Friday, July 31, 2015

Know Your Ankylosaurs: Gondwana Edition

Last time, I talked about the ankylosaurids of China, and today we're talking about Gondwanan ankylosaurs. Gondwana basically refers to the continents of today's southern hemisphere; when the supercontinent Pangaea broke apart, it split into two large continents – Laurasia in the north, and Gondwana in the south. Gondwana includes South America, Africa, Australia, and Antarctica, and, somewhat nonintuitively, India (India kind of beelined into Asia from Australia and that's why we have the Himalayas). Almost all of the ankylosaurs we know about are from the Laurasian continents, which means that the few found in Gondwana are phylogenetically and biogeographically interesting: do they represent southern branches of the ankylosaur family tree, or new migrations into Gondwana from Laurasia? Let's take a closer look:

Minmi paravertebra and Minmi sp.
Minmi is the iconic Australian ankylosaur. Most people, when they think of such things, think of the spectacular referred skeleton with agood skull and in situ armour.

The Smithsonian has a cast of the specimen - here's a section of the ribcage, showing some of the osteoderms in their original arrangement.

Sadly, the holotype is extremely fragmentary and has few elements to make a diagnosis with. Originally, one of the most striking features of Minmi paravertebra was the presence of paravertebral elements, thin rod-shaped bones along the dorsal vertebrae. These were originally interpreted as ossified tendons of the dorsal muscles, and although these are cool to see in Minmi, they are not really unique to Minmi or even to ankylosaurs, since ossified tendons are ubiquitous throughout Ornithischia. One unusual aspect of these ossified tendons is that one set has a flattened, expanded front end. These were interpreted as possible ossified aponeuroses (aponeuroses are sheets of connective tissue in between muscles and tendons). This particular aspect of the ossified tendons IS very unusual, because ossified aponeuroses are extremely rare in animals. While I was hunting around for information about ossified aponeuroses, I came across a very odd case study about mouse deer (Tragulus) – the males completely ossify the aponeuroses above their pelvis and back, creating a carapace-like structure! This is super weird and I would love to investigate this further at some point.

Ossified aponeuroses have since been identified in the European nodosaur Hungarosaurus, which poses a bit of a problem for Minmi: since this feature was one of the only diagnostic characters for Minmi, and since it is now found in an animal that is very unlikely to be Minmi given the spatial and temporal distance between the two, Minmi paravertebra is left without diagnostic characters. A sticky situation that will hopefully be resolved in the future by people who have spent time with the original fossil material!

Did you know that the first dinosaur discovered in Antarctica was an ankylosaur? Cryolophosaurus might get all the buzz, but Antarctopelta was first to the press. Antarctopelta is a very interesting little ankylosaur, which I had the chance to study during my visit to Argentina back in 2011. The material is fragmentary but tantalizing, with some pieces of the pelvic armour that are reminiscent of ankylosaurs like Stegopelta and Glyptodontopelta from North America. Unfortunately, in the course of my research I noticed that some of the bones attributed to Antarctopelta and used to help diagnose the taxon didn't quite seem like they came from an ankylosaur. The material was found on an ancient beach strandline with some marine fossils mixed in, and it looks like some of the material originally interpreted as ankylosaurian might be better interpreted as belonging to a mosasaur and a plesiosaur. In the end, we weren't left with any diagnostic characters for Antarctopelta and we should consider that a nomen dubium for now, but there's definitely an Antarctic ankylosaur and I hope at some point some better material is recovered so we can determine the best name for this guy.

