| Protostegids | |
|---|---|
 | |
| Life restoration ofArchelon ischyros | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Testudines |
| Suborder: | Cryptodira |
| Clade: | Panchelonioidea (?) |
| Family: | †Protostegidae Cope, 1873[3] |
| Type species | |
| †Protostega gigas Cope, 1872 | |
| Genera | |
Protostegidae is a family ofextinctmarine turtles that lived during theCretaceous period. The family includes some of the largest sea turtles that ever existed. The largestArchelon had a head one metre (39 in) long. Like most sea turtles, they had flattened bodies andflippers for front appendages; protostegids had minimalshells likeleatherback turtles of modern times.
As some of the firstmarine turtles, the protostegids set the general body plan for future species of sea turtles. They had 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 possessed oar-like front appendages especially evolved for swimming in the open ocean. Similar to the still-extant, possibly closely relatedDermochelyidae, protostegids possessed extremely reducedcarapaces. Some specimens had skeletal protrusions from their ribs almost wrapping around their 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 was their almost-disproportionately large heads. Specifically, some specimens ofArchelon have been found with heads one metre (39 in) long. In addition, the members of the family had somewhat reduced plastrons, as well.[5]
While all members of the family areextinct,palaeoecological studies on the members of the family have provided some insight into the ecological roles of the Protostegidae. Analysis of fossil organs of some protostegids has revealed entire stomachs containing fossilized shells ofinoceramids clams,[6] and the first evidence ofgastroliths was reported in 2024.[7] The turtles, in turn, are postulated to have been preyed upon by the major predators of the time. Fossil protostegids have been found with tooth impressions from the largelamnid sharks of the time.[8] Two specimens ofProtostega gigas have been discovered to have tooth marks from largesharks. In addition, teeth of the extinct sharkCretoxyrhina mantelli have been found embedded in at least oneProtostega skeleton.[9]
The family's oldest members include anEarly Cretaceous (Valanginian) taxon described on the basis of limb bones and shell remains from theRosablanca Formation ofColombia,[2]Desmatochelys padillai, known from the specimens recovered from the Early CretaceousPaja Formation of Colombia[10] andSantanachelys gaffneyi, known from a specimen excavated fromBrazil in 1998. The latter 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.[4] As with most largefauna of theera, the Protostegidae died out during the events of theCretaceous–Paleogene extinction event that led to the extinction of thedinosaurs.[11]
The exactphylogenetic position of protostegids among turtles is uncertain. Some phylogenetic studies determine theleatherback turtles in the familyDermochelyidae to be their closest living relatives, with both these families beingmonophyletic.[4][12] Conversely, the phylogenetic analyses conducted by Joyce (2007) and Anquetin (2012), which included one protostegid species (Santanachelys gaffneyi), recovered the family as only distantly related to leatherback turtles. Joyce (2007) recovered Protostegidae as basaleucryptodiran turtles lying outside thecrown group ofCryptodira (the least inclusive clade containing all living cryptodirans) and closely related toSolnhofia parsonsi;[13]Santanachelys had a similar phylogenetic position in the analysis conducted by Anquetin (2012), who stressed that future studies should include more protostegids to confirm this phylogenetic placement.[14] If confirmed, these results would prove that protostegids weren't close relatives of leatherback turtles (or in fact any living cryptodirans), but instead "represent an independent lineage of marine turtles that originated in the Late Jurassic".[13] The analyses conducted by Sterli (2010) and Sterli & de la Fuente (2011) recoveredSantanachelys (and, presumably, the entirety of Protostegidae) as even more distantly related to living cryptodirans; it was found to be basal turtle lying outside the crown group of turtles (the least inclusive clade containing cryptodirans andpleurodirans).[15][16]
A phylogenetic analysis conducted by Cadena and Parham (2015) recovered Protostegidae within the crown group of Cryptodira; specifically the family was recovered as belonging to Chelonioidea and more closely related to theleatherback sea turtle thancheloniids are.[10]
In 1888, the Belgian zoologistGeorge Albert Boulenger published his classification of theTestudinata within the 9th edition of theEncyclopædia Britannica. The genusProtostega was placed within the familySphargidae under the suborderAthecae, and the family Protostegidae was named byEdward Drinker Cope in 1873.[3] A year or so later, the entire suborder was downgraded byKarl Alfred von Zittel into a family within theCryptodira.[17]
In 1994, Hirayama proposed a three-family subdivision of thesea turtle superfamily based oncladistic analysis; Protostegidae was given full, formal family status in the system, containing most of theextinct genera, includingArchelon, and a previously undescribed protostegid.[18] The unidentified specimen was fully described in 1998, as the speciesSantanachelys gaffneyi. The genusSantanachelys 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.[4]
