Arcusaurus pereirabdalorum: the little sauropodomorph that could

Adam Yates, myself, and Johann Neveling published on another new sauropodomorph dinosaur from South Africa in the Journal of Vertebrate Paleontology in 2011.  The dinosaur is named Arcusaurus pereirabdalorum.  Before I go any further, let me explain the origin of the name.  With each dinosaur that our team has unearthed and described, we wanted to give nods to various cultures and people in South Africa.  For example, our previous sauropodomorph dinosaur, Aardonyx celestae, gives a nod both to the Afrikaans language (one of 11 official languages in the country) with “Aard” (the word for Earth; hence an Aardvark is literally an “earth pig”) and to Celeste Yates for her amazing preparation of the material.   Arcusaurus means “rainbow reptile,” and we felt this was fitting for South Africa, the Rainbow Nation.  The species came at my insistence (and of course with no resistance) because of the two people who bothered to smack the rocks and reveal this little guy to world (me being silly — their hard work is responsible for this nice specimen).  Here’s a (mostly) true telling of the story:

On an overcast day in March of 2006, at the Spion Kop Heelbo quarry in Free State, South Africa, myself, Adam Yates, Johann Neveling, and an intrepid band of students, postdocs, and volunteers were busy clearing debris and chipping away at the channel sands of the Upper Elliot Formation.  In 2004, we had previously unearthed some other bones … I’ll leave you hanging on those until that publication.

Suffice it to say we were digging out some nice bones.  Some very delicate, small, and brittle (read here: pain in the butt) bones were coming out of the eastern portion of the quarry.  We didn’t quite know what they were, but it was painstaking to deal with them and nothing very definitive was coming to light.  Enter Lucy Pereira and Fernando Abdala, a student and post-doc, respectively, in 2006 at the University of the Witwatersrand, Johannesburg.  Their help was priceless in the quarry, though neither one of them was a dinosaur fanatic — Lucy was studying ancient hominids, whereas Fernando was studying cynodonts, close relatives to the common ancestors of mammals.

At some point during the day, a hammer struck rock, the rock split open, and I heard from Lucy, “Matt, I think we have a skull!”  I went over to have a look, and here’s what I saw:

Arcusaurus skull as found in the field by Lucy Pereira and Fernando Abdala, March, 2006.  Scale bar = 10 cm.

Here is a close-up of the part and counterpart as found in the field:

Arcusaurus Skull Part in field closeup.
One Arcusaurus skull part in field, closeup. Each block on the scale bar = 1 cm.
Arcusaurus counterpart jaw.
The counterpart of the Arcusaurus skull, showing the lower jaw (dentary bone) to good effect. Each scale block = 1 cm.

This was a very exciting moment for all of us.  A tiny skull, preserving enough of the details for a reconstruction, and looking very much like a “prosauropod” sauropodomorph.  As it would turn out, this find was significant for evolutionary reasons I describe shortly.

Of course, after you find something like that and map the loose part, you have to get your picture taken.  First, the two people who made this discovery possible:

Lucy Pereira
Lucy Pereira – co-discoverer of Arcusaurus, March 2006. She is pictured here with one part of the skull.
Fernando Abdala
Fernando Abdala – co-discoverer of Arcusaurus, March 2006.  He is pictured here with part of the skull.

And now the picture of self-indulgence:

Matthew Bonnan
Matthew Bonnan basking in the glow of Arcusaurus, March 2006.

After beautiful preparation of the skull material by Charles Dube (a preparator at Witwatersrand), Adam Yates was able to skillfully reconstruct what the skull of this critter probably looked like:

Arcusaurus pereirabdalorum skull reconstruction.
Arcusaurus pereirabdalorum skull reconstruction, (c) Yates, Bonnan, Neveling, and Journal of Vertebrate Paleontology, 2011. Scale bar = 50 mm.

