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Protostega
Protostega
Protostega
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Protostega
Temporal range: Late Cretaceous, 83.5 Ma
↓
Mounted skeleton
Scientific classification Edit this 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 an extinct genus of sea turtle containing a single species, Protostega gigas. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas (Hesperornis zone, dated to 83.5 million years ago[2]), time-equivalent beds of the Mooreville Chalk Formation of Alabama[3] and Campanian beds of the Rybushka Formation (Saratov Oblast, Russia).[4] Fossil specimens of this species were first collected in 1871, and named by Edward 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 giant Archelon,[6] and one of the three largest turtles of all time alongside Archelon and Gigantatypus.[7]

Protostega skeletal reconstruction in the Rocky Mountain Dinosaur Resource Center, Woodland Park, Colorado

Discovery and history

[edit]

The first known Protostega 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, when E. D. Cope found and collected the first identified specimen of Protostega 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]

Paleoenvironment

[edit]

The Late Cretaceous was marked by high temperatures, with large epicontinental seaways.[11] During the Mid-to-Late Cretaceous period the Western 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 of Protostega gigas have been found.[14][15]

Life restoration

Description

[edit]

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 upper Taylor 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's Protostega gigas discovery revealed that their shell had a reduction of ossification that helped these huge animals with streamlining in the water and weight reduction.[17] The carapace 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 a Dermochelys,[18] creating better movement of their limbs.

Body structure of fossilized Protostega

Edward Cope described Protostega 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 the Protostega genus.[8] The premaxillary beak was much shorter than that of Archelon.[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 the Cheloniidae 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 likely omnivores 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]

Classification

[edit]

The classification of Protostega 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 genus Dermochelys 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 that Protostega gigas was an intermediate form between Dermochelys and Cheloniidae.[19]

Paleobiology

[edit]

Examining the bone tissue microstructure (osteohistology) of Protostega revealed growth patterns similar to modern leatherback 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. If Protostega 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, comparing Protostega to its more basal relative Desmatochelys 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 large mosasaurs like Tylosaurus. 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]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Protostega

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from Grokipedia
Protostega is a genus of extinct marine turtles belonging to the family Protostegidae, known primarily from the species P. gigas, which inhabited the Western Interior Seaway during the Late Cretaceous period, approximately 86 to 83 million years ago. These turtles were among the largest marine reptiles of their time, reaching lengths of up to 3.4 meters and weights exceeding 2,000 pounds, making P. gigas the second-largest sea turtle ever known, surpassed only by its close relative Archelon. The genus was first described in 1871 based on specimens discovered in the Smoky Hill Chalk deposits of , with the type specimen collected by paleontologist and additional finds attributed to . Fossils indicate that Protostega possessed a broad, flattened up to 1.2 meters wide, elongated flippers for powerful swimming, and a large head adapted for an aquatic lifestyle, though lacking the hooked seen in . Recent histological studies reveal that Protostega exhibited rapid growth rates similar to modern leatherback turtles (Dermochelys coriacea), achieving 85% of adult humeral length by age 8 and potentially reaching within a , suggesting elevated metabolic rates suited to the warm, epicontinental seas of the . This adaptation likely contributed to its success in the diverse marine ecosystem of the , where it coexisted with mosasaurs, plesiosaurs, and early bony fishes before going extinct during the end- mass extinction.

Discovery and Naming

Initial Discovery

The first known specimen of Protostega, a fragmentary fossil cataloged as YPM 1408, was collected on July 4, 1871, during a Scientific Expedition to western , led by paleontologist . This discovery occurred on the south side of the , approximately eight miles east of Fort Wallace, in exposures of the Smoky Hill Chalk Member of the Niobrara Chalk Formation. The Smoky Hill Chalk represents marine deposits from the , a vast shallow sea that divided during the , facilitating rich preservation of marine vertebrates in fine-grained chalk sediments. These deposits played a pivotal role in early paleontological surveys of the American West, as expeditions like Marsh's targeted the Cretaceous outcrops for novel fossils amid the competitive "Bone Wars" era of discovery. The specific horizon yielding the Protostega specimen dates to the early stage, approximately 83.5 million years ago, within the Hesperornis biostratigraphic zone. Due to its fragmentary condition—consisting of isolated bones that were not immediately diagnostic—the specimen was not formally recognized or described at the time of collection, highlighting possibilities for initial misidentification amid the era's focus on more complete remains like mosasaurs. This oversight reflected the challenges of field in remote chalk badlands and the intense rivalry between and his contemporary , who later examined similar material. Cope formally named the genus Protostega in 1872 based on a more complete specimen he collected later that year from the same formation.

