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| Protostega | |
|---|---|
 | |
| Mounted skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Testudines |
| Suborder: | Cryptodira |
| Family: | †Protostegidae |
| Genus: | †Protostega Cope, 1872 |
| Type species | |
| †Protostega gigas Cope, 1872 | |
Protostega ('first roof')[1] is anextinctgenus ofsea turtle containing a single species,Protostega gigas. Its fossil remains have been found in theSmoky Hill Chalk formation of westernKansas (Hesperornis zone, dated to 83.5 million years ago[2]), time-equivalent beds of theMooreville Chalk Formation ofAlabama[3] andCampanian beds of theRybushka Formation (Saratov Oblast,Russia).[4] Fossil specimens of this species were first collected in 1871, and named byEdward Drinker Cope in 1872.[5] With a total length of 3.9 metres (13 ft), it is the second-largest sea turtle that ever lived, second only to the giantArchelon,[6] and one of the three largest turtles of all time alongsideArchelon andGigantatypus.[7]
The first knownProtostega specimen (YPM 1408) was collected on July 4 by the 1871 Yale College Scientific Expedition, close to Fort Wallace and about 5 months before Cope arrived in Kansas. However, the fossil that they found was never described or named.[8] It was not named until 1872, whenE. D. Cope found and collected the first identified specimen ofProtostega gigas in the Kansas chalk in 1871. A variety of bones were found in yellow Cretaceous chalk from a bluff near Butte Creek.[9][10]
TheLate Cretaceous was marked by high temperatures, with large epicontinental seaways.[11] During the Mid-to-Late Cretaceous period theWestern Interior Seaway covered the majority of North America and would connect to the Boreal and Tethyan oceans at times.[12][13] Within these regions are where the fossil ofProtostega gigas have been found.[14][15]
Protostega is known to have reached up to 3–3.9 m (9.8–12.8 ft) in length.[2][6][4] A specimen from the upperTaylor Marl is even larger, at 2 m (6.6 ft) in carapace length and 4.2 m (14 ft) in total length.[16][6] Despite lacking its head and three limbs, it is well-preserved.[16] Cope'sProtostega gigas discovery revealed that their shell had a reduction ofossification that helped these huge animals with streamlining in the water and weight reduction.[17] Thecarapace was greatly reduced and the disk only extending less than halfway towards the distal ends of the ribs. Cope described other greatly modified bones in his specimen including an extremely long coracoid process that reached all the way to the pelvis and a humerus that resembled aDermochelys,[18] creating better movement of their limbs.
Edward Cope describedProtostega gigas as having a large jugal that reached to the quadrate along with a thickened pterygoid that reached to the mandibular articulating surface of the quadrate.[1] The fossil featured a reduction in the posterior portion of the vomer where the palatines meet medially.[1] Another fossilized specimen showed that a bony extension, that would have been viewed as a beak, was lacking in theProtostega genus.[8] The premaxillary beak was much shorter than that ofArchelon.[18] In front of the orbital region the skull was elongated with a broadly-roofed temporal region. The jaws of the fossil showed a large crushing surface.[18] The quadrato-jugal was triangular with a posterior edge that was concave, with the entire bone being convex from distal view. The squamosal appeared to have a concave formation on the surface at the upper end of the quadrate. In Cope's fossil the mandible was preserved almost perfectly and from this he recorded that the jaw was very similar to theCheloniidae and the dentary had a broad for above downward with a concave surface, marked by deep pits in the dentary.[19]Cope concluded that these animals were most likelyomnivores and consumed a diet of hard shelled crustacean creatures, due to the long symphysis of its lower jaw.[18]Protostega also likely fed on seaweed and jellyfish or scavenged on floating carcasses as well, like modern turtles.[6]
The classification ofProtostega was complicated at best. The specimen that Cope discovered in Kansas was hard to evaluate with the preservation condition. The fossil shared many characteristics with the genusDermochelys and the family Cheloniidae. Cope wrote about the characteristics that distinctly separated this particular species from the two controversial groups. The differences he described were that the fossil had a reduced or lacking amount of dermal ossification on the back, the articulation of the pterygoid and quadrates, the presence of a presplenial bone in the jaw, a lack of an articular process on the back side of the nuchal, simple formation of the radial process on the humerus, and a peculiar bent formation of the xiphiplastra. He concluded thatProtostega gigas was an intermediate form betweenDermochelys and Cheloniidae.[19]
Examining the bone tissue microstructure (osteohistology) ofProtostega revealed growth patterns similar to modernleatherback sea turtles with rapid growth to large body size. Leatherbacks lack a typical reptile metabolism, instead having high resting metabolic rates and the ability to hold a body temperature higher than their surroundings. IfProtostega had similar bone growth patterns to leatherbacks, it is hypothesized that they both had a similar metabolism. This rapid growth to adult body size in sea turtles would also indicate rapid growth to reproductive maturity, which would have been a great advantage in their survival. However, comparingProtostega to its more basal relativeDesmatochelys shows that not all protostegids had the same growth patterns. This indicates that rapid growth to large size evolved late within the lineage, perhaps in response to the evolution of largemosasaurs likeTylosaurus. Given uncertainties in the phylogenetic placement of protostegids relative to living sea turtles, it is unclear if the evolution of rapid growth rates and possible elevated metabolism were convergent with modern leatherbacks or if the two were more closely related.[20]
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