06 November 2015

Fossil Vertebrate(s) of the Month: Pelycosaur Extravaganza!

Pelycosaurs (Edaphosaurus, Dimetrodon, Sphenacodon, & Ophiacodon)
From The Age of Reptiles by Rudolph Zallinger
I've been negligent about posting fossil vertebrates of the month since the beginning of the school year, so I've made November's installment an especially hearty three-course Thanksgiving feast of paleontological delicacies on which to gorge yourself.  Bon appetit!

Soup & Salad: Rudolph Zallinger & The Age of Reptiles
As long as paleontologists have been finding and describing fossils, they have relied on paleoartists to bring these organisms to life.  There have been several such artists through time, but none have had more of an impact than Rudolph Zallinger.  Born in Russia, raised in Seattle (where you can still see some of his work), and educated at Yale, Zallinger led a fascinating life.  He created several works of scientific illustration and paleoart, but two have played an especially important role in public understanding of science.  His 1965 depiction of human evolution, widely known as the March of Progress, became one of the greatest (and most endlessly parodied) icons of evolutionary biology, despite the fact that it does not do justice to the true branching complexity of human evolution (as Zallinger himself well knew).  Before this, though, Zallinger had painted the most impressive single work of paleoart ever created.  In 1941, the director of the Peabody Museum decided to liven up the museum's Great Hall (home to, among other things, the original specimen of the recently resurrected genus Brontosaurus) and selected Zallinger, still a student at the time, to carry out his vision.  The result, completed in 1947, was the 1,760 square-foot mural The Age of Reptiles (later to be joined by the smaller Age of Mammals).  As a fresco, the animals, plants, and scenery depicted in the painting remain as stunning today as they were in the '40s.  So impressive is the mural that, until the release of Jurassic Park, it probably had more impact on the public image of prehistoric life than any other work (it is, for example, widely believed that Zallinger's lumbering Tyrannosaurus had a direct influence on the design of Godzilla).  While much of what has been written about the mural has focused on dinosaurs (the subjects of the mural, incidentally, that are now the most outdated), Zallinger gets high paleoecological marks for including a wide range of organisms (especially plants) in his work and for placing them in a landscape that reflected the conditions that were thought to have existed on Earth during a given period.  For the Permian section of the mural, shown in the picture above, filling in the foreground in front of a series of soaring red cliffs (seriously, check out an uncropped image of the mural: no reconstructions of extinct organisms ever got a more dramatic backdrop), Zallinger chose for his main subjects reptiles that had been emerging from the Red Beds of Texas since the 1870s.  They are, from left to right, Edaphosaurus, Dimetrodon, Sphenacodon, and Ophiacodon.  As an artist, Zallinger no doubt chose to depict them because they were visually striking.  One of his strengths as a paleoartist, though, was his understanding of the organisms he was painting, so he was no doubt aware that these animals have an evolutionary history every bit as impressive as their appearance.

Main Course: Pelycosaurs
The four large animals depicted in the Permian section of The Age of Reptiles are known collectively as pelycosaurs, often misunderstood as a lineage of dinosaurs.  In fact, pelycosaurs are neither a lineage (the group is paraphyletic) nor are they dinosaurs (the nearest - though still quite distant - relative of dinosaurs in this picture is the small Araeoscelis scuttling along the rocks below the feet of Dimetrodon) .  Look at the skull of any of the pelycosaurs depicted in the mural and you'd see a single opening behind the eye.  This is known as the synapsid opening and it has lent its name to the large group of whose evolutionary tree pelycosaurs form the base: the Synapsida, the taxon that includes all mammals and our extinct relatives (as I frequently point out to students, this means that pelycosaurs are more closely related to us than to dinosaurs).  While often misidentified, two of the animals depicted by Zallinger, Edaphosaurus and Dimetrodon, are among the most widely recognized organisms in paleontology due to the characteristic sails on their backs.  Despite their superficial similarity, the two genera are actually not particularly close relatives.  Edaphosaurus was an edaphosaurid, a family remarkable for its stubby teeth indicating one of the earliest occurrences of herbivory among land-living vertebrates.  Teeth are also one of the remarkable features of the larger Dimetrodon, which belonged to the family Spenacodontidae (named after Spenacodon, the third of the four large pelycosaurs in Zallinger's mural).  Unlike Edaphosaurus, the teeth of Dimetrodon were serrated and clearly adapted for a carnivorous diet.  The name Dimetrodon ('two sizes of teeth') reflects the most important aspect of sphenacodontid dentition: teeth vary in size and shape along the jaw, with large teeth at the front and smaller teeth towards the rear.  While this may seem a minor point, differentiated and specialized teeth are one of the defining characteristics of mammalian evolution, and as our distant relatives, sphenacodontids represent the earliest expression of this hugely important trait.  Edaphosaurus and Dimetrodon, then, represent significant innovations in synapsid evolution - herbivory and heterodont dentition, respectively - but it should come as little surprise that much of the attention that has been lavished on them by paleontologists and the general public has focused on the elongated vertebrae and the sails they supported, which may (or may not) have implications for another defining characteristic of mammals.

Dessert: Warm-Bloodedness
It's easy to take warm-bloodedness (both endothermy - the ability internally regulate our own body temperature - and homeothermy - the ability to maintain a constant internal temperature) for granted, as it's the basis of our metabolism, as it is for other mammals and for birds.  However, warm-bloodedness is only really widespread in those two groups and, just as much recent work has focused on the evolution of endothermy in the dinosaurian ancestors of birds, there has long been in interest in identifying its origins in our own lineage as well.  Such research has often centered on the sails of edaphosaurids and sphenacodontids, as one of the leading hypotheses for their function is that they acted as a means of absorbing solar heat to power metabolism (the main alternative hypothesis is that the sails were used as display structures of some sort).  If this were the case, it would imply that, while not fully capable of endothermy, being able to maintain an active lifestyle would have been advantageous to pelycosaurs.  Thus, just as the teeth of Edaphosaurus and Dimetrodon are harbingers of the later diversity of mammalian diets, perhaps their sails were likewise precursors to the warm-bloodedness of modern mammals.  Early tests of this hypothesis focused primarily on the ability of the sails to absorb solar heat or, in the case of the unusual knobs on the spines of Edaphosaurus, to create a turbulent airflow around the sail that would allow a more efficient uptake of energy.  More recently, several analyses of bone structure have shed new light on the question.  One working group in particular (that includes former UW paleontologist Adam Huttenlocker) has looked into the histology of Sphenacodon, Edaphosaurus, and Dimetrodon vertebrae.  This group has shown that not only is there no compelling support for pelycosaur sails acting as solar collectors, but that in many cases the morphology of pelycosaur spines does not mesh with what one would expect if the sails were primarily adapted to absorbing energy.  Very recently, an analysis of the fourth pelycosaur depicted by Zallinger - Ophiacodon, seen in the mural lounging on the riverbank below Sphenacodon - has added another histological datum to the debate.  This study suggests that Ophiacodon produced fibrolamellar bone, fast-growing tissue that is often associated with warm-blooded metabolisms.  It's worth noting that the study is still in peer-review and that the correlation between fibrolamellar bone and endothermy may not always hold true for extinct taxa, but if these findings hold up, they could push the evolution of warm-bloodedness further down the synapsid tree than had been previously suggested.

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