Type Specimen and Subsequent Finds

The genus Protostega was formally named by Edward Drinker Cope in 1872, based on a partial skeleton (AMNH FR 1503) collected from the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas, serving as the type specimen for P. gigas. This material, consisting primarily of plastron elements and limb bones, was described in Cope's publication in the Proceedings of the Academy of Natural Sciences of Philadelphia. Subsequent discoveries expanded knowledge of Protostega gigas beyond the initial 1871 collection. In Kansas, paleontologist Charles H. Sternberg recovered multiple partial skeletons from the Smoky Hill Chalk during the late 19th and early 20th centuries, including well-preserved carapaces and limbs now housed in institutions such as the Carnegie Museum of Natural History (e.g., CMNH 1420 and 1421). These specimens contributed detailed insights into the turtle's skeletal variation and abundance in the Western Interior Seaway. Additional finds from Alabama include isolated bones and partial remains from the late Santonian to early Campanian Mooreville Chalk Formation (Selma Group) in the Black Belt region, indicating a southeastern extension of the species' range. Further broadening the geographic distribution, isolated bones attributable to Protostega gigas were reported from the Rybushka Formation at the Beloe locality in Saratov Province, , along the Upper Volga River, marking the first confirmed records outside and suggesting transatlantic dispersal capabilities. Among these post-type discoveries, the largest known specimen of P. gigas, with an estimated total length of 4.2 meters, was collected from the Upper Taylor Group near , , in the late and is now in the Perot Museum of Nature and Science. This incomplete but substantial , lacking the head and some , has been pivotal in establishing upper limits for body size estimates in the genus, highlighting its position as one of the largest marine turtles.

Taxonomy and Classification

Etymology and Species

The genus name Protostega was coined by in 1871, combining "prōtos" (first) and "stegos" (roof or covering), in reference to the primitive, foundational structure of its relative to later marine turtles. The species epithet "gigas" derives from the Latin word for "giant," highlighting the exceptional size of the type specimen, which measured over 3 meters in length. Protostega is currently regarded as monospecific, with P. gigas Cope, 1871 as the sole valid ; this consensus stems from comprehensive revisions that have synonymized all other proposed taxa under it. Historically, Oliver P. Hay proposed two additional in 1908—P. potens and P. advena—based on partial skeletons from the in , distinguishing them primarily by plastron morphology and proportions. Rainer Zangerl further expanded the genus in 1953, recognizing P. potens and introducing P. dixie and P. latus as distinct from the Selma Formation in and related deposits, citing variations in shape, size, and proportions. Post-2000 studies have invalidated these taxa, treating P. potens, P. advena, P. dixie, and P. latus as junior synonyms or ontogenetic/individual variants of P. gigas, often due to overlapping diagnostic features and insufficient differentiation in fragmentary material. This monospecific status was formalized by Hooks (1998) in a of and reaffirmed in subsequent analyses, such as those incorporating European material.

Phylogenetic Relationships

Protostega is classified within the family Protostegidae, part of the superfamily Dermochelyoidea, and occupies an intermediate evolutionary position between the modern leatherback turtle (Dermochelys coriacea) and the hard-shelled sea turtles of the family . This placement reflects its role as a stem-group representative of Chelonioidea, basal to the crown groups and , highlighting the diversification of marine turtles during the . Phylogenetic analyses consistently recover Protostegidae as monophyletic, with Protostega gigas as the sole valid species within its genus, positioned outside the crown-group Chelonioidea. Key synapomorphies supporting the phylogenetic position of Protostega include reduced dermal in the and plastron, an elongated temporal region in the , and highly modified paddle-like limbs adapted for marine propulsion. These features align Protostegidae closely with in their loss of rigid shell plating but distinguish them from the more ossified , suggesting convergent adaptations for open-ocean lifestyles. Such traits underscore Protostega's transitional morphology in the evolution of chelonioid sea turtles. Cladistic analyses, such as those by Sterli et al. (2018), positioned Protostega as sister to Archelon within Protostegidae, based on expanded character matrices including cranial and postcranial features. More recent studies, including those incorporating osteohistological data, support this topology, with Protostega and Archelon forming a derived clade in Protostegidae and emphasizing stem Chelonioidea affinities. These updates reinforce Protostegidae's exclusion from crown Chelonioidea, aligning fossil evidence with the K–T boundary radiation of modern sea turtle lineages.

