living invertebrate species and a fascination of mankind for centuries, relatively little is known about the species. This is due to their apparent preference for inhabiting very deep-water ocean environments.

                    Giant squid are known to feed on other deep-water species such as fish. They also eat other squid species and even smaller members of their own species. Like other squid species, they use their tentacles – with the serrations found on its ‘sucker’-like features – to grip prey. These will bring the prey towards their powerful beaks that conceal their radula, a combination of small teeth and a modified tongue. The radula shreds the prey before it enters the esophagus and, ultimately, the digestive tract of squid. When giant squid are pulled up in fishing nets they are typically alone, suggesting that they are generally solitary and hunt alone.

                    Apart from other giant squids, the only known predators of adults of the species are sperm whales. These predatory whales will dive thousands of feet below the surface while hunting squid. Although it has not been confirmed, there is some evidence that pilot whales also feed on the giant cephalopod as well. Juvenile giant squid will fall victim to other large species, such as various deep-sea shark species and other predators of the deep.

                    Reproduction

                    Due to their remote environment, little is known about the reproductive cycle of the giant squid. The exact mechanism of copulation is much debated but undoubtedly occurs when the male transfers sperm to the egg mass on the female. Like other squid, females produce large quantities of eggs that she will carry around with her, eventually ‘laying’ them in several groups or ‘capsules’.

                    Once hatched, the young giant squid are independent, subsisting in their larval form on nutrients inside their egg sac. Eventually, they will grow to juvenile squid, reaching sexual maturity after about three years. Despite their large size, giant squids live for approximately 5 years.

                    The species is currently listed as “Least Concern” on the IUCN Red List of Threatened Species. With so little known about their life cycle and behavior, the current status of giant squid populations is unclear

                    Fun Facts about Giant Squid!

                    Giant squid, like all cephalopods, are fascinating and interesting creatures. From their ability to spray ink at predators to their jet propulsion strategy for mobility, there is no shortage of fun facts about the giant squid to explore.

                    Rocket Squid

                    Like all cephalopods, giant squid use jet propulsion for mobility. This is a very effective strategy and allows these species to move quickly and accelerate rapidly, perhaps to avoid prey. The jet is produced by the contraction of the large mantle muscles, forcing water through a narrow funnel-like organ. This is then repeated by drawing water into the cavity before once again forcing out again through a small opening of the respiratory system. This unique propulsion system has never been observed in giant squid due to the difficulty of observing the species in its natural environment. However, given their large size, it is likely to be a remarkable and fascinating display.

                    Inked Out

                    Like other squid and most cephalopods, giant squid have the ability to produce and distribute an ink-like substance. They will secrete it from their ‘ink sac’, unique organs in which they produce the substance. It is blackened by a molecule called melanin, a natural pigment that occurs in most organisms including plants, fungi, and bacteria. This ink has been used by humans for millennia for various purposes including in drugs and medicine.

                    This trait is used to distract and deter predators and is often used in combination with their jet propulsion abilities. They can spray ink at their would-be predator then move away rapidly, leaving the animal disoriented and unable to locate the evasive invertebrate.

                    A True Giant

                    In the case of the giant squid, it seems to have truly earned its name. In fact, the species is held up as an example of deep-sea gigantism, or abyssal gigantism, an observation that invertebrate species that live in the deep sea tend to be larger than their shallow-water relatives. As these habitats are so difficult to study due to their general inaccessibility to humans, it is not entirely clear why this phenomenon occurs. However, various proposed explanations include variations in temperature, where the colder temperature dictates the larger body morphs. The scarcity of food and the lack of predation in the deep sea may also create selective pressures that lead to this discrepancy in size.

                    At up to 43 ft (13 m) long, the giant squid is not generally capable of destroying a ship and threatening its crew the way the myths surrounding them suggest. However, it is one of two of the largest invertebrate species on the planet, making it a true giant by any measure.


                    Asexual Ascomycota and Basidiomycota

                    Reproduction of the fungi in this group is strictly asexual and occurs mostly by production of asexual conidiospores (Figure). Some hyphae may recombine and form heterokaryotic hyphae. Genetic recombination is known to take place between the different nuclei.

                    Aspergillus niger is an asexually reproducing fungus (phylum Ascomycota) commonly found as a food contaminant. The spherical structure in this light micrograph is a conidiophore. (credit: modification of work by Dr. Lucille Georg, CDC scale-bar data from Matt Russell)

                    The fungi in this group have a large impact on everyday human life. The food industry relies on them for ripening some cheeses. The blue veins in Roquefort cheese and the white crust on Camembert are the result of fungal growth. The antibiotic penicillin was originally discovered on an overgrown Petri plate, on which a colony of Penicillium fungi killed the bacterial growth surrounding it. Other fungi in this group cause serious diseases, either directly as parasites (which infect both plants and humans), or as producers of potent toxic compounds, as seen in the aflatoxins released by fungi of the genus Aspergillus.


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