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The Anapsida: turtles and relatives_

The oldest anapsids were small insect eaters. During the Permian and Triassic, some unusual anapsids came on the scene. The most diverse of these were the procolophonids (Fig. 16.19a), small animals with triangular skulls and broad teeth adapted to a diet of tough plants and insects.

The turtles appeared first in the Late Triassic, being represented by Proganochelys (Fig. 16.19b). Modern turtles have no teeth, but Proganochelys still had some on its palate. The skull is solid, and the body is covered above and below by a bony shell. Turtles live on land, in ponds (Fig. 16.19c) and in the sea. Some marine turtles of the Cretaceous reached 3 m in length.

Our current understanding of amniote evolution (see Fig. 16.18) has been challenged by new molecular studies that suggest turtles might in fact be modified diapsid reptiles; if this is so, and it is still controversial (Lee et al. 2004), then the clade Anapsida might no longer have any meaning.

A world of synapsids_

The first synapsids, known from the Late Carboniferous and Early Permian, are grouped loosely as "pelycosaurs". Most of these were small- to medium-sized insectivores and carnivores with powerful skulls and sharp, flesh-piercing teeth. Some later pelycosaurs, like Dimetrodon (Fig. 16.20a), had vast sails supported on vertical spines growing up from the large orbit

Figure 16.19 Fossil and recent anapsid reptiles: (a) skull of the Triassic procolophonid Procolophon; (b) skull of the Triassic turtle Proganochelys; (c) a fossilized snapping turtle, with the head (bottom right) and skeleton separated from the carapace, from pond sediments filling an impact crater at Steinheim, Germany. (a, based on Carroll & Lindsay 1985; b, based on Gaffney & Meeker 1983.)

Figure 16.19 Fossil and recent anapsid reptiles: (a) skull of the Triassic procolophonid Procolophon; (b) skull of the Triassic turtle Proganochelys; (c) a fossilized snapping turtle, with the head (bottom right) and skeleton separated from the carapace, from pond sediments filling an impact crater at Steinheim, Germany. (a, based on Carroll & Lindsay 1985; b, based on Gaffney & Meeker 1983.)

Figure 16.20 Synapsids of the Permian: (a) the carnivorous pelycosaur Dimetrodon; (b) the carnivorous gorgonopsian Lycaenops; and (c) the herbivorous dicynodont Dicynodon. (a, based on Gregory 1951/1957; b, c, courtesy of Gillian King.)

vertebrae, perhaps used in controlling body temperature. The pelycosaurs also include a number of groups that adapted to plant eating, among the first herbivorous land vertebrates.

Synapsids radiated dramatically in the Late Permian as a new clade, the Therapsida. The most astonishing carnivores were the gor-gonopsians (Fig. 16.20b) with their large, wolf-like bodies and massive saber teeth that they probably used to attack the larger thick-skinned herbivores. The dicynodonts had bodies shaped like overstuffed sausages, and no teeth at all, or only two tusks (Fig. 16.20c). They were successful herbivores and some of the first animals to have a complex chewing cycle that allowed them to tackle a wide variety of plant foods. Late Permian therap-sids are common in the continental sediments of the Karoo Basin in South Africa and the Urals in Russia. At the end of the Permian, at the time of a major mass extinction in the sea (see p. 170), most of these animals died out. The gorgonopsians disappeared and the dicyn-odonts were nearly wiped out - extinction on a huge scale.

The cynodonts were an important Triassic synapsid group. The Early Triassic form Thri-naxodon (Fig. 16.21a) looked dog-like. In the snout area of the skull, there are numerous small canals that indicate small nerves serving the roots of sensory whiskers. If Thrinaxodon had whiskers, it clearly also had the potential for hair on other parts of its body, and this implies insulation and temperature control. Cynodonts evolved along several lines during the Triassic, and gave rise to mammals, such as Megazostrodon (Fig. 16.21b), in the Late Triassic and Early Jurassic.

