Monday, December 31, 2012

Dino (and Mammal)-Eating Crocs Part 3: The First Neosuchians

*Before I start, I must apologize for misusing the term Crocodilian in my last post concerning these animals. Crocodilian is used to define all the members of the Crocodilia, which include living crocodiles and all their closest extinc relatives, and not the Notosuchian + Neosuchian branch Metasuchia. Instead, the more broad term in considered to be Crocodylomorph, which is what I will use in this post.
Image of Stomatosuchus inermis, a large Crocodylomorph from mid Cretaceous Africa.
Its toothless lower jaw and possibly pelican-like throat pouch are just one example of diversity in the Neosuchia.
As I had shown in the previous post concerning Notosuchians, during the Mesozoic all kinds of these reptiles were running around under the feet of dinosaurs, and in some cases, directly competed with them for resources, territory, and living space. However, the other group of Crocodilimorphs, known as the Neosuchia, were just as, if not more diverse as their Neosuchid brethren, and exploited another type of environment; the water. Such a high level of diversity should really be expanded longer than a single blog post, but in an effort to save time, I'll be just quickly scanning though a few of the various groups and families.

The earliest known Neosuchian, called Calsoyasuchus valliceps, dates back to the early Jurassic, and predates the Neosuchia by almost 90 million years. However, despite being so old, Calsoyasuchus is a fairly derived form of Crocodilimorph, being a member of the Goniopholididae family, and suggests the Neosuchia originated even earlier. Goniopholididae were the first group of semi-aquatic Neosuchians, and would've resembled modern crocodilians in many ways. However, they lacked many defining characteristics, such as a well-developed secondary palate which allows them to breath while their body is submerged, and would've probably needed to lift most of their head out of the water to get a gulp of air.

More primitive Neosuchians only occur later in the fossil record. The Atoposauridae are often thought to be the most primitive family within the Neosuchia, but they don't appear until the late Jurassic, leaving another 30 million year gap between them and Calsoyasuchus. They seem to have been primarily land-living animals that resembled armored monitor lizards, and were probably filling a similar niche to them in the shadows of dinosaurs. As time went on, however, this certainly did change. While both the Goniopholididae and the Atoposauridae represent the earliest offshoots to the group, the second offshoot, however, has been getting more press with the discovery of Dakosaurus; the Thalattosuchia, or sea-crocodiles.

Image showing just a few species of Metriorhynchids with a diver for scale.
Despite the image portraying a 4.5m Dakotosaurus, some species may have rivaled
Plesiosuchus in size.
The Thalattosuchia are as far as we know the only crocodilians adapted towards a fully pelagic existence. They evolved during the mid Jurassic and persisted into early Cretaceous times, and are separated into two families; the very crocodilian-like Teleosauridae and the very fish and whale-like Metriorhynchidae. The Teleosauridae  have elongated jaws, and are thought to have been primarily shallow water piscivores, although some members like Machimosaurus have their teeth regularly found embedded within turtle shells. They show numerous adaptations for diving, however, such as big neck muscles and short, compact bodies, but still hold onto the elongated crocodile-like tail of their ancestors. The Metriorhynchidae were much more whale-like in appearance as well as feeding methods, sporting killer whale-like teeth, and many having shorter porpoise-like snouts. These were presumably the toothed whales of the age, and being equipped with well-developed salt glands, fish-like tails, and flippers, they lived a fully aquatic existence. They are also currently believed to be the only members of the Archosauria to be ovoviviparous, in that they likely held onto the young in the body and gave live birth to them.

However, despite often being considered members of the Neosuchia, some recent cladistic analysis of these animals have suggested that they may be more primitive, perhaps even outside of the Metasuchian branch altogether, and thus I shouldn't be including them in this post. For a more in-depth review of this as well as any information you may want to know about these creatures, see Darren Naish's recent article concerning these animals.

Next are possibly one of my favorite families among the primitive members of the Neosuchian branch, the Mahajangasuchidae. So far the family has only been identified by two species, both from Africa, Mahajangasuchus insignis from Madagascar and Kaprosuchus saharicus from the Sahara. They are characterized by their short and deep jaws, horn-like projections over the eyes, and fused nasal bones, the latter trait they share with the tyrannosauridae. They also both show adaptations in their skulls that suggest a largely if not fully terrestrial existence, such as more laterally positioned forward-facing eyes. In the case of Kaprosuchus, it had six pairs of elongated teeth and an armored nose, which may have allowed the animal to kill large, land-based prey.

