‘Non-dinosaurian dinosauromorphs’ certainly has a poetic ring to it. It’s the name given to the phylogenetic grade of animals more closely related to dinosaurs than to pterosaurs, though not quite dinosaurs themselves. Why aren’t they dinosaurs? Well, to be a dinosaur you have to be in the defined boundaries of Dinosauria, which means in the group of animals containing Passer domesticus (house sparrow) and Triceratops horridus, their most recent common ancestor and all of its descendants. To get in you need morphological character states. If you really want to know all the character states that may define Dinosauria let’s go to Schultzy’s for a beer, but suffice it to say it’s all in the hips (and humeri and tibiae and ankles). Compared to the true dinosaurs, of which there are well over 1,000 extinct species and over ~10,000 living birds, the non-dinosaurian dinosauromorphs are relatively unknown. The species we have fossils for all lived in the Triassic Period (252-201,000,000 years ago) between around 247-208,000,000 years ago.
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| Some non-dinosaurian dinosauromorphs from North America. The larger animal is a silesaurid, the most diverse clade of non-dinosaurian dinosauromorphs. Image from sciencemag.org. |
What’s remarkable is that of the 12 known species, nine of them have been described since 2003(!), the other three being known since the 1970s. This means that the outgroups to dinosaurs, their body plan & size, skeletal anatomy, diet, and temporal & geographic ranges, are only now capable of contributing to our macroevolutionary understanding of where the ‘true dinosaurs’ came from.
The lion’s share of the newly described taxa belongs to a single, so far monophyletic clade called Silesauridae. These animals challenged much of what we thought we understood about the dinosaur family tree. Other non-dinosaurian dinosauromorphs were bipedal and likely carnivorous, which is the same as the earliest members of the major clades of true dinos. However, the silesaurids (even the oldest and ‘basal-most’ members of the clade) seem to be quadrupedal and, perhaps more surprising, adapted to a life of possible herbivory! Also, the non-dinosaurian dinosauromorphs lived alongside their dino cousins for tens of millions of years and are found all over Pangea, both north and south. So why is this so cool? Well it implies that rather than being some new-fangled group of reptiles that stood upright and moved quickly, dinosaurs were part of a larger evolutionary event.
The Triassic starts out right at the end of the largest extinction event we can see in the fossil record (with up to 96% species extinction in some places!) and true dinosaurs are first found up to 24 million years later. Due to this discrepancy, and the patterns of radiation observed in other tetrapod groups, it was thought that the evolution of dinosaurs was dissociated from the extinction recovery interval. Fossils of non-dinosaurian dinosauromorphs, like silesaurids, can be found between five and 10 millions years after the extinction event, and they pull the radiation of animals like dinosaurs and pterosaurs back to that long ago as well. So the evolution of dinosaurs and their kin may be a part of the terrestrial recovery process after the largest mass extinction event of all time. Everyone knows about the extinction event that wiped out the non-bird dinosaurs and the pterosaurs, but isn’t it cool that their initial evolution may have been in the wake of an even bigger one!? What the happenings of the universe giveth; the happenings may take away (keep that in mind mammals).
The paper that I recently published in the Journal of Vertebrate Paleontology describes a new taxon of silesaurid from our fieldwork in Zambia that is in a three-way tie for being the oldest dinosauromorph of any kind (it’s tied with another silesaurid (Asilisaurus)and possibly the oldest true dinosaur (Nyasasaurus) from equivalent rocks in Tanzania, also the results of our fieldwork). I named the new animal Lutungutali sitwensis. The village near where we worked is called Sitwe and the people there speak Bemba, a major Bantu language. I found a Bemba dictionary online that only contained entries for A-H (the dictionary now contains A-O). After looking up ‘ancestor’, ‘crocodile’, ‘ancient’, ‘bird’, ‘animal’, etc., I decided on ‘lutungu’ (translated to ‘hip’) and ‘tali’ (translated to ‘high’) because one of the diagnostic features of Lutungutali is the height of the iliac blade (hip bone) over the acetabulum (where the femur goes in), so it has ‘high hips.’
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| The 2011 team scouring a small hill for many, many silesaurid bones. Note the terrifyingly ‘lion-colored’ grass that surrounds us. |
One of the outcomes of the work of the Sidor lab in Zambia and Tanzania has been to shed light on the evolution of the early relatives of dinosaurs. The reason that paleontologists didn’t used to think of dinos in the light of the largest extinction event was that the rocks traditionally used to study that time interval in South Africa and Russia do not have dinosaur, or even crocodilian, relatives. We went to Zambia and Tanzania to increase the geographic sampling for the post-extinction time interval and were very surprised to find so many fossils of dinosaur relatives. The holotype of Lutungutali was discovered in 2009, and in 2011 I couldn’t find any more of it. However, in 2011 we did find a bonebed of around a dozen juvenile silesaurids (cute!) and some partial femora belonging to absolutely huge individuals. The new specimens cannot yet be directly referred to Lutungutali (or the Tanzanian Asilisaurus), but research continues and we’ll be back to Zambia for a long season during summer 2014!
Here’s the paper!
~Brandon