The transition from basal synapsid to mammal is marked by an extraordinary shift of the jaw joint into the middle ear. Reptiles typically have six bones in the lower jaw and the articular bone articulates with the quadrate in the skull (Fig. 16.21c). In mammals, on the other hand, there is a single bone in the lower jaw, the dentary, which articulates with the squamosal (Fig. 16.21d). The reptilian articular-quadrate jaw joint became reduced in Triassic cynodonts, and moved into the middle ear passage. That is why we have three tiny ear bones, the hammer, anvil and stirrup, which transmit sound from the ear drum to the brain, while reptiles have only one, the stirrup or stapes.

lumbar vertebrae lumbar vertebrae

(d) dentary

Figure 16.21 Transition to the mammals: (a) the Early Triassic cynodont Thrinaxodon; (b) the Early Jurassic mammal Megazostrodon; and (c, d) skulls of an early synapsid (c) and a mammal (d) to show the reduction in elements in the lower jaw and switch of the jaw joint. (a, based on Jenkins 1971; b, based on Jenkins & Parrington 1976; c, d, based on Gregory 1951/1957.)

(d) dentary

Figure 16.21 Transition to the mammals: (a) the Early Triassic cynodont Thrinaxodon; (b) the Early Jurassic mammal Megazostrodon; and (c, d) skulls of an early synapsid (c) and a mammal (d) to show the reduction in elements in the lower jaw and switch of the jaw joint. (a, based on Jenkins 1971; b, based on Jenkins & Parrington 1976; c, d, based on Gregory 1951/1957.)

Dinosaurs and mammals_

People usually think of the dinosaurs as precursors of the mammals. Dinosaurs famously ruled the Earth for 160 Myr of the Mesozoic, and then were replaced by the mammals 65 Ma. However, as we have seen, the mammals arose in the Late Triassic, about the same time as the first dinosaurs. So, both groups evolved side by side through the Late Triassic, Jurassic and Cretaceous - the dinosaurs as large to very large beasts, and the mammals generally scuttling unobtrusively through the undergrowth. Our understanding of how both groups evolved has changed enormously in recent years, and this is discussed in Chapter 17.

1 How has the application of cladistics (see p. 129) affected our ideas about basal vertebrate phylogeny? Look at older books and papers, and compile simple trees of Agnatha, Placodermi, Chondrichthyes, Acanthodii and Osteichthyes as accepted in 1960, 1970, 1980, 1990 and 2000.

2 Read about the typical Devonian fishes of either the Orcadian Basin in Scotland or Miguasha in Canada, and attempt to reconstruct a food web (see p. 88): what eats what?

3 How has the discovery of seven- and eight-fingered tetrapod fossils from the Late Devonian changed our views about the development of fingers and toes? Read about older and newer views on development (embryology) of limb buds and digits, and find out about the Hox genes (see p. 148).

4 How did global environments change through the Carboniferous and Permian, and how did this affect tetrapod evolution?

5 How did mammal-like characters appear in the synapsids of the Permian and Trias-sic? Draw up a simple cladogram of 10 key synapsid genera, and mark on the acquisition of key apomorphies.

Armstrong, H.A. & Brasier, M. 2004. Microfossils, 2nd edn. Blackwell, Oxford. (Chapter on conodonts.)

Benton, M.J. 1991. The Reign of the Reptiles. Crescent, New York.

Benton, M.J. 2003. When Life Nearly Died. Norton, New York.

Benton, M.J. 2005. Vertebrate Paleontology, 3rd edn. Blackwell, Oxford.

Carroll, R.L. 1987. Vertebrate Paleontology and Evolution. Freeman, San Francisco.

Clack, J.A. 2002. Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press, Bloomington, IN.

Cracraft, J. & Donoghue, M.J. 2004. Assembling the Tree of Life. Oxford University Press, New York.

Gould, S.J. (ed.) 2001. The Book of Life. Norton, New York.

Long, J. 1996. The Rise of Fishes. Johns Hopkins University Press, Baltimore.