The Argentinian ankylosaur
Finally, I also had the chance to study the only described ankylosaur from Argentina. This is also a fairly fragmentary specimen, and it came from a channel lag deposit so it's possible that more than one individual is represented. There are osteoderms, some vertebrae, and a femur, and all are very small – about the same size as the juvenile Anodontosaurus (originally described by Coombs as Euoplocephalus) from Alberta. The femur is interesting because it has some very prominent ridges running lengthwise on it, which seem to be intermuscular lines; these are present but very faint on some other ankylosaurs, and I haven't encountered anything like that in other ankylosaurs. There also may be fragments of the cervical half rings preserved as part of this specimen, since there are some unusual curved osteoderms with multiple peaks and keels. These don't bear any resemblance to other half rings I've looked at, and cervical half ring morphology seems to be taxonomically informative for ankylosaurs. Together, the weird intermuscular lines and unusual cervical half ring fragments might be enough to diagnose the Argentinian specimen as a new taxon, although we withheld from doing so at present.

Here's the specimen on display at the Museo Carlos Ameghino in Cipoletti!

There have been reports of some possible ankylosaur material from India and Madagascar, although much of this material is either very fragmentary (a single tooth from Madagascar), or has not been described (material from India). Stay tuned to find out more about how these rare ankylosaurs fit into the big picture of ankylosaur evolution!

Next up: a grab bag of everybody else!

Arbour VM, Currie PJ. In press. Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs. Journal of Systematic Palaeontology.

Friday, July 24, 2015

Know Your Ankylosaurs: China Edition

I'm in Utah digging up dinosaurs! But also, one of the last big chunks of my PhD thesis has just been published online at the Journal ofSystematic Palaeontology. They are generously allowing free access to the paper through the end of August, so head on over and grab a copy while it's free! This time, I'm taking all of the knowledge gained from my previous taxonomic revisions, adding in some more taxa, and doing a revised phylogenetic analysis building on previous analyses to see how everyone shakes out and to learn a little bit more about ankylosaurid biogeography. I'll cover some of the taxonomic stuff over the next few posts, and finish off with the big picture of ankylosaurid evolution.


I've talked previously about the ankylosaurs of Mongolia, but I've also had the opportunity to study some of their friends from across the border in China. In particular, I got to see lots of specimens of Pinacosaurus, both from the Alag Teeg bonebed in Mongolia, and from Bayan Mandahu in China. Because Pinacosaurus specimens are relatively abundant and usually well preserved, there has already been lots of descriptive work on this taxon, including on the skull (and here, and here), hands and feet, and general postcrania

Baby Pinacosaurus are so teeny tiny! This one is from Bayan Mandahu and was collected during the Canada-China Dinosaur Project back in the 1980s.

I've discussed just a few new points about Pinacosaurus, especially about how we tell the two species of Pinacosaurus apart. Pinacosaurus grangeri is known from lots of specimens, almost all of which are juveniles; it has relatively short horns at the back of its skull, a constriction in the snout between its nose and its eyes, and a notch in the rough ornamentation above each nostril. Pinacosaurus mephistocephalus is known from just one specimen (also a juvenile), and it has long squamosal horns, no constriction in its snout, and no notch in the ornamentation above each nostril (it looks like it does on one side, but I think this is just damage given that it is not present on the other side). Both species are known from Bayan Mandahu, and so it is reasonable to ask whether or not these could represent the same taxon – given the differences in skull morphology, I suspect we're not looking at intraspecific variation here, although more specimens of P. mephistocephalus would be very helpful in this regard!

Crichtonsaurus becomes Crichtonpelta

Crichtonsaurus is another cool ankylosaur that has received surprisingly little attention given its Jurassic Park affinities. Two species have been named: Crichtonsaurus bohlini (the type species), and Crichtonsaurusbenxiensis. Crichtonsaurus bohlini is, unfortunately, a very incomplete jaw that does not bear any diagnostic features, and so we argue that Crichtonsaurus should be considered a nomen dubium. Crichtonsaurus benxiensis, on the other hand, is a great specimen with a really good skull and a fair bit of the postcrania, and the skull has some unique features that make it easy to distinguish from other taxa, most specifically the upturned quadratojugal horns. We've proposed the new name Crichtonpelta benxiensis for this material – Crichtonsaurus was a good name and we wanted to keep the replacement name similar, so now we have Crichton's shield instead of Crichton's lizard.

During the Flugsaurier symposium in 2010, while I was visiting Beijing and the IVPP, we took a field trip out to Liaoning and visited the Sihetun Fossil Site. It has a cool museum, including a mounted Crichtonpelta skeleton! I don't think this specimen has been described, but it does corroborate certain aspects of the holotype skull. Crichtonpelta seems to lack discrete caputegulae (tile-like ornamentation) on its skull, which gives it a similar appearance to Pinacosaurus. I don't think the osteoderms have been placed quite correctly on this skeletal mount – I think they've been tipped on their sides so that the keel forms part of the 'base', giving it a somewhat stegosaur-like appearance.

Liaoningosaurus and Chuanqilong

I'm going to talk more about Liaoningosaurus in a few months, but it is one cool little ankylosaur! At only about 30 cm long, the holotype is one of the smallest known ankylosaur specimens and probably represents a very young individual. There may be a few osteoderms in the cervical/scapular region, but that's about it. I've previously argued that the putative plastron in this specimen is more likely skin impressions, which is still pretty cool because we don't have a lot of belly skin for ankylosaurs. 

Liaoningosaurus! YAY!

I also wanted to give a shout out to here to Chuanqilong, a larger ankylosaur from Liaoning that was described last summer and which didn't make it into my thesis but which I did include in the revised phylogenetic analysis in the final paper.

Here's Chuanqilong, from Han et al. (2014).

Dongyangopelta, Taohelong, and Sauroplites

Let's finish off this post today with a triad of interesting but enigmatic ankylosaurs. Dongyangopelta and Taohelong are relatively new entries to the world of ankylosaurs, with both taxa appearing in 2013. Neither are particularly complete, but they are interesting because both species possess chunks of fused osteoderms, which would have been found over the pelvis and which are most commonly encountered in nodosaurids and 'polacanthids/polacanthines', and are presently unknown in ankylosaurids – and indeed, Yang et al. described Taohelong as the first example of a polacanthine from Asia. Nodosaurids (including 'polacanthines' as basal taxa within this clade) have been tentatively identified from Asia previously (an interesting but fragmentary specimen from Japan may be a nodosaurid), but to find a Polacanthus-like animal in Asia is unexpected and very interesting. The two species can be differentiated based on the morphology of these pelvic shield pieces. Dongyangopelta comes from the Chaochuan Formation, and another ankylosaur, Zhejiangosaurus, had been named from that formation in 2007; it may eventually shake out that Dongyangopelta is a junior synonym of Zhejiangosaurus, but in the absence of overlapping diagnostic material we opted to keep these taxa separate for now.

Pelvic shield fragments - Dongyangopelta redrawn from Chen et al. (2013), Taohelong redrawn from Yang et al. (2013), and Sauroplites redrawn from Bohlin (1953).

Sauroplites, on the other hand, is a very old name that has been largely overlooked in recent assessments of ankylosaurs. The material was originally described by Bohlin in 1953, but sadly the whereabouts of the original material is unknown today (although there are casts at the American Museum of Natural History). I think Sauroplites was overlooked for a while because it's based off of osteoderms alone, and it's hard to assess diagnostic characters in osteoderms sometimes because they vary so much along the body. This is partly why I like cervical half rings and pelvic shields – in these structures, you can understand the positions of the osteoderms on the body and directly compare patterns and morphologies across different taxa. Supposedly, the osteoderms for Sauroplites were preserved in their original positions when the specimen was excavated, and if so, it's a bit surprising that more of the skeleton was not preserved. Bohlin correctly identified some of these pieces as elements of the sacral armour, and the morphology of these pieces can be used to differentiate Sauroplites from Taohelong and Dongyangopelta, and we consider Sauroplites to be a valid, but poorly known, taxon. It's good to revisit poorly figured and fragmentary taxa from time to time, because new discoveries might help put those pieces in context.

Next time: we head south! See you then!

Arbour VM, Currie PJ. In press. Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs. Journal of Systematic Palaeontology.