Machaeracanthus peracutus Shark Spines

August's Fossil of the Month is a remarkably preserved set of pectoral spines from the ancient spiny shark, Machaeracanthus peracutus, which is exact the specimen John Strong Newberry illustrated and described in his 1889 publication of Paleozoic Fishes of North America

Very few fish today have dorsal spines and none that we know of have pectoral spines. However, in the Mesozoic and Palaeozoic it is not uncommon to find fish guarded with spines on their fins. The earliest "spiny sharks", or acanthodians, predate sharks by 50 million years and had cartilaginous skeletons with bony-based fins and dentin spines. They were so-named "sharks" because they had the general shape of a shark with paired fins and an upturned tail.

The spines and scales are often all that can be found of spiny sharks due to the fact that their skeletons were mostly made of cartilage. This specific specimen from Machaeracanthus peracutus from Auburn, New York was represented in the publication because it displays a pair of spines that were fossilized roughly in the position they would be in on the pectoral fins of the spiny shark, thus providing evidence that they were connected with the fins. The presumed pectoral position and wear of the spines provide theories that they could have prevented the fish from sinking into the mud when resting on the ocean bottom, and/or been used as weapons. 

These spines are each 5 inches long with an arched and gradually tapering edge. They are extremely smooth with softly articulated longitudinal striation and a well defined ridge running the full length of the spines that is triangular in cross-section on one side and rounded on the other. 

 

 

Shark Spines & Original 1889 Publication
Machaeracanthus peracutus (Newberry, 1857)
Middle Devonian
Auburn, New York

(Reproduction image credit: spinops.blogspot.com)