3:10 to Humor
3:10 am
This insight into the chondrichthyan condition brought to you by The Western Nostril.
Tip of the toupee to the good bloggers at Drawn!
April 2, 2008
August 26, 2005
Friday Dead Animal Blogging
12:19 am
Spiny-Brush Shark Edition
At the dawn of the Carboniferous period, around 350,000,000 years ago, great changes were taking place in the waters of the world. Many once-diverse and successful groups of fish hadn’t made it past the end of the previous period (the Devonian), so a plethora of aquatic opportunities had just become available for the fish that were fortunate enough to survive.
One of these fortunate groups were the sharks and their relatives, collectively known as chondrichthyans or “cartilaginous fish” for their most distinctive feature, a skeleton made largely of cartilage. Chondricthyans exploded into a fantastic diversity of forms during the Carboniferous, many of them odd and bizarre. Some developed tooth-rows like buzzsaw blades or pinking-shears, some grew winglike fins from the sides of their necks, some evolved long, antler-like pincers on their snouts, and some developed large clusters of spines atop their heads and over their dorsal fins. This last group is known as the stethacanthids, or spiny-brush sharks.
Stethacanthus (shown here in purple) is the most-familar of the group, and the most commonly illustrated. Its strange, wedge-shaped dorsal fin topped with dozens of sharp denticles must have cut quite a profile in the Carboniferous seas. Several suggestions have been made about the purpose of this unusual feature. Some have suggested that, from above, the two patches of tooth-like denticles on its fin and head might have appeared to be the gaping maw of a much larger shark. Others have thought that it might have served as a sort of velcro attachment point, allowing Stethacanthus to hitch a ride on larger fish, as remora do with sharks today. Unfortunately for these hypotheses, the spiny-brush of Stethacanthus and the closely-related Akmonistion (at left) were not particularly flexible and probably wouldn’t be of much use in those suggested situations.
One clue comes from the fact that all known stethacanthid specimens that bear the spiny-brush are male (this is easy to tell in most sharks, because males have prominent claspers behind their pelvic fins). Clasper-less (female) stethacanthid specimens are known, but none have been found with the dorsal spiny-brush. Some researchers have classified them as different animals, but it seems just as likely that they are female Stethacanthus or Akmonistion. Fossils of close stethacanthid relatives named Falcatus and Damocles support this idea.
Falcatus (”hook/sickle”) and Damocles (named after the king with a sword hanging over his head) have even more extensively-modified dorsal fins that curve forward over their heads like prongs, with the spiny denticles forming barbs near the tip. A spectacular fossil of two Falcatus from the Bear Gulch Limestones of Montana preserves a prongless, clasperless female was preserved biting the prong of a claspered (and presumably excited) male in what appears to be Paleozoic foreplay. The drawing shown here depicts a Damocles couple in a similar romantic moment.
Because the prongs of Falcatus and Damocles are modified versions of the same spiny-brush that adorns Stethacanthus and Akmonistion, it seems probable that all these sharks used their fins for similar purposes—as distinctive structures that males could use to advertise their prowess, and where females could focus some toothy affection during courtship and mating.
This artwork was used in PowerSharks!, “the Fintastic game for card sharks everywhere” developed by Ray Troll and myself.