Good tidings and well-wishes!
Though I don’t ordinarily extend my coverage to fish, I can’t completely disregard these most abundant of vertebrates. Part of the reasoning behind that statement stems from the fact that one of my most popular posts of all time was a brief little snippet I typed up last August which linked to an article I’d stumbled across on Pharyngula that discussed the reproductive organs of arthrodires, the group most famous for producing Dunkleosteus during the late Devonian. This taught me two very valuable lessons:
1: NEVER underestimate the awesome power of PZ Meyers.
2: Dunkleosteus terelli has got to be the most beloved fossilized fish of all time! (Screw you, Tiktaalik roseae and Carcharocles megalodon!)
Since D. terelli itself receives more than enough press coverage, I thought that I’d use this week’s ‘Spotlight’ segment to discuss one of the armored fish’s most unusual relatives: Titanichthys agassizi from the Late Devonian of Morocco and the Midwestern U.S.
A member of the placodermi class, the anthrodira order is chiefly defined by the presence of two bony structures known as ‘gnathal plates’ in each side of the upper jaw: the anterior superognathal and the posterior inferognathal, both of which would slide over a third set of plates known as the inferognathals (which resided in their lower jaws) during a bite from the animal. Like all placoderms, anthrodires lacked teeth, so these gnathal plates would have been razor-sharp in life and presumably worked like a massive pair of scissors with which the animal in question could shear through its prey with devastating force, as a computer-animated Dunkleosteus demonstrates in the following clip from the BBC’s miniseries “Sea Monsters”.
As Philippe Janvier writes in “Early Vertebrates”
“The arthrodires are divided into three major groups: the Actinolepidoidei…the Phylctaenii,…and the Brachythoraci. Only the second and third of these groups possess an articulation device between the skull-roof and the thoratic armour. It consists of a condyle on the anterior dorsolateral plate of the thoratic armour, and a corresponding fossa in the paranuchal plate of the skull-roof.”
For the purposes of this article, we need only concern ourselves with the latter of these three groups. According to this same volume, the Brachythoraci “are easily recognizable by their thick dermal bones with over-lapping or sutured margins. The inferognathal comprises a biting part and a large, posterior non-biting blade…The median dorsal plate has a well-developed ventral keel and the internal surface of the nuchal (‘neck’) plate has two deep pits in which are inserted a paired process of the neurocranium.”
Right then, having said all that, what’s so special about Titanichthys?
Remember the description which I utilized earlier to describe the biting mechanisms employed by the vast majority of arthrodires such as Dunkleosteus? Well, it turns out that the ‘razor sharp’ cutting edges weren’t especially sharp in Titanichthys. In fact, they were downright dull. Compare, if you please, the following skull reconstructions of Dunkleosteus (on the left) and Titanichthys (on the right).
The second pair doesn’t exactly appear to be the ‘jaws of death’, now does it? In fact, these gnathal plates would have been less than useless as shearing devices, which would render mastication essentially impossible.
However, I think it’s fairly safe to assume that Titanichthys wasn’t feeding through a straw…
The question remains: how did Titanichthys obtain sufficient food with such a puny arsenal in its mouth? The leading school of thought maintains that this massive fish (estimated to have achieved lengths of four to six meters) would have used its proportionally-large maw to literally inhale small, anchovy-like fish and (possibly) krill-like zooplankton. The rudimentary gnathal plates would have presumably been used when the animal closed its mouth for the purposes of retaining all entrapped food matter whilst the excess water it would have consumed in the process of predation was evicted, a technique exercised by a mata mata turtle (Chelus fimbriatus) in the following clip:
Additionally, it’s worth mentioning that Titanichthys sported a pair of extraordinarily large pectoral fins with which it would have theoretically balanced itself while engulfing its miniscule prey. This assumption is based on this region’s similar anatomy in the modern basking shark (Cetorhinus maximus) which dines in a similar fashion.
May the fossil record continue to enchant us all!