Monday, January 28, 2013

The Ecological Importance and Awesomely Gigantic Reptiles of the Cerrejon Formation, part II

Dyro Straits
While T. cerrejonensis evolved to rule with brawn, crocodylomorphs known as dyrosaurids championed in adapting to a completely new environment. The majority of dyrosaurids dwelled in shallow, coastal environments, preying on small fish; however, the two species of dyrosaurid uncovered from the Cerrejon display some remarkable adaptations for living in calm, freshwater shallows.

A. guarjiraensis swims below T. cerrejonensis. Reconstruction by Danielle Byerley.
Acherontisuchus guajiraensis, a relatively large species measuring up to 6.5m (~21ft), was found with a bone found in most dryosaurids and crocodylomorphs known as the ischial shaft. This bone anchors the muscles which allow the animal to control its buoyancy and pitch in the water, an indispensable task for life in the rolling waves of the coasts. However, in A. guajiraensis, this bone is much more slender than in other species, indicating a lesser need and ability to control its movement in rough waters. The placid waters of the Cerrejon wetlands were free of waves or strong currents, leading to the shrinking of this bone. A. guajiraensis was also found to have various physiological adaptations for living in an inland freshwater environment, as well as adaptations found in extant crocodilians which help the limbs support the animal’s entire weight. A. guajiraensis, unlike other dyrosaurids, could therefore easily navigate amongst the river systems in which it thrived, both on land on in the water. The long, slender snout of A. guarjiraensis, a feature typical of most dyrosaurids, indicates a specialized piscivorous diet; it likely fed on eels and lungfishes in the sluggish rivers. (Bloch et. al, 2011)

The holotype skull of C. improcercus. From Hastings et. al, 2010. Scale bar represents 10cm.
Unlike A. guarjiraensis, which was large-bodied, slender-snouted, and specialized for a diet of fish, Cerrejonisuchus improcerus was a small, short-skulled generalist. This little dyrosaurid, the shortest-bodied species yet discovered, reached lengths of only 2.2m (~7ft) which, while still large for a reptile, is small compared to the rest of the family. Its short snout and uncurved teeth, both of which are very unusual for a dyrosaurid, indicate a diet much more general than that of A. guarjiraensis. C. improcerus likely fed on small amphibians, reptiles, and mammals.

While T. cerrejonensis and other huge reptiles get media attention for their extraordinary size, C. improcerus was touted as simply being “snake food.” Various articles about the discovery of this dyrosaurid merely mention that it was “devoured” by the fifty-foot-long super-serpent. C. improcerus, as we know, was much more than just a prey item. It evolved in an environment unexploited by crocodilians or other aquatic carnivorous reptiles, and its real beauty lies not in its edibility, but its adaptability.

Carbonemys: the football-headed car-turtle
While giant snakes and dyrosaurids are all unique and interesting in their own right, no species of the Cerrejon Formation is as downright bizarre as Carbonemys cofrinii.

Edwin Cadena of the University of Florida, discoverer of C. cofrinii. The huge thing on the table is its shell.
Before we get into just how awesome C. cofrinii was, we need to delve into the phylogeny of turtles. The order Testudines is divided into two suborders: Cryptodira and Pleurodira. The former subfamily includes turtles which are able to retract their heads and necks backwards into their shells in classic turtle fashion. The latter, the pleurodires, cannot retract in this way: instead, they tuck their necks into their shells horizontally, and include species known as side-necked turtles. C. cofrinii is a pleurodire of the family Podocnemididae, which includes extant side-necked turtles found in Madagascar and South America.

What nightmares are made of when you're a baby snake or dyrosaurid. Reconstruction by Liz Bradford.
With a streamlined shell the size of a Smart Car and a skull as big as a regulation American football, C. cofrinii was an absolutely massive animal, a product of the hothouse climate which led to the evolution of the other massive reptiles with which it shared its environment. Side-necked turtles today are notorious for eating just about everything they can fit in their mouths, equipped with powerful jaws that can break open the shells of snails, claws that can tear apart carrion, and necks strong enough to drag birds into the water. An animal the size of C. cofrinii would have no trouble chowing down on the hatchlings of other reptiles, including dyrosaurids and snakes, as well as preying upon fish and browsing on vegetation.

Disappearance of the Cerrejon wetlands
The hot and humid environment of the Paleocene did not endure. Within a few million years, the climate became cooler and drier, and this led to the disappearance not only of the Cerrejon swamp, but of rainforests in North America, Europe, and Asia. Just as the hot climate favored large reptiles, cooler temperatures favored smaller ones, and eventually the tremendous reptiles went extinct. The entire family of dyrosaurids went extinct, while boids closely related to Titanoboa still include the largest extant snakes, the anacondas. Certain species of South American pleurodires still grow to large sizes, but none compare with the vast Carbonemys. Thankfully, the coal swamps of ancient Colombia preserved a remarkably detailed account of life on the stagnant wetlands of the Paleocene, and we are constantly discovering more about this alien environment.

Bloch, J.I.; Hastings, A.K.; and Jaramillo, C.A. 2011. A new longirostrine dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of north-eastern Colombia: biogeographic and behavioural implications for New-World Dyrosauridae. Paleontology 54, 1095–1116.

Gomez-Navarro, C.; Herrera, F.; Jaramillo, C.A.; Labandeira, C.C.; Wilf, P.; and Wing, S.L. 2009. Late Paleocene fossils from the Cerrejon Formation, Colombia, are the earliest record of Neotropical rainforest. Proceedings of the National Academy of Sciences of the United States of America.

Herrera, F.; Jaramillo, C.; and Wing, S. 2005. Warm (not hot) tropics during the late Paleocene: first continental evidence. AGU Fall Meeting Abstracts, 608.

Niinemets, U.; Peppe, D.J.; Royer, D.R.; and Wheeler, E.A. 2012. Roles of climate and functioning traits in controlling toothed vs. untoothed leaf margins. American Journal of Botany 99, 915-922.

No comments:

Post a Comment