Other bones were also found within 50 cm of the (now holotype) skull including some vertebral and limb elements.  Although we did not have anything as complete as Aardonyx, we can be about as certain as scientists can that this new animal was a juvenile.  We determined this given the size of the skeletal materials and the lack of fusion / completeness of various joint surfaces.  In particular, the lone sacral vertebra (part of the backbones between the hips) preserved is lacking its neural arch (the part that covers the spinal cord in life).  This isn’t due to a break in the specimen — there are roughened scars where the neural arch would have attached, and this suggested to us that this sacral vertebra fell apart postmortem and prior to burial.  In adult dinosaurs, the arch and body of the vertebra fuse together — given the lack of fusion and the loss of the neural arch in our specimen, we can be very confident that Arcusaurus was a juvenile.  This pattern of fusion is observed in modern alligators and crocodiles.

Juvenile alligator vertebra unfused
The neural arch and body of a juvenile alligator vertebra, showing the scars where the two unfused pieces attach.  Compare this to the figure of the Arcusaurus sacral vertebra below.
Arcusaurus sacral vertebra
The sacral vertebra (body) from Arcusaurus in A, cranial, B, left lateral, C, caudal, D, right lateral, E, ventral, and F, dorsal views. Note in (E) the scars (SNA) where the neural arch would have attached — compare this to the juvenile alligator vertebra in the figure above. (c) Yates, Bonnan, Neveling & Journal of Vertebrate Paleontology, 2011.

We lucked out in that despite not having a lot of this dinosaur (some of the skull, some vertebrae, and bits of limbs), we had enough of the right bones to run a cladistic analysis to see where this animal came out on the sauropodomorph family tree.  It turns out that Arcusaurus may have been a relict from an earlier time.  In the Late Triassic, the prosauropod sauropodomorphs (those that stood on their hindlimbs only) were a diverse and widespread group of dinosaur herbivores.  By the Early Jurassic (where we find Arcusaurus), however, most of the prosauropod sauropodomorphs had gone extinct, with just a few hangers-on.  Arcusaurus has an interesting mix of features that suggest not only that it was a survivor from a by-gone time but that it was a survivor of one of the earliest lineages of prosauropod sauropodomorphs.  In cladistic parlance, if we are correct, Arcusaurus would be the only known non-plateosaurian sauropodomorph to survive into the Jurassic period.

Cladistic placement of Arcusaurus within Sauropodomorpha.
Cladistic placement of Arcusaurus within Sauropodomorpha.  Note that Arcusaurus falls outside the Plateosauria, making it a primitive (relict) surviving prosauropod.  Anchisauria is the larger clade to which the classic sauropods belong. (c) Yates, Bonnan, Neveling, and Journal of Vertebrate Paleontology, 2011.

Overall, Arcusaurus is part of a local and distinctive fauna in the Early Jurassic of South Africa.  Along with animals like Aardonyx, we are finding that the Early Jurassic of South Africa was not simply a monolithic desert sprinkled with small prosauropod sauropodomorphs like Massospondylus.  Instead, there were pockets of diversity and evolutionary novelty in this region of the world that deserve closer inspection.

There are many people to acknowledge: The following people participated in the field work at Spion Kop: Z. Ali-Jinnah, N. Barbolini, N. Bremmer Jr., J. Cisneros, G. DeVilliers, C. Dube, S. Fowell, J. Hancox, K. Lalla, C. McCrae, R. M¨ orsner, M. Nicolas, L. Norton, S. Potze, N. Sithole, and C. Vasconcelos. The holotype specimen was skillfully prepared by C. Dube.  And again, of course, we are indebted to Lucy Pereira and Fernando Abdala!

Our funding: The field work was funded by the National Geographic Society (CRE no. 7713–04).  I was supported in part by a Faculty Mentor grant from the College of Arts and Sciences at WIU and a Center for Innovation in Teaching Research, Faculty Research Developmental Activities Award.

One thought on “Arcusaurus pereirabdalorum: the little sauropodomorph that could

  1. Pingback: Like finding a unicorn | The Evolving Paleontologist

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