Dinosaurs - Appalachiosaurus montgomeriensis

Introduction

Pronounced ap-uh-LAY-chee-oh-SOR-us mont-gom-uri-en-sis.

The genus name Appalachiosaurus means 'lizard from Appalachia', in reference to the mountain region where the fossils were found. Montgomeri honours Alabama's Montgomery county and ensis is the Latin suffix meaning 'from'.

Appalachiosaurus montgomeriensis is the first and most complete tyrannosaur known from the eastern USA. It is smaller than its relatives Albertosaurusand Gorgosaurus.

Identification

The only known individual, the type specimen, was a juvenile that was about two-thirds fully grown.

Appalachiosaurus montgomeriensis had a long, narrow skull with a shallow jaw and probably short, stunted arms with two fingers. Although the forelimbs of Appalachiosaurus are poorly known, large tyrannosaurids are characterised by proportionally small forelimbs and two-fingered hands, so it's believed that's the case for this species. Early reconstructions gave it long arms with three fingers.

Similar to the arrangement in the Asian Alioramus altai, this North American tyrannosaur had six low crests along the top of its long snout.

Although the original specimen was found in 1982, it wasn’t until 2005 that palaeontologists determined that the material represented an entirely new genus and species of small tyrannosaur, naming it Appalachiosaurus montgomeriensis.

Habitat

During much of the Late Cretaceous, North America was divided by the Western Interior Seaway. Eastern tyrannosaurs like Appalachiosaurus and Dryptosaurus had no interactions with their more advanced western cousins.

Large seeds found in Alabama suggest Appalachiosaurus lived in a rainforest-like environment with a closed canopy.

Distribution

Appalachiosaurus montgomeriensis lived in a densely wooded rainforest in what is now the Appalachian region in Montgomery County, Alabama, USA, during the Late Cretaceous, about 80-76 million years ago.

Feeding and diet

Appalachiosaurus montgomeriensis was a predatory dinosaur that lived among a small community of eastern North American dinosaurs.

Experts believe the moderate size of this dinosaur, and the type of habitat it lived in, would have forced Appalachiosaurus to be an ambush predator. The densely wooded forests may have prevented Appalachiosaurus from reaching its potential top speeds,; instead it may have waited amongst the trees for prey to approach. Estimates suggest it may have only reached a maximum speed of around 13 km per hour when chasing prey in enclosed areas, but it could possibly have engaged in long distance pursuits.

Detailed studies of this species' skull and jaw have revealed more information on its feeding strategies. Based on bite force estimates, Appalachiosaurus likely didn't habitually crush bone like Tyrannosaurus rex, but focused on the flesh of prey. Its teeth were also well equipped for raking down and backwards into flesh. Evidence suggests Appalachiosaurus would have been able to dispatch prey of equal or greater body size to itself - finds near Appalachiosaurus fossils include large and heavily armoured dinosaur species. However, because of the small skull it likely used lateral shaking to dismember prey rather than repeated biting.

Other behaviours and adaptations

The type specimen of Appalachiosaurus, as well as one of Albertosaurus, bears evidence of bite marks from the large crocodilian Deinosuchus. The hunter was also the hunted (or possibly scavenged!).

Fossils description

It's not often that dinosaurs are dug up in the southeastern USA, so the discovery of Appalachiosaurus was big news. The fossil, believed to be of a juvenile, measured about 7 metres long, and the living dinosaur probably weighed a bit less than a ton.

The type specimen was found by Auburn University geologist David King in eastern North America, Montgomery County, Alabama, in July 1982. It is about 40 percent complete and includes parts of the skull and mandible (lower jaw), several vertebrae, parts of the pelvis, and both hind limbs. It was named in 2005 by palaeontologists Thomas Carr, Thomas Williamson and David Schwimmer.

As with all Late Cretaceous eastern North American dinosaurs, the fossils of Appalachiosaurus montgomeriensis were found in marine sediments, more specifically chalk, made of fine clay and marine plankton that formed at water depths of over 100 hundred metres. Scientists believe that Appalachiosaurus died on land before its carcass was washed out to sea, perhaps during a storm. This theory is based on fossil plankton and other marine creatures which were entombed in the rock alongside the dinosaur. Also, some of the heavy bones were missing, probably lost while the carcass floated before burial.

Evolutionary relationships

Scientists are yet to agree on this species' position within the superfamily.

The first phylogenetic analyses using cladistics, performed before the animal was named, found Appalachiosaurus to be a member of the Albertosaurine subfamily of Tyrannosauridae, which also includes Albertosaurus and Gorgosaurus. The original published description in 2005 also included a cladistic analysis, and claimed A. montgomeriensis to be a basal tyrannosauroid outside of Tyrannosauridae.

More recently, it has again been proposed that the species is member of the Albertosaurine subfamily, and at least one scientist considers it a possible species of Albertosaurus as it is structurally similar to its western cousin.

References

Carr, T.D.; Williamson, T.E.; Schwimmer, D.R. (2005). "A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama". Journal of Vertebrate Paleontology 25 (1): 119–143.

Jovanelly T.J. & Lane L. (2012), "Comparison of the Functional Morphology of Appalachiosaurus and Albertosaurus", The Open Geology Journal 6: p. 65-71.