The Upright Ape: A New Origin of the Species

by Aaron G. Filler, MD, PhD

Figure 8-1 Locomotor Stance in the Tetrapods

Figure 8-1 Locomotor Organization in Land Vertebrates

 A – The early tetrapod Eryops. At a very early point in the emergence onto land, fins became five fingered digits and the spine demonstrated its five regions.

B, C, D – Walking in amphibians and some reptiles involves body undulation as in a fish, with the limbs far out to the side. In the evolution of mammals, the limbs shift to a new position below the body.

Further Details:

A - Eryops is now considered a semi-aquatic amphibian. It lived in the early Permian about 295 million years ago and demonstrates many of the features of the earliest land tetrapods including a vertebral column with differentiation into different regions that could support the standing weight of the animal out of water on all four limbs.

B - The limbs are out to the side in a salamander, but in mammals the proximal and distal portions of the limbs are both typically directly below the animal.

C - A progressive transition from the horizontally directed proximal limbs of amphibians and early synapsid reptiles towards the configuration in therian mammals is shown by the intermediate organizations in these mammal-like reptiles. C1 shows the relatively horizontal orientation in a Pelycosaur from the early Permian around 270 mya. C2 shows the more vertically oriented configuration in the cynodont Cynognathus. C3 is the transversodontid cynodont Massetognathus. C4 is a lateral view of the cynodont Thrinaxodon. C5 is the chiniquodontid cynodont Probelesodon. The cynodonts in C2, C3, C4 and C5 are from the Triassic period of around 210 mya near the time of origin of the mammals.

D - Demonstrates the configuration in a typical modern cursorial (running) eutherian mammal using a carnivore as an example.

Figure credits -

A - Photograph by Aaron Filler. © Copyright Aaron G. Filler (2007) The Upright Ape: A New Origin of the Species. Franklin Lakes, New Jersey; New Page Books.

B & D- From: Kardong, K. V. (2005). Vertebrates : Comparative Anatomy, Function, Evolution, pg. 345, fig 9.31. New York, McGraw-Hill Higher Education. © McGraw-Hill reproduced with their permission.

C1 & C2- (limb) – From: Jenkins, F. A. (1971). The Postcranial Skeleton of African Cynodonts; Problems in the Early Evolution of the Mammalian Postcranial Skeleton. New Haven, Peabody Museum of Natural History Yale University. Drawing reproduced by permission of Yale Peabody Museum.

C2- (lateral view) - From: Romer, A. S. and A. D. Lewis (1973). The Chanares (Argentina) Triasic reptile fauna. XIX. Postcranial materials of the Cynodonts Probelesdon and Probainognathus. Breviora 407: 1-26. (drawing reprodcued by permission of Harvard Museum of Comparative Zoology).

C3 – From: Jenkins, F. A. (1970). The Chanares (Argentina) Triassic reptile fauna. VII. The postcranial skeleton of the traversodontid Massetognathus pascuali (Therapsida, Cynodontia) Breviora 352: 1-28. (drawing reprodcued by permission of Harvard Museum of Comparative Zoology).

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