Wednesday, February 20, 2013

A very large aquatic Palaeophiine snake from Morroco's Eocene


Fossil snakes of the Palaeophiinae snakes are relatively poorly known, but they inhabited the oceans, lakes, rivers and possibly the land from the Maastrichtian to the Late Eocene. This subfamily includes the two genera Palaeophis and Pterosphenus. Palaeophiinae includes a wide range of species of various sizes. The largest species known to date is considered to be Palaeophis colossaeus estimated to be about 9 m long, whereas the smallest species P. casei  is estimates at about 1–1.3 m long. This subfamily includes various species illustrating three ecological grades. The ‘primitive’ Palaeophis grade comprises species with vertebrae only slightly modified for an aquatic life, whereas the ‘advanced’ Palaeophis includes species with vertebrae clearly modified for an aquatic life The third grade is comprised only of Pterosphenus, whose vertebrae are highly modified for an aquatic life.

Palaeophis maghrebianus described by Arambourg in 1952 is one of the better documented species, but it has not been revised since its original description. It is only known from the Ypresian Phosphates of Morocco. And, it is the only squamate discovered in the Ypresian of Moroccan Phosphates to date. This large snake is considered aquatic based on its vertebral morphology. However, it belongs to the ‘primitive’ Palaeophis grade.

The discovery of new isolated vertebrae and three articulated vertebral segments, which correspond to the longest vertebral segments known for a palaeophiine, enabled Houssaye and colleagues (2013) to obtain new anatomical data about this species which is reported in a forthcoming article the journal Paleontology.

Houssaye et al. (2013) examined the  microanatomical and histological features of some vertebrae to obtain new information about the biology and palaeoecology of this snake.

Within palaeophiids, P. maghrebianus appears to be relatively poorly adapted to an aquatic life with the vertebrae only weakly compressed laterally. These vertebrae appear peculiarly compact, as a result of a combined inhibition of primary bone remodelling and of additional deposits of secondary bone during remodelling. A high vertebral compactness has also been observed in the living marine snakes of the genus  Hydrophis and the freshwater homalopsid snake Enhydris bocourti. These data are thus consistent with an aquatic mode of life for this taxon. Palaeophis was a near-shore and estuarine dweller that lived in shallow marine, brackish and possibly freshwater environments.   Eocene mangrove environments were inhabited by  the ‘advanced’ palaeophiid Pterosphenus. But, Pterosphenus was found also in an open marine environment. Palaeophis, even those of the ‘advanced’ group, were likely not pelagic animals. Shallow aquatic environments including mangroves were especially favorable to Palaeophiidae.

This new specimen has vertebrae much larger than the previously known for this species. Vertebrae reach 3.3 cm in length; as compared to a maximal length of 1.9 cm in a 5.9-m long specimen of Python reticulatus. Palaeophis maghrebianus was probably much longer than the biggest extant snakes. Along with P. colossaeus, it becomes one of the two longest palaeophiids, assuming that the number of vertebrae was similar in these two species.

A dense vascular network was observed in P. maghrebianus vertebrae. Vascularization is generally absent or consists of a few simple vascular canals radially oriented in extant squamates. Vascularization is observed in only the largest species of extant squamates and the degree of vascularization in P. maghrebianus is much higher than in the common anaconda, Eunectes murinus or the reticulated python, Python reticulatus. This important vascular network indicates that P. maghrebianus was growing much faster and thus had a much higher metabolic rate than the largest extant snakes.

Citations
Arambourg. C. 1952. Les vertébrés fossiles des gisments de phosphates (Maroc- Algérie- Tunisie). Service Geologique du Maroc, Notes et Mémoires 92:1-372.

HOUSSAYE, A., RAGE, J.-C., BARDET, N., VINCENT, P., AMAGHZAZ, M. and MESLOUH, S. (2013), New highlights about the enigmatic marine snake Palaeophis maghrebianus (Palaeophiidae; Palaeophiinae) from the Ypresian (Lower Eocene) Phosphates of Morocco. Palaeontology. doi: 10.1111/pala.12008.

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