Anatomy and Description

Size and General Morphology

Protostega gigas attained a substantial size for a marine turtle, with typical total body lengths of 2.7 to 3.4 meters based on complete and partial skeletons from the Smoky Hill Member. The largest known specimen (DMNH 1999) reached 3.4 meters, derived from scaling of limb bones such as the and in the largest known individuals. Weight estimates, calculated through skeletal scaling and allometric regressions comparable to those used for modern sea turtles, fall between 400 and 900 kilograms, reflecting its robust yet buoyant build suited to oceanic environments. The overall body plan of Protostega was highly streamlined, featuring a hydrodynamically efficient form with a dorsoventrally flattened profile that minimized drag during . Its and plastron exhibited reduced compared to more rigid-shelled , allowing greater flexibility and aiding in control through adjustable body volume in water. This leathery, less mineralized shell structure parallels adaptations seen in the modern leatherback turtle (Dermochelys coriacea), though Protostega's was proportionally larger. Limb proportions further emphasized its marine specialization, with elongated foreflippers that, when spread, spanned up to 4.7 meters in the largest specimens, providing powerful via paddling motions. These flippers, supported by elongated humeri and radii, were broader and more paddle-like than those of terrestrial , enhancing maneuverability and speed in open water; individual foreflippers were approximately 1.5-1.6 times the length of the hind flippers. In comparison to extant sea , Protostega was significantly larger than most —such as the green turtle (Chelonia mydas), which reaches only about 1.5 meters—but smaller than its contemporary relative ischyros, which could reach up to 4.6 meters in total length.

Skeletal Features

The of Protostega was characterized by a leathery texture, lacking the rigid seen in many other , and consisted of thin, elongated neural plates numbering 10 to 12 along the midline, which were often crested and covered the only partially. These neural plates were paper-thin and prone to crushing in fossils, with the remaining free for much of their distal length, contributing to a flexible dorsal . Peripheral plates were reduced in number and size, typically around 22 but variably developed, forming an incomplete bony margin that emphasized the overall lightweight construction of the shell. The plastron consisted of 9 plates, comprising paired epiplastrons, hyoplastrons, hypoplastrons, and xiphiplastrons flanking a single entoplastron, with intricate digitations along the borders of the hyo- and hypoplastrons that enhanced flexibility compared to the more fused plastrons of hard-shelled taxa. This arrangement allowed for greater ventral mobility, as the plates were not fully co-ossified, and radial striations on elements like the hypoplastron indicate a structure suited to an active aquatic lifestyle. The exhibited an elongated rostrum formed by distinct premaxillaries, maxillaries, and prefrontals, measuring approximately 58 cm in length when complete, with a narial opening about 7.5 cm long and 5.5 cm wide. Large temporal openings were defined by a prominent parieto-squamosal arch, providing space for robust adductor musculature, while the overall cranial profile was narrow and elevated. The vertebral column included 8 cervical vertebrae, 10 dorsal vertebrae with centra lengths increasing from 5.5 cm anteriorly to 9 cm mid-series, and an extended caudal series with chevron facets on the posterior centra facilitating articulation with hemal elements for tail-based propulsion in water. These facets, along with the short, wide cervical bodies, supported a streamlined axial skeleton. The appendicular skeleton was adapted into broad paddles, with forelimbs spanning up to 250 cm when extended and hind limbs 190 cm, featuring hyperphalangy in the manus and pes where digits contained extra phalanges beyond the typical reptilian formula (e.g., front: 2-3-3-3-3, totaling 14 elements). The longest phalanges in the third manual digit reached 43 cm, enhancing paddle surface area akin to that in extant leatherback turtles.

Paleobiology

Growth and Physiology

Histological analysis of Protostega long s, including humeri and femora, reveals a fibrolamellar microstructure characterized by high vascularization and spongiose tissue, indicative of rapid skeletal growth rates similar to those observed in modern leatherback sea turtles (Dermochelys coriacea). Lines of arrested growth (LAGs), interpreted as cyclical growth marks, are present in specimens, with counts up to eight LAGs suggesting individuals reached ages of approximately 8–9 years at death, during which humeral length doubled from about 18 cm to 35 cm. This pattern of sustained, variable but fast deposition implies that Protostega potentially attained sexual and skeletal maturity within 10 years, enabling early reproduction in a high-energy marine lifestyle. The absence of an external fundamental system (EFS) in the analyzed bones further supports that these individuals had not yet ceased growth, consistent with ongoing rapid ontogeny into adulthood. Bone microstructure also points to an elevated metabolism in Protostega, with the highly vascularized tissue suggesting endothermic-like traits and gigantothermy, where large body size helped retain heat. Such physiological strategies mirror those of leatherback turtles, which exhibit regional endothermy.

