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The cynodont Thrinaxodon survived the great Permian extinction. In the world of the archosaurs, cynodonts became small and evolved many mammalian traits as they adapted to hunting at night and steering clear of dangerous archosaurs, including birds. The tritheolodonts (Banner above) evolved by the end of the Triassic. They were just on the cusp of classification as a mammal.
Figure 11‑9. Early Triassic carnivorous cynodont, Thrinaxodon (251-247 Ma). Credit: Nobu Tomura. Used here per CC BY-SA 2.5.
Thrinaxodon was a carnivorous cynodont that was the size of a cat. It appeared in the fossil record just after the Great Permian extinction and survived in the early Triassic until 247 Ma. It had a semi sprawling posture between reptiles and mammals (Figure 11‑9). It was the size of a cat and lived in burrows. It already had many mammal features such as large sagittal crest, canine and post canine teeth, double occipital chondral, and open ribs.
Two primary mammal characteristics are hair and mammary glands; however, these characteristics are not preserved in the fossil record. Mammalian characteristics preserved in bony fossils include teeth that include incisors, canines, premolars, and palate, secondary palate which allows eating and breathing simultaneously. The also have an erect gait, larger brain, and unique bones in the ear. The cynodonts gradually acquired more mammalian characteristics such as an enlarged dentary, reduced reflected lamina, bony secondary palate, complex postcanine dentition, reduced lumbar ribs, and parasagittal gait.[1] Their large skulls and teeth indicate that they were carnivores.
Step by step, paleontologists can see the shift from peglike teeth in reptiles to differentiated teeth in mammals, and therapsids had well differentiated teeth by the Late Permian; the reptilian jaw with five bones and a single bone in the ear evolved to the single boned mammalian jaw with three bones in the ear, the malleus, incus, and stapes.[2] [3] Reptiles produce multiple sets of teeth in their lifetime. Mammals have only two sets of teeth in their life, which enables precision in biting.
One of the significant skeletal changes that occurred in Cynodontia was the loss of lower ribs (Figure 11‑10). This was probably linked to the development of a diaphragm (expands and contracts the lungs), which was probably needed for increased oxygen transfer into the body and support of a higher metabolism rate.[4] In addition to supplying an increased rate of metabolism, heavy breathing is important in mammals because most heat in mammals is dissipated through breathing. Other physiological developments that increased energy production and transfer were fully separated pulmonary (breathing) and systemic circulatory systems, expanded cardiac output, increased blood volume and blood oxygen capabilities, and increased aerobic tissue enzymatic activities in order to sustain high rates of energy production during physical exertion.[5] Early Triassic cynodonts (Figure 11‑10) were dog-sized predators with a mix of features between reptiles and mammals. During the middle and late Triassic, small cynodonts showed further progression to mammal-like features.[6]
Figure 11‑10. Early to mid Triassic cynodont, Cynognathus craterootus. Credit: Nobu Tomura. Used here per CC BY-SA 2.5.
One early to middle Triassic cynodont, Cynognathus (245 to 230 Ma), had three types of teeth: incisors, canines, and shearing check teeth (more similar to mammal teeth). It was approximately the size of a wolf and bore a resemblance to modern mammals (Figure 11‑10). Ninety percent of the lower jaw was constructed from a single bone (versus the five bones of early synapsids); thus, the jaw was well on its way to the single bone construction of mammals.
Paleontologists can track small changes from species to species. The Probainogntathidae (Figure 11‑11) were Late Triassic cynodonts that had a development in the jaw joint that aligned it with later mammals (Figure 11‑12 and Figure 11‑13). This type of small change is why it is placed in the line of mammals (Figure 11‑14) while cynognathus is not. The sizes of different Probainogntathidae ranged from rat to small dog.
Figure 11‑11. Late Triassic carnivorous cynodont, Probainognathus (235-221 Ma). Credit: Nobu Tomura. Used here per CC BY-SA 4.0.
Figure 11‑12. Probainognathus skull. Credit: Ghedoghedo. Used here per CC BY-SA 4.0.
