Sand Sharks
Earlier this year I spent some time in birding in Morocco, an amazingly diverse country that ranges from the snow-covered peaks of the High Atlas Mountains to the deserts of the Sahara. So when a paper was published recently about the incredibly well-preserved skeleton of a prehistoric shark found in the Anti-Atlas Mountains it caught my eye. That’s right, a shark skeleton in what now the edge of the Sahara Desert. The Anti-Atlas Mountains are part of the larger Atlas mountain range, but they lie south and east of the High Atlas, which prevent moisture from entering the region and contribute to a very arid environment. On their southeastern front the Anti-Atlas Mountains descend to the Sahara, with the fossil site located near present-day Erfoud. During the Late Devonian period when this specimen lived however, from 360-370 million years ago, the area was a marine sea with depths to 400 meters. Sharks carry cartilaginous skeletons that rarely fossilize the way bony skeletons do, so most prehistoric sharks are known only from their mineralized teeth. A well-preserved full skeleton is a highly important find. The low-oxygen marine sediment at the bottom of this Devonian sea likely helped to preserve this shark specimen in such exquisite detail.
The nearly complete skeleton of one individual, and several additional skulls, were imaged with computerized axial tomography (CT scanning) to recreate the size and structure of the animal when alive (see figure). The new shark belongs to the genus Phoebodus, which until now consisted only of tooth remains, and represents a previously unknown species now named Phoebodus saidselachus. The skeletal structure suggests the shark was approximately 1.2 meters in length, with an elongated body and a long snout, making it look similar to the unrelated frilled sharks that exist today --- bizarre creatures in their own right. P. saidselachus had the distinctive tricuspid (three-pointed) teeth of related ancient sharks, and carried spines at the base of its dorsal fins. The animal had a small lower jaw with a weak bite force, suggesting it fed by capturing prey whole with a rapid snapping motion. This specimen from near the origin of sharks is the earliest known “eel-like” shark; it broadens our understanding of shark evolution and increases the known diversity of early shark species.
Reconstruction of the likely appearance of the extinct shark Phoebodus saidselachus. (Image from Frey et al.)
The paper is: Frey L, Coates M, Ginter M, Hairapetian V, Rücklin M, Jerjen I, and Klug C. (2019) The early elasmobranch Phoebodus: phylogenetic relationships, ecomorphology and a new time-scale for shark evolution. Proceedings of the Royal Society B, 286:20191336. DOI:10.1098/rspb.2019.1336