Diet and Behavior

Protostega gigas possessed a toothless but robust adapted for crushing, suggesting a primarily durophagous diet focused on hard-shelled mollusks such as inoceramid bivalves, with possible inclusion of softer prey like , crustaceans, and based on anatomical comparisons to related protostegids. Gut contents from a closely related Early Cretaceous protostegid (cf. Notochelone) containing fragments of inoceramid shells encased in phosphatic material confirm that such bivalves formed a regular part of the diet in the family, likely ingested after being bitten into segments along with associated soft tissues. This feeding strategy aligns with the turtle's powerful jaws, enabling it to exploit benthic and pelagic resources in the . Locomotion in Protostega was dominated by its enlarged foreflippers, which provided strong propulsive force for sustained open-ocean swimming, while the lightweight, flexible —lacking extensive —enhanced hydrodynamic efficiency and reduced drag. The species likely undertook long-distance migrations across marine environments, similar to modern sea turtles, with the ability to haul itself onto coastal margins for nesting behaviors inferred from and trackway analogs in related taxa. No exists for coordinated group movements, supporting an inferred solitary punctuated by periodic nesting events. Fossil evidence reveals significant predation risks for Protostega, with multiple specimens exhibiting bite marks and embedded teeth from the large shark , indicating lethal or scavenging interactions that often targeted the limbs and shell. These injuries show no signs of healing in some cases, suggesting fatal attacks on live individuals, while possible encounters with mosasaurs are hypothesized from in the fossil record but lack specific bite trace confirmation for this . The absence of gregarious fossils or trace evidence further implies that Protostega did not rely on social grouping for defense, relying instead on its size and speed for evasion.

Distribution and Paleoecology

Geographic Distribution

Protostega gigas fossils are primarily known from the of , with abundant remains recovered from the Smoky Hill Chalk Member of the in western , . The type specimen, described by in 1871, originates from Gove County, , within this formation. Additional material referred to Protostega sp. has been found in the Pierre Shale Formation, including the Pembina Member in southern , , and equivalents in and , , with protostegid remains reported from the latter. Secondary occurrences are documented in eastern , notably the Mooreville Chalk Formation (part of the Selma Group) in , USA, where isolated bones and shark-bitten elements, including specimens with embedded teeth from the shark Cretoxyrhina mantelli, have been collected. In , the species is recorded from the Rybushka Formation at the Beloe Ozero locality in Saratov Province, , marking the first confirmed finds outside based on isolated cranial and postcranial elements. The temporal range of Protostega gigas is late Santonian to early stages of the , spanning approximately 86 to 80 million years ago, with the majority of specimens, including those from the Smoky Hill Chalk, dating to around 83.5 million years ago. This distribution pattern, extending from the across the Atlantic to the Tethyan margins in , indicates trans-Atlantic dispersal facilitated by open marine connections via the Tethys Sea, supporting a cosmopolitan paleobiogeography for the species during the Santonian–Campanian.

Habitat and Associated Fauna

Protostega inhabited the shallow to mid-depth waters of the , an epicontinental sea that spanned central during the (late Santonian–early stages, approximately 86–80 million years ago). This environment featured depths up to ~200–500 meters overall, with the deposits indicating water depths of around 50–150 meters in the central basin. Surface water temperatures averaged 30 ± 2.7 °C, reflecting a warm, influenced by high global sea levels and conditions, while bottom waters could be cooler and occasionally dysoxic due to oxygen fluctuations driven by and stratification. The seaway's high productivity supported diverse , with nutrient influx from surrounding landmasses promoting blooms of and that formed the base of a complex . Similar paleoenvironmental conditions extended to Tethyan marine settings, where comparable protostegid turtles thrived in warm, shallow epicontinental seas; for instance, Protostega gigas specimens from the Rybushka Formation in Saratov Province, , suggest analogous with tropical waters and high faunal diversity. In the , coexisted with a rich assemblage of marine vertebrates, including large predatory mosasaurs such as Tylosaurus proriger, short-necked plesiosaurs like Polycotylus, and giant teleost fish including Xiphactinus audax and various sharks. Smaller cheloniids, such as Toxochelys species, also shared this , occupying similar neritic zones. As a mid-level in this high-productivity ecosystem, Protostega likely foraged on abundant soft-bodied and schooling , while serving as potential prey for apex predators like