Shubin, N.H. 2008. Your Inner Fish. Pantheon, New York.

Zimmer, C. 1999. At the Water's Edge. Touchstone, New York.

References

Aldridge, R.J., Briggs, D.E.G., Smith, M.P. et al. 1993a. The anatomy of conodonts. Philosophical Transactions of the Royal Society of London B 340, 405-21.

Aldridge, R.J., Jeppsson, L. & Dorning, K.J. 1993b. Early Silurian oceanic episodes and events. Journal of the Geological Society of London 150, 50113.

Alexander, R.McN. 1975. The Chor dates. Cambridge University Press, Cambridge.

Armstrong, H.A. & Brasier, M. 2004. Microfossils, 2nd edn. Blackwell, Oxford.

Carroll, R.L. 1987. Vertebrate Paleontology and Evolution. Freeman, San Francisco.

Carroll, R.L. & Lindsay, W. 1985. The cranial anatomy of the primitive reptile Procolophon. Canadian Journal of Earth Sciences 22, 1571-87.

Coates, M.I., Jeffery, J.E. & Ruta, M. 2002. Fins to limbs: what the fossils say. Evolution and Development 4, 390-401.

Daeschler, E.B., Shubin, N.H. & Jenkins Jr., F.A. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440, 75763.

Donoghue, P.C.J. & Purnell, M.A. 2005. Genome duplication, extinction and vertebrate evolution. Trends in Ecology and Evolution 20, 312-19.

Furlong, R.F. & Holland, P.W.H. 2004. Polyploidy in vertebrate ancestry: Ohno and beyond. Biological Journal of the Linnean Society 82, 425-30.

Gaffney, E.S. & Meeker, L.J. 1983. Skull morphology of the oldest turtles: a preliminary description of Proganochelys quenstedti. Journal of Vertebrate Paleontology 3, 25-8.

Review questions

Further reading

Gagnier, P.-Y. 1993. Sacabambaspis janvieri, vertébré Ordovicien de Bolivie: 1. analyse morphologique. Annales de Paléontologie 79, 19-69.

Grande, L. 1988. A well preserved paracanthopterygian fish (Teleostei) from freshwater lower Paleocene deposits of Montana. Journal of Vertebrate Paleontology 8, 117-30.

Gregory, W.K. 1951/1957. Evolution Emerging, Vols 1 and 2. Macmillan, New York.

Hou, X.-G., Aldridge, R.J., Siveter, D.J. et al. 2002. New evidence on the anatomy and phylogeny of the earliest vertebrates. Proceedings of the Royal Society B 269, 1865-9.

Jenkins Jr., F.A., 1971. The postcranial skeleton of African cynodonts. Bulletin of the Peabody Museum of Natural History 36, 1-216.

Jenkins Jr., F.A. & Parrington, F.R. 1976. The postcranial skeletons of the Triassic mammals Eozostrodon, Megazostrodon and Erythrotherium. Philosophical Transactions of the Royal Society B 173, 387-431.

Lee, M.S.Y., Reeder, T.W., Slowinski, J.B. & Lawson, R. 2004. Resolving reptile relationships: molecular and morphological markers. In J. Cracraft & M.J. Donoghue (eds) Assembling the Tree of Life. Oxford University Press, Oxford, pp. 451-67. Moy-Thomas, J.A. & Miles, R.S. 1971. Palaeozoic

Fishes, 2nd edn. Chapman and Hall, London. Shu, D.-G., Luo, H.L., Conway Morris, S. et al. 1999. Lower Cambrian vertebrates from South China. Nature 402, 42-6. Shubin, N.H., Daeschler, E.B. & Jenkins Jr., F.A. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440, 764-70. Sweet, W.C. & Donoghue, P.C.J. 2001. Conodonts: past, present and future. Journal of Paleontology 75, 1174-84.

Trewin, N.H. 1986. Palaeoecology and sedimentology of the Achanarras fish bed of the Middle Old Red Sandstone, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 77, 21-46.

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