Owner Comments:
After a bit of a long wait, we finally have the next animal in our little “Animals that are not Dinosaurs” subset, where we look at animals that many people think are dinosaurs which are not. This time, we will look at Bathygnathus borealis. This animal was recently reclassified as Dimetrodon borealis, but we’ll use the old name here to avoid confusion as this is the name which appears on the coin.
As far as Bathygnathus borealis as an animal, we actually know very little as the only known fossil remains of this animal are a single partial upper jawbone found on Prince Edward Island in Canada. As there are some very sharp teeth in the jawbone, it can be determined that Bathygnathus borealis was carnivorous and for many years, that was it. When this jawbone was found in 1845 and described in 1854, it was thought that this bone was the lower jaw of a theropod dinosaur and was hailed as Canada’s first dinosaur. This classification was one of early paleontology’s biggest errors and one that would stand for almost 50 years. When this jawbone was reexamined in 1905, it was found to be something else entirely, but we have to look at some evolutionary history to see what a colossal error this early classification of Bathygnathus borealis truly was.
312 million years ago, during the Late Carboniferous, the first Amniotes (animals which could lay amniotic eggs on land instead of in the water like amphibians) evolved from earlier reptilomorph amphibians. In the blink of an eye evolutionarily (within a few million years), the ancestral Amniotes split into two groups. The first group were the Synapsids, characterized by a distinctive opening in the skull right behind the eyes. The second were the Sauropsids, which had either two skull openings behind the eyes, in the case of reptiles or none at all, in the case of what are known as parareptiles. Throughout the rest of the Paleozoic Era, the Synapsids would be the dominant land animals, evolving a huge number of forms, such as Bathygnathus borealis and many others while the Sauropsids continued to evolve into new forms in the shadow of the Synapsids.
The Paleozoic Era ended with a devastating mass extinction, the Permian-Triassic Extinction Event, which was the most numerically severe mass extinction in the history of the Earth, with over 92% of all species disappearing over the span of a few million years. The Synapsids were devastated by this event and they would never truly recover numerically from their Permian Era high point, though they would survive and they continue to do so in the present day in the form of mammals. Sauropsids took over and, in the form of dinosaurs, ruled the earth throughout the Mesozoic Era. While many people think that the end-Cretaceous event 66 million years ago was the end of dinosaur and, as a result, reptilian dominance, this is not truly the case. The surviving Synapsid lineage of mammals was also devastated by the end-Cretaceous event as well and they took longer to recover than many groups of Sauropsids did. As odd as this sounds to many of us who were in school before the true identity of birds was known, the dinosaurs actually weathered the end-Cretaceous extinction better than mammals did.
For much of the Cenozoic Era, dinosaurs still ruled as huge, predatory birds dominated the food chains in much of the Earth, including North and South America, Europe and New Zealand. Dinosaurs were also beginning to see success in Antarctica in the form of Penguins. Where dinosaurs didn't re-inherit the Earth, other large reptilian predators such as Crocodilians and Monitor Lizards filled apex predatory niches in places like Australia. In the oceans, where Sea Turtles were the only reptilian survivors, fish in the form of sharks took over the ruling spot, which they still hold today. Even as mammals recovered, their evolution was shaped by their relationships with these large, non-mammalian predators.
Mammals began their post-Cretaceous Extinction diversification during the Paleogene Period of the Cenozoic (66 to 23 Million Years Ago) in the shadows of these predators, and they mostly remained small. It wasn't until the Neogene and Quaternary Periods (23 Million Years Ago to the Present) that mammals really came into their own. Africa, in particular, seems to have been a refuge for mammals to evolve new forms, which would be expected in looking at the previous listing of where reptiles and fish still ruled. South Asia is another place where many modern mammal lineages evolved and even with huge monitor lizards ruling the food chain, Australia became a safe haven for marsupials to evolve, diversify and thrive with limited competition from both placental mammals and avian dinosaurs, who both out-competed marsupials in almost every other ecosystem on Earth, with some exceptions such as the Virginia Opossum. Indeed, the extinction of the large monitor lizards such as Megalania at the end of the Pleistocene around .04 million years ago allowed for the evolution of predatory marsupials such as the now-extinct Thylacine and the Tasmanian Devil.
The glaciation events which happened during the Quaternary especially helped mammals spread into new niches as many of the large, reptilian predators became extinct or restricted in range by the changing climate. An example of this is the demise of the Bathornids and Phorusrhacids, two families of very large, very nasty predatory ground birds that ruled the food chains in North and South America through much of the early Cenozoic, due to climate change in the later Cenozoic allowed for large predatory mammals such as saber-toothed cats (along with many of their less dentally enhanced relatives) and certain species of canids to have their chance as ground-based apex predators in the Americas. However, the moment at the top was fleeting for many of these mammalian predators as the end of the glaciers and the arrival of humans meant the end for most of them. The large, predatory birds had been a 60 million year continuation of the dinosaur dynasty. By comparison, many of the predatory mammals which replaced them during the Ice Age were extinct less than 2 million years later. The dinosaurs which could fly were relatively unaffected by both the coming of the glaciers and the receding of them
Even considering all this, however, it can be argued that, humans aside, Sauropsid dominance continues today as there are over 20,000 extant species of reptiles (of these, over 10,000 are birds and thusly dinosaurs) and only 5,700 species of mammals. Though mammals today are indeed a successful and diverse group of animals that fill many ecological niches, their present numbers and diversity fall short of the success levels that earlier Synapsid groups had in the Late Paleozoic. It is also important to note that mammals are the group of Amniotes which have the largest percentage of species threatened with extinction through human activities at the present time…..once again, the Synapsids are being threatened more than the Sauropsids by a mass extinction. Only this time, the threat is a fellow Synapsid rather than an anoxic event or a meteor strike.
When the fossil of Bathygnathus borealis was re-examined in 1905, it was found to be the upper jaw of a Pelycosaur, which is an evolutionary grade of Synapsid. So, Bathygnathus borealis is not a dinosaur. In fact, despite how it looks, Bathygnathus borealis isn’t even a reptile. It’s a Pelycosaur-grade Synapsid, which means that it’s far more closely related to mammals like you and me than it is to any true reptile, such a dinosaur. Pelycosaurs also lived tens of millions of years before dinosaurs even existed, so there is no chance that they even lived side by side with them. By the time dinosaurs came along in the Late Triassic, Pelycosaurs had already evolved into Therapsids, the advanced Synapsid group that includes mammals, among other groups. Because Pelycosaurs went extinct by evolving into Therapsids, they are an excellent example of what is termed pseudoextinction, where an animal went “extinct” by evolving into a new form known as a descendent taxon. For many years, theropod dinosaurs were considered pseudoextinct because they were thought to have evolved into birds, which were considered a new animal in older taxonomies. However, as there are very few differences between birds and other Maniraptorans that are not literally skin deep, birds are still dinosaurs and, thusly, dinosaurs are simply just not extinct rather than pseudoextinct like Pelycosaurs.
Well, I hope that you have enjoyed meeting one of our earliest and most mysterious relatives here in the form of this beautiful coin! And, remember that Bathygnathus borealis and any other Synapsids, no matter what they looked like, are not and never were true reptiles. They are a distinct lineage of amniotes, with their own distinct history and characteristics that ended up being steps along the way to the emergence of all the mammals you see today, including us!