Figure 11‑13. Probainognathus dentition. Credit: AG Martinelli. Used here per CC BY-SA 4.0.
Martinelli AG, Soares MB, Schwanke C - Martinelli AG, Soares MB, Schwanke C (2016) Two New Cynodonts (Therapsida) from the Middle-Early Late Triassic of Brazil and Comments on South American Probainognathians. PLoS ONE 11(10): e0162945. doi:10.1371/journal.pone.0162945
Fossils of the jaw and teeth are the best preserved and are often the only fossil remains of animals. Patterns are retained from one generation to the next. Thus, details of the jaw and order and pattern of teeth clearly show the path of evolution from one clade to the next. This is why paleontologists can interpret of the identity of fossils based on a few teeth.
The Trithelodontidae evolved from the Probainognathidae, followed by the Tritylodontidae and the Mammaliaformes in the Late Triassic (Figure 11‑14). They had many mammal characteristics and are considered the last of the nonmammal synapsids. The Trithelontidae and Tritylodontidae are classified as Mammaliamorpha.
Figure 11‑14. Cladogram of the evolution of mammals from cynodonts in the Triassic and Early Jurassic . Wikipedia.
Figure 11‑15. Oligokyphus, a trytilodont from the Early Jurassic of England, pencil drawing. Credit: Nobu Tomura. Used here per CC BY-SA 2.5.
Oligokyphus was a tritylodont from the early Jurassic in England (Figure 11‑15). Sinocodon (208 Ma) was a late Triassic mammaliamorph.
Morganucodon (Late Triassic - Early Jurassic)[7] (Figure 11‑16) was a 10 cm long, nocturnal, insectivore, trithelodont. Scientists classify it as a mammaliaformes (closer to mammals than mammaliamoropha). It had nearly mammalian teeth and jaw. Because it had a short lifespan and had a slower metabolism than mammals, some paleontologists do not classify it as a mammal; however, other paleontologists classify it as the first mammal. Scientists do not know whether they laid eggs or gave birth to immature young, but evidence indicates that they fed their young by lactation (milk). It had glands that aid in grooming, which means that it probably had hair. The primary diet was insects, as with many of the mammals that followed it in the Jurassic and Cretaceous. One of the key advances in the evolution of mammals was the mammalian ear, with its complex bone structure that amplifies sound much better than the reptilian ear.
Figure 11‑16. Mammaliaformes (trithelodont) – Morganucodon (205 Ma). Credit: Funkmonk. Used here per CC BY-SA 3.0.
Morganucodon had three bones near the jaw joint, but in a reduced form, that would eventually form the three bones in the mammalian inner ear. The trithelodont back teeth had evolved ridges called cusps, which were more efficient at chewing food and were the precursors to molars. It also lived in the Early Triassic.
References
References
[1] Botha, Jennifer, The Cynodontia, National Museum in Bloemfontein, South Africa, Accessed in 2006 at http://www.nasmus.co.za/PALAEO/jbotha/the_cynodontia.htm
[2] Michael Benton, The Mammals, In Vertebrate paleontology. Edited by M. Benton, (Wiley, 2000), 295.
[3] Michael Benton, Evidence of Evolutionary Transitions, ActionBioscience.org article (2001), Accessed at http://www.actionbioscience.org/evolution/benton2.html
[4] Botha, Cynodontia.
[5] John Ruben, and Terry Jones, Selective Factors Associated with the Origin of Fur and Feathers, American Zoologist, 40 (2000) no. 4: 585-596.
[6] Michael Benton, E. Cook, E. and J. Hooker, Mesozoic and Tertiary Fossil Mammals and Birds of Great Britain, Geological Conservation. Review Series (2005), No. 32, Joint Nature Conservation Committee, Peterborough, 215. Publication available at http://jncc/defra.gov.uk/PDF/V32chap1Part1.pdf
[7] Benton, Mesozoic.
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Mammaliaformes (trithelodont) – Morganucodon (205 Ma). Credit: Funkmonk. Used here per CC BY-SA 3.0.
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