Skull of the therapsid Lycaenops on display at The Field Museum. Photo by Ken Angielczyk.
Ancient synapsids are interesting for many reasons, but we hope our fieldwork this year in Brazil will help to address a particular problem in synapsid evolution. The oldest synapsids are found in parts of North America and western Europe, and these areas were located in a narrow band near the equator at the time these animals were alive (about 300 million years ago, in the Carboniferous Period of Earth history). The synapsid fossil record in these areas continues up to about the end of the Early Permian Period (roughly 275 million years ago), and although a number of synapsid species are present, they are mostly members of early lineages (colloquially known as pelycosaurs) with rather lizard-like body plans. To continue to trace synapsid history after this time, we need to look at the fossil record preserved in younger rocks in other geographic areas, traditionally South Africa and European Russia.
Paleogeographic map showing the reconstructed positions of the continents in the Late Carboniferous Period of Earth history (approximately 300 million years ago). All synapsid fossils known from this time have been found in the narrow belt near the equator that is highlighted in yellow. The yellow box shows the approximate position of the Parnaíba Basin at this time. Base map courtesy of Ron Blakey, Colorado Plateau Geosystems, Inc.
Both of these areas were located at relatively high latitudes at the time and, with a few exceptions, the synapsid fossils found in the rocks in these areas are different than the ones from North America and western Europe: they tend to be more closely related to mammals and they begin to take on a more mammal-like appearance. Likewise, although the South African and Russian synapsids clearly are related to the older synapsids found in North America and western Europe, they don't seem to have direct ancestors in the latter areas. Thus, there is information missing from the known fossil record about the origin of these younger synapsids.
Paleogeographic map showing the reconstructed positions of the continents in the Late Permian Period of Earth history (approximately 255 million years ago) Areas where therapsid fossils have been found are highlighted in yellow; the richest and most studied deposits are in southern Africa and Russia. Base map courtesy of Ron Blakey, Colorado Plateau Geosystems, Inc.
There have been a number of hypotheses presented by paleontologists about the cause of this missing information. One common explanation is that there is a time gap of several million years between the rocks in North America and Europe on the one hand, and those in South Africa and Russia on the other. Under this scenario, the earliest therapsids must have evolved and dispersed to high latitude areas during the missing time. However, the time gap has been steadily shrinking as our age estimates for the rocks in the different areas become more refined, so missing time is at best an incomplete explanation. An alternative is that therapsid ancestors have been found in North America and/or Europe, but have not been recognized as such. For example, the late paleontologist Everett Olson suggested that a number of fossils from North America represent early members of therapsid groups, but more recent scrutiny of the fossils suggests that they represent pelycosaurs instead. A third explanation is that the origin of therapsids occurred in a different geographic area, one that either does not preserve rocks with fossils from this time or that has a fossil record that has not been thoroughly studied. The recent discovery of the very early therapsid Raranimus in China suggests that incomplete geographic sampling may indeed be an important factor contributing to the uncertainty surrounding the early history of therapsids. If that's the case, then it is necessary to explore fossiliferous rocks of approximately the right age in new geographic areas to see if we can find evidence of early therapsids.
As we'll see, the rocks preserved in the Parnaíba Basin of northeastern Brazil appear to be the right age to preserve early therapsid fossils. The area also was ideally located to catch early therapsids or their ancestors if they were dispersing from equatorial North America to southern Africa. So any synapsid fossils we might find during the course of our fieldwork could be very important!