Question

Describe the plesiadapiform mammals. In what ways do they resemble modern primates? In what ways are they different?

Answer 1

Plesiadapiform mammals -

The group of fossils that is the focus of debates over primate origins is called plesiadapiform. This is an incredibly diverse group including more than 140 named species arranged into 11 different families . The first record appears as the non avian dinosaurs were going extinct about 65 million years ago , near the beginning of Paleocene . Some plesiadapiform persist well into the eocene , with the last species going extinct around 37 million years ago ( Silcox and Gunnell 2008 ) .

Similarities with modern primates -

Plesiadapiforms share some traits with living primates, including long fingers well designed for grasping, and other features of the skeleton that are related to arboreality (Bloch & Boyer, 2002). One species, Carpolestes simpsoni, even had a divergent big toe with a nail (Bloch & Boyer, 2002). Dentally, plesiadapiforms look quite similar to definitive primates, with broad talonid basins and a similar pattern of cusps and crests. However, no known plesiadapiform exhibits the features of living primates associated with specialized vision, such as the postorbital bar or convergent orbits. Even though they exhibit some features for primate-like grasping, known plesiadapiforms still retain claws on most of their digits. These contrasts have led some workers (e.g., Martin, 1968; Cartmill, 1974) to suggest that plesiadapiforms do not belong in the Order Primates. Others (e.g., Kay et al., 1990; Beard, 1990) have further argued that plesiadapiforms are not most closely related to primates, but instead are closer kin to another order, Dermoptera.

Difference -

The most comprehensive analysis to date of the relationships among plesiadapiforms, primates, and closely related mammals is by Bloch and colleagues (2007; Figure 3). The results of this analysis support the idea that plesiadapiforms are more closely related to primates than to any other group. These authors argued that plesiadapiforms should therefore be considered stem primates; they adopted the name Euprimates (Hofstetter, 1977) for living primates and any fossil forms that exhibit all of the features of modern primates listed above. It is worth noting, however, that not all researchers are convinced by Bloch et al.'s (2007) results. These workers generally equate the Order Primates with Euprimates, excluding plesiadapiforms from the order. This leads to a very different conception of what constitutes primate origins than discussed here (e.g., Soligo & Martin, 2007), with the order not appearing until nearly 10 million years later, when the first forms that share traits such as the postorbital bar and nails on most digits (e.g., adapoids and omomyoids) are first found in the fossil record (e.g., Rose et al., 2012).