Kaprosuchus has been getting some attention ever since its discovery, and has been portrayed on National Geographic's special When Crocs Ate Dinosaurs, as well as the British television series Primeval. Despite not really realizing it, Mahajangasuchus has gotten some screen time as well. In the 2005 remake of King Kong by Peter Jackson, they portrayed in a short scene a crocodile-like creature chasing after the lead heroine Ann Darrow before getting eaten by a T-rex-like animal. The creature was later confirmed in a "Natural History Book" about Skull Island to be known as Foetodon ferrus, but looking over its anatomy, its overall build and size shows an uncanny resemblance to Mahajangasuchus, and I wouldn't be surprised if it was based on the latter.

Image comparing the giant extinct Pholidosaurid Sarcosuchus to a living crocodile.
Their has been a debate as to what Sarcosuchus was eating; mostly land-based dinosaurs,
 or mostly fish. I personally find the later more likely. Sorry Sarco fans...
The Tethysuchia are a group of crocodilians which evolved near the end of the Jurassic, and survived into the Paleogene epoch. Within this group their are two families, the Pholidosauridae and Dyrosauridae. The Pholidosauridae, which includes Pholidosaurus, Meridiosaurus, and the infamous "Super Croc" Sarcosuchus, which was one of the largest Crocodylomorph to have ever existed. They were primarily long-jawed predators of fish with widened, bulbous noses, but some species of bucked this long-jaw trend entirely; Oceanosuchus had jaws similar to a modern day Alligator. The Philosauridae went extinct about 90 million years ago, likely after suffering heavy losses after the Cretaceous Thermal Maximum (CTM).

The Dyrosauridae were similar to the Pholidosauridae in many aspects, and are their sister group, but came about 20 million years later in geological time. They appeared about 70 million years ago and survived the K/T event that wiped out the dinosaurs. Most had, like the  Pholidosauridae, elongated jaws with numerous teeth, suggesting again, a mainly piscivorous lifestyle, though some species, such as Phosphatosaurus, had more traditional crocodilian-like teeth and jaws. This group survived up until their extinction in the Eocene, likely being victims of another small extinction event hat occurred at this time, although their disappearance could also be explained by competition withthe more advanced crocodilians.

Next come quite possibly, in my opinion, the most extreme members of any Crocodylomorph lineage. The Stomatosuchidae were a small family, like the Mahajangasuchidae, and have only two currently accepted genera. The first discovered was Stomatosuchus, which was unearthed by Ernst Stromer during one of his expeditions to Egypt, but was sadly lost along with the original Spinosaurus and Carcharodontosaurus remains during WW2. The second was Laganosuchus, which was described and discovered by Paul Sereno in 2009.

The original remains of Stomatosuchus were not very complete, but it suggested that the animal was over 30ft in length. Laganosuchus was probably smaller, reaching a little over 20ft. The upper jaw had tiny teeth, but the lower jaw may have been toothless (although Laganosuchus has small teeth in it's lower jaw). While Sereno has suggested that these animals may have been ambush predators, waiting jaws open for something to swim into them, I'm sceptical of this. The wideness of the jaws would produce a lot of drag in water, preventing them from being closed effectively at high speeds. Any rapid movement of such wide jaws would've also produced currents that would alert the fish of the danger through it's lateral lines, if not carry the fish out of their jaws entirely.

I instead believe that, since their jaws show similarities to living whales, these animals may have been hunting fish through filter-feeding. Perhaps they actively followed after schools of fish, swallowing them in gigantic mouth fulls. It would've certainly been a niche that they could fit into nicely, seeing as the only other creatures hunting like this at the time were gigantic fish in the family pachycormidae, and even these huge fish were likely feeding on tiny plankton rather than other fish species. So it seems the stomatosuchidae had this unique lifestyle and feeding habits to keep all to themselves. (although another  Crocodylomorph family, the aegyptosuchidae, may have been doing similar things)

So their we go. I know it is a bit short, but I told everyone I'd try to get this done before the end of the month, and I was running out of time.

Still, I'm not done yet, and I'd really like to get into some more crocodilians next time, including more modern crocodilians that are all scattered across the past. I've also been facinated by a species of land-living (if not arboreal) crocodilan that may have crossed paths with early Homo sapiens, and you can be sure it will be talked about sooner or later. ;)

1 comment:

  1. Interesting. Is the terrestrial crocodile you are talking about a member of the Mekosuchus genus? I don't know if you have read it, but this article could perhaps be of some interest: