Week Of Wonders: Sebecus

19 12 2009

Good tidings and well-wishes!

Terrestrial crocodylomorphs have been gracing the various postings of this blog since its earliest days and thus, I felt it would be quite fitting to throw the spotlight upon one of these magnificent creatures for the conclusion of ‘The Theatrical Tanystropheus’s first week-long ‘special’. So, without further ado (boy, that’s a first!), I now present the star of today’s installment of my ‘Week Of Wonders’ series:  Sebecus icaeorhinus.

A crocodylomorpha 'family portrait'. Sebecus itself can be observed crouching down in the stream on the left (more on that later).

As per the general protocol of this space, a discussion concerning the animal’s taxonomic affiliations is in order. Though you wouldn’t know it from looking at modern specimens, the crocodylomorpha has undergone a fascinating evolutionary history featuring dozens of intriguing players which ranged from streamlined aquatic predators to turtle-crushing behemoths to stout terrestrial omnivores. Until fairly recently, the superorder was divided into the suborders Eusuchia, Mesosuchia, Thalattosuchia, and Protosuchia. However, a handful of papers published earlier this decade have since re-arranged this antiquated setup.

The crocodylomorpha's place within the sauropsida class.

Under the new system, Sebecus and its relatives have been placed within the Mesoeucrocodylia taxa, which includes the Eusuchia, Thallatosuchia, and now-obsolete Mesosuchia suborders. The group is united by the following characteristics:

-The secondary palate is expanded toward the rear.

-The vertebrae are amphicoelous (concave at both ends).

When we attempt to more specifically define Sebecus‘ taxonomic place, however, problems emerge. There simply isn’t enough space for me to describe the current state of the Sebecosuchia suborder, so I’ll send everyone to ‘Why I Hate Theropods‘ for a nice overview. In 2007, sebecosuchians were treated to a fairly extensive revision which, among other things, dissected the Sebecus genus by re-naming “S. huilensisLangstonia huilensis, and “S. querejazus” Zulmasuchus querejazus (These updates along with several others put forth that year [see above], have effectively reduced the genus to a single species: S. icaeorhinus).

A sebecosuchian tooth

Nonetheless, the Sebecosuchia is characterized by the following traits (from ‘palaeos.com’):

“Active terrestrial predators. Tall, narrow rostrum [(‘snout’)]; sides of rostrum tall and nearly vertical; teeth long, curved, laterally compressed  (very similar to theropod teeth) [(in point of fact, they’re so similar that until the late 1930’s, several paleontologists used them as evidence behind the idea that non-avian theropods had survived the K-T event)] ; 4 teeth on premaxilla; 10-11 teeth on maxilla; no enlarged maxillary teeth; teeth widely spaced, intercalate; posterior ends [(‘rear ends’)]of maxillae meet on palate anterior to [(‘in front of’)]palatine; broad maxilla forms sides of rostrum, and narials forms flat dorsal [(‘top’)] portion, terminating in a premaxilla with substantial diastema; maxilla and premaxilla do not overlap; no maxillary fenestra [(‘holes’)]; nares [(‘nasal openings’)] face antero-laterally [(‘forward and to the side’)] or dorso-laterally [(‘upward and to the side’)]; maxilla and, especially, premaxilla deeply sculptured with deep pits connected by channels; rostrum widens abruptly in front of orbits [(‘eye sockets’)]; orbit relatively small; angular and surangular large, long and strongly curved dorsally [(‘upwards’)].” 

Sebecus skull (heavily reconstructed).

The suborder is furthermore divided into three families: the Baurusuchidae, the Bretesuchidae, and the Sebecidae. Naturally, Sebecus falls within the latter alongside Langstonia, Ayllusuchus, Barinasuchus, & Ilchunaia.

A modern Alligator modified to resemble a juvenile Sebecus.

As with many taxonomic identification systems, the characters mentioned earlier almost exclusively refers to cranial features. How, then, can we assert that Sebecus and its relatives were terrestrial predators? Well for starters, the comparatively rare limb bones (and the even scarcer articulated skeletons) of these beasts tend to bear a much greater resemblance to those of rauisuchians than to the traditional crocodilian physique (in fact, their proportions suggest that they likely severed all ties to any sort of liquid habitat). However, the skull itself offers a significant amount of evidence towards the land-based hunter interpretation, for the teeth of these crocs conform to a ziphodont mold, meaning that they’re compressed from side to side, slightly recurved, and serrated along their edges: a pattern which frequently found in terrestrial carnivores, but is rarely seen in their aquatic counterparts.

A pair of Baurusuchus square off, exhibiting the generally-accepted terrestrial crocodylian stance.

In his excellent 2002 book, “King Of The Crocodylians: The Paleobiology Of Deinosuchus”, David R. Schwimmer (not to be confused with the actor who famously portrayed a geeky pseudo-paleontologist) writes (note that, for the sake of modern accuracy, any mention of ‘Sebecus‘ should be replaced with ‘Sebecus and its kin’) (NOTE: Certain parts of the following quote are debatable and/or inaccurate. See the ‘comments’ section for details) :

“The sebecosuchids were… the last terrestrial crocodylomorphs in the fossil record, and the namesake genus Sebecus apparently survived and flourished in isolation in South America, when it was separated from both Africa and the Northern Hemisphere during much of the Tertiary period. Sebecus filled the role of dominant large predator there until the Pleistocene, when North American predators (eg: big cats) entered South America via the Central America land bridge When Sebecus went extinct, less than 1.0 Million Years ago, it was the last remaining mesoeucrocodylian below the neosuchian grade- and at the same time the last crocodylomorph below the eusuchian grade!”

May the fossil record continue to enchant us all!

Advertisements

Actions

Information

4 responses

20 12 2009
Adam Pritchard

A fine tribute to an under recognized group of monsters. Any idea where that “family portrait came from?

21 12 2009
tanystropheus

Thanks for the hefty compliment, Adam!
All I know about the ‘family portrait’ is that it’s a John Sibbick painting which has appeared in a few popular books.

21 12 2009
johannes

> The sebecosuchids were… the last terrestrial crocodylomorphs in the fossil > record,

Some of the Australasian island mekosuchines survived into historic times, and even on the Australian mainland, *Quinkana* survived into the late Pleistocene.

> and the namesake genus Sebecus apparently survived and flourished in isolation > in South America,

It’s also known from Messel in Germany

> when it was separated from both Africa and the Northern
> Hemisphere during much of the Tertiary period. Sebecus filled the role of
> dominant large predator

I hate theropod suprematists as much as anybody else ;), but I fear I will have to play the devil’s advocate here: Terror birds are more abundant in the fossil record.

> there until the Pleistocene,

According to palaeos, *Sebecus* disappears in the Miocene

> when North American predators (eg: big cats) entered South America via the
> Central America land bridge

The mass extinction in late Pliocene SA was probably caused by a bolide impact, not by invaders from the north, the only pre-impact laurasitherian “carnivore” of note in SA was the procyonid *Chapalamania”; a peaceful, Panda-like planteater. BTW, as mentioned above, AFAIK there is no fossil record of Pliocene sebecids.

21 12 2009
tanystropheus

Johannes,

Many thanks for your informative commentary on Dr. Schwimmer’s quotation (like yourself, I disagree with many of his assertions)! I’ve updated this post accordingly.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s




%d bloggers like this: