This picture gallery picks up where the Chapter 9 gallery left off. Again, the animals shown here represent key points in our ancestral evolution. It is impossible to know exactly which fossil species were our ancestors, so each image here should be construed with some degree of abstraction. Dates are very grossly rounded. These particular specimens are selected as basal embodiments of their clades, so they are known to be pretty closely related to our line of descent.
300 MYA: Synapsids are one of the three living lines of amniotes. They are distinguished from anapsids and diapsids (today’s reptiles and birds) by objective features like holes in the skull. Synapsids were originally reptile-like, but gradually evolved into mammals.
250 MYA: Cynodonts came about halfway through the reptile-mammal transition. The early cynodonts were the ancestors who lived through the worst of the P-T Extinction, while almost all other tetrapods died around them. They had noticeably more erect legs than earlier synapsids, and their transition to warm-bloodedness was probably mostly complete. 1
200 MYA: It is difficult to know for sure, but fur may have evolved in pre-mammalian cynodonts.
200 MYA: Mammals are recognizable in fossils dating back a little further than the 200 million year mark, based on skeletal definitions such as the teeth, jaws, and middle ear. Mesozoic mammals were tiny, and they evolved very slowly.
150 MYA: Eutheria was a transitional form. The earliest eutheria laid eggs, but its descendants all give live birth today.
70 MYA: Placental mammals give live birth. The earliest placental species has actually been modeled in an amazing supercomputer team project that analyzes fossils and reconstructs their common ancestors. 2 This hypothetical critter, the Eve of all wombs, has been named Shrewdinger by popular vote. Her “official” image is copyrighted, so I can’t duplicate it here, but the story is too cool to pass up. Here’s the link to that picture and a video describing the project. She basically looked like a shrew:
70 MYA: Primates emerged as part of the early Cenozoic mammal radiation. We are defined mostly by our grasping hands and forward-looking eyes.
50 MYA: Simians are active during the day. They have larger brains and more advanced social structures than earlier primates. Catarrhines in particular have opposable thumbs and human-like teeth, but for the long canines. The tail started to lose its function as a useful limb and gradually became weaker and shorter.
Chapter 8 Margin Notes (Blog Posts)
- Kemp, Mammal-like reptiles and the origin of mammals, Academic Press, 1982, ISBN 0-12-404120-5, p. 301. ↩
- O’Leary et al, “The placental mammal ancestor and the post-K-Pg radiation of placentals”, Science vol. 339 (2/08/2013), pp. 662 – 667, http://www.stonybrook.edu/sb/images/features/oleary130208.pdf (accessed 5/23/16). ↩
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