Protostegidae

Protostegidae is a family of extinct marine turtles that lived during the Mesozoic Era. The family includes some of the largest sea turtles that ever existed. The largest, Archelon, had a head a meter long. While like most sea turtles, they had flattened bodies and had flippers for front appendages, protostegids had minimal shells like leatherback turtles of modern times.

Anatomy

As some of the first marine turtles, the protostegids set the general body plan for future species of sea turtles. Protostegids have a generally depressed turtle body plan, complete with four limbs, a short tail and a large head at the end of a relatively short neck. Like other sea turtles, they possess oar-like front appendages specially-evolved for swimming in the open ocean. Similar to the closely-related and still-extant Dermochelyidae, protostegids possess extremely reduced carapaces. Some specimens have skeletal protrusions from their ribs almost wrapping around the turtles' bodies in place of a complete shell. Like modern sea turtles, protostegids had sharp beaks. One of the defining characteristics of the members of the family are their almost-disproportionately large heads. Specifically, some specimens of Archelon have been found with heads that were a meter long. In addition, the members of the family had somewhat reduced plastrons as well.

Ecology

Trophic ecology

While all members of the family are extinct, palaeoecological studies on the members of the family has provided some insight into the ecological roles of the Protostegidae. Analysis of fossil organs of some protostegids has revealed entire stomachs containing fossilized shellfish. The turtles themselves are postulated to have been preyed upon by the major predators of the time. Fossil protostegids have been found with tooth impressions from the large lamnid sharks of the time. Two specimens of Protostega gigas have been discovered to have tooth marks from large sharks. In addition, teeth of the extinct shark Cretoxyrhina mantelli have been found embedded in at least one Protostega skeleton.

Evolutionary history

The family's oldest member is Santanachelys gaffneyi, known from a specimen excavated from Brazil in 1998. The species first appeared during the Early Cretaceous. As an early sea turtle, Santanachelys had several unspecialized characteristics such as distinguishable digits in its flipper-like arms. Later relatives' flippers were completely fused together for more efficient swimming. As with most large fauna of the era, the Protostegidae died out during the events of the Cretaceous–Tertiary extinction event that led to the extinction of the dinosaurs. Through phylogenetic analysis, it has been determined that the closest living relatives of this particular family are the leatherback turtles in the family Dermochelyidae, both of which are monophyletic.

Taxonomic history

In 1888, the Belgian zoologist George Albert Boulenger published his classification of the Testudinata within the ninth edition of the Encyclopaedia Britannica. The genus Protostega was placed within the family Sphargidae under the suborder Athecae. A year or so later, the entire suborder was downgraded by Karl Alfred von Zittel into a family within the Cryptodira.

In 1994, Hirayama proposed a three-family subdivision of the sea turtle superfamily based on cladistic analysis; Protostegidae was given full, formal family status in the system, containing most of the extinct genera including Archelon and a previously undescribed protostegid. The unidentified specimen was fully described in 1998, as the species Santanachelys gaffneyi. The genus (Santanachelys) was appended to the family after the new species was described. This specimen was later to be analyzed to be the family's oldest member.

References

Further reading