Let's turn our attention from hadrosaur skin to ankylosaur
skin, a topic which has received surprisingly less attention in the published
literature than I would have thought. I should qualify that statement, however,
by saying that by 'ankylosaur skin' I mean ankylosaur skin impressions, because
ankylosaur dermal elements are well known and the focus of many a paper – I refer
of course to osteoderms, which form within the dermis of the skin and which
give ankylosaurs their spiky and armoured appearance.
For a couple of years now I've been keeping notes about
occurrences of skin impressions in ankylosaurs, which eventually lead to a
paper by myself, Mike Burns, Phil Bell, and Phil Currie. We reviewed the
morphology of scale patterns in the few specimens that preserve skin, and found
that there were some intriguing differences in scalation between different
ankylosaurs.
The holotype of Scolosaurus cutleri, NHMUK R5161, has the
best preserved integument for any North American ankylosaur, and has loads of scale impressions lying overtop of the in
situ osteoderms. In Scolosaurus, the scales form rosettes around the
osteoderms. The largest scales are generally found closest to the osteoderms,
but some large scales are scattered in between the osteoderms as well.
Underneath the scales, small ossicles (little osteoderms less than 1 cm in
diameter, but usually only 2-4 mm wide) fill the spaces between the larger
osteoderms.
Scolosaurus is hard to photograph well, sorry!
In contrast, a very unusual specimen (ROM 813) has a
completely different morphology. This specimen includes unusual long,
rectangular osteoderms that aren't present in NHMUK R5161. The scales are on
average much smaller, don't form much of a rosette pattern around any of the
osteoderms, and are more uniform in size overall. ROM 813 is a little bit
difficult to interpret because it is partially disarticulated (which is also
intriguing given that such large portions of the integument are intact), but
our best guess for the preserved portions is shown here.
Another super cool thing about ROM 813 is that it preserves the epidermal covering of an osteoderm, and it is the only example of this in an ankylosaur that I know about. In the photo below, the smooth side of the osteoderm is the epidermal scale, and the rough side of the osteoderm is the true bony part of the osteoderm.
Moving over to Mongolia,
a specimen referred to Tarchiagigantea lacks the small pavement of ossicles seen in the Albertan ankylosaurs,
and the epidermal scales are huge and more rectangular. In the portion of the
integument preserved, osteoderms are separated by only one row of scales.
There's enough overlapping material between these specimens
to allow us to compare scale patterns among different ankylosaurs, and the
differences support the hypothesis that these are different taxa.
Unfortunately, right now we can't assign ROM 813 to any known ankylosaurid
taxon from Alberta – this could represent the postcrania of
Euoplocephalus
tutus, or
Dyoplosaurus acutosquameus, or (less likely) a new taxon of
ankylosaurid from the Dinosaur Park Formation. I think it's safe to say that
the differences between
Scolosaurus and ROM 813 represent true taxonomic
differences, a finding that is in line with previous work by Phil Bell on
scalation differences between Saurolophus angustirostris and Saurolophus osborni.
Illustrations by Lida Xing and via PLOS ONE.
One more comment about ankylosaur skin: In 2010 I had the
opportunity to study the holotype of
Liaoningosaurus paradoxus, and very
interesting little ankylosaur from the Liaoning Formation of China. The
original authors described
Liaoningosaurus as possessing a ventral plastron
(bony shield, like that found in turtles), which would have been a highly
unusual anatomical feature given that no other ankylosaurs possess a plastron.
Having looked at this specimen, I think a better interpretation for the
plastron is that this is a segment of skin impressions from the belly region –
there didn't seem to be any bony texture around the edges of this area, and the
pattern is more consistent with scales than any osteoderms in other
ankylosaurs.
Belly scales for Liaoningosaurus. The scale bar is in millimetres.
Papers!
