Community College (LRCCD)

Geology & Earth Science Instructor: Arthur Reed, P.G.

Happy Fossil Friday!

Friday March 19, 2021

Our destiny was set before the rise of the dinosaurs

Before the great extinction at the end of the Permian, a group of tetrapods (four limbs with a vertebrae) had moved away from other tetrapods as the bases of the subclass Synapsida. These included interesting animals like the Dimetrodon shown below. Some in this class evolved into borrowing mammals in the Mesozoic (while hiding from the dinosaurs!) and eventually led to all modern-day mammals including humans. Below is a short article from The Fossil Non-Mammalian Synapsid Collection at ‘The Field Museum of Natural History’ in Chicago, and a link to an informative PBS video.

Photograph of a skeleton of the early non-mammalian synapsid (ancient mammal relative) Edaphosaurus on display at the Field Museum of Natural History. Credit: Photograph by Ken Angielczyk

Video: ‘The Synapsids Strike Back’

Introduction

Amniote tetrapods (i.e., those terrestrial vertebrates that produce eggs in which the embryo is surrounded by a series of extra-embryonic membranes) in the modern world can be divided into two great lines of descent, the Reptilia and the Synapsida. Extant reptiles include lizards, snakes, turtles, the worm-like amphisbaenians, crocodiles, and birds, while monotreme, marsupial, and placental mammals are the extant representatives of Synapsida. The reptile and synapsid lineages both descend from a common ancestor, but that divergence is ancient, occurring in the Carboniferous Period of Earth history (approximately 315 Mya). The first true mammals appear in the fossil record about 200 Mya, near the end of the Triassic Period. If we consider a phylogenetic tree that shows patterns of descent from common ancestors, we can see that there is a large number of extinct members of the synapsid lineage that existed between the origin of Synapsida and the appearance of mammals.

Simplified phylogeny showing relationships among tetrapods. Groups with living members are shown in bold; extinct groups are in plain type. Modified from Angielczyk (2009).

A single temporal opening around which jaw muscles attach is a feature shared by all synapsids. A. location of the temporal opening in the Early Permian synapsid Dimetrodon (FMNH UC 40). B. location of the temporal opening in a human skull. From Angielczyk (2009).

These extinct synapsids are often referred to as “mammal-like reptiles” because some have a superficially reptilian appearance. However, all are descendants of a common ancestor that existed after the divergence between Synapsida and Reptilia, which means they are all more closely related to extant mammals than to any reptile. A more accurate name for these extinct species is “non-mammalian synapsids,” which reflects the fact that they are members of the synapsid lineage, but are not mammals.

Non-mammalian synapsids are an extremely important part of the fossil record because they document the evolutionary history of many of the distinctive features of mammals, such as the presence of a bony secondary palate, the incorporation of bones from the lower jaw into the middle ear, teeth with complex occlusion patterns, and upright limbs. Morphological features, such as the presence of a single opening behind the eye socket around which jaw musculature attaches, help us recognize members of the synapsid lineage in the fossil record. An introduction to non-mammalian synapsids can be found in Angielczyk (2009).