Thursday, 26 June 2014

A blunt-snouted Dryosaurid from the Palaeocene of Columbia.

Crocodylomorphs arose during the Triassic and diversified throughout the Mesozoic, reaching a great diversity of forms by the end of the Cretaceous. However they group were badly affected by the End-Cretaceous Extinction, with only two groups surviving into the Tertiary; the still surviving Crocodylians and the predominantly marine Dryosaurids, which underwent a major adaptive radiation in the early Neogene, before going extinct at the end of the Eocene.

In a paper published in the journal Historical Biology on 23 May 2014, Alexander Hastings of the Florida Museum of Natural History at the University of Florida and the Geiseltalmuseum at Martin Luther Universität Halle-Wittenberg, Jonathan Bloch, also of the Florida Museum of Natural History, and Carlos Jaramillo of the Smithsonian Tropical Research Institute describe a new species of Dryosaurid from the Middle Palaeocene Cerrejón Formation of northeastern Columbia.

The new species is named Anthracosuchus balrogus, where ‘Anthracosuchus’ means ‘Coal-Crocodile’ and ‘balrogus’ refers to the Balrog from JRR Tolkein’s Lord of the Rings, a fearsome creature unearthed in a deep-mine. The species is described from four skulls and a variety of post cranial materia, all unearthed within the Cerrejón Coal Mine in Guajira Department in northeastern Colombia. All come from a single layer of fine-grained grey clay beneath Coal Seem 90, that has been dated to 58-60 million years old.

Anthracosuchus balrogus, first specimen, from Cerrejón locality in northeastern Colombia, middle-late Palaeocene: (A, B) in dorsal view, (C, D) in ventral view and (C) insert displays an in situ tooth in lingual and anterior views; scale bar is 1 cm. bf, bone fragment; bo, basioccipital; bsp, basisphenoid; en, external nares; eo, exoccipital; ep, ectopterygoid; f, frontal; j, jugal; l, lacrimal; max, maxilla; m1, 4, 5 and 8, maxillary teeth/alveoli; n, nasal; or t, orbital tuberosity; ost, osteoderm (displaced); o t, occipital tuberosities; p, parietal; pal, palatine; pm2–4, premaxillary teeth/alveoli; pmx, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; sq, squamosal; stf, supratemporal fenestra; vert, vertebra (displaced). Scale bar is 10 cm. Hastings et al. (2014).

The Cerrejón Formation is thought to have been laid down on a fluvial (river) floodplain with abundant vegetation, part of an ancient river system draining into the Caribbean Sea. It has yielded abundant plant fossils, representing the earliest known examples of tropical rainforest from the Americas, as well as numerous vertebrate fossils, including Side-necked Turtles, Giant Boid Snakes and a diverse assemblage of Dryosaurids, though to represent the first examples of post-Cretaceous freshwater members of the group at a time when they were rapidly diversifying.

Anthracosuchus balrogus is morphologically distinct from any other known Dryosaurid, with a short, almost square skull, and a number of tuberous growths on the snout, forward of the eye sockets. Anthracosuchus balrogus also has a much reduced tooth-count for a Dryosaurid (or any form of Crocodylomorph), with only eight teeth on each side, and appears to show a degree of heterodont dentition; i.e. its teeth were not all the same, but rather it had different shaped teeth in different parts of its mouth.

Second specimen of Anthracosuchus balrogus from Cerrejón locality in northeastern Colombia, middle-late Palaeocene: (A, B) skull in dorsal view, (C) associated tooth in lingual view, (D) associated tooth in posterior view and (E, F) skull in ventral view. bo, basioccipital; en, external nares; eo, exoccipital; eo a, articulation for exoccipital; ep, ectopterygoid; f, frontal; j, jugal; l, lacrimal; max, maxilla; m1–8, maxillary teeth/alveoli; n, nasal; orb, orbit; or t, orbital tuberosity; ot, occipital tuberosities; p, parietal; pal, palatine; pm2–4, premaxillary teeth/alveoli; pmx, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; qc, quadratic condyle; sq, squamosal; stf, supratemporal fenestra. Scale bar for A–B and E–F equals 10 cm. Scale bar for C–D equals 1 cm. Hastings et al. (2014).

Hastings et al. interpret the short jaws and reduced tooth number of teeth seen in Anthracosuchus balrogus as an adaptation to a durophagous diet (diet of hard or tough items), and further suggest that it may have been responsible for bite marks seen on some large Turtle specimens from the Cerrejón Formation.

See also…



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