Ankylosaur week, day 3: Aletopelta

Originally discovered in 1987 by Bradford Riney during a palaeontological surveying trip, the only known specimen of Aletopelta coombsi [shown here] is from an outcrop of the Campanian Point Loma Formation at Carlsbad, California. It's one of several ankylosaur specimens whose remains come from marine sediments: an occurrence which has led several palaeontologists to suggest that some ankylosaurs frequented beaches, or even that they swam in the shallows eating marine plants, like armour-plated manatees. Of course these ideas are possible but difficult to confirm (actually, it might be possible to test them by studying the isotopes preserved in ankylosaur teeth, but no-one has done this yet, to my knowledge. And, for what it's worth, no I don't think that ankylosaurs were semi-aquatic). However it got there, the Aletopelta specimen lay upside-down on the Cretaceous seafloor, its decomposing skeleton acting as a reef that became colonised by molluscs and other invertebrates. Judging by their shed teeth, sharks fed on the carcass.

Aletopelta was originally described as a nodosaurid (see day 2 article for more on ankylosaur classification), mostly because its armour was argued to resemble that of the poorly known Stegopelta landerensis (Coombs & Deméré 1996). These authors also concluded that the specimen couldn't be identified more precisely than this. In a reassessment, Ford & Kirkland (2001) argued that the Carlsbad ankylosaur more resembled ankylosaurids than nodosaurids in forelimb, pelvic, foot and armour morphology, and they also argued that the specimen was diagnostic, with the polygonal, mosaic-style armour pattern over the pelvis, the hindlimb proportions and other characters being unique to the specimen. They therefore gave it a name. Aletopelta means 'wandering shield' and refers to the fact that the southern California microplane has wandered north since the Cretaceous, carrying the Carlsbad ankylosaur with it.

A large and tall, subtriangular scute from the specimen probably came from near the midline of the shoulder region where it would originally have been one of a pair. Tall, broad-based triangular scutes with hollow bases seem to have decorated the tail. An unusual feature of Aletopelta, seen elsewhere among ankylosaurs only in Euoplocephalus, is the presence of just three metatarsals (and hence three toes). Other ankylosaurs have four or five, so if the one known foot of Aletopelta is complete it shows that this ankylosaur had a more reduced foot than that normally present in the group. As in Stegopelta and Glyptodontopelta, there were no small scutes separating the large pelvic scutes in Aletopelta, and this and other similarities might suggest that these ankylosaurids may have been close relatives. Not all experts agree that Aletopelta is distinct: in a major recent revision of ankylosaurs it was listed as a nomen dubium (Vickaryous et al. 2004). The adjacent illustration by Tracy Ford depicts Aletopelta as it may have looked in life (total length c. 5 m).

And more tomorrow...

Refs - -

Coombs, W. P. & Deméré, T. A. 1996. A Late Cretaceous nodosaurid ankylosaur (Dinosauria: Ornithischia) from marine sediments of coastal California. Journal of Paleontology 70, 311-326.

Ford, T. L. & Kirkland, J. I. 2001. Carlsbad ankylosaur (Ornithischia, Ankylosauria): an ankylosaurid and not a nodosaurid. In Carpenter, K. (ed) The Armored Dinosaurs. Indiana University Press (Bloomington and Indianapolis), pp. 239-260.

Vickaryous, M. K., Maryanska, T. & Weishampel, D. B. 2004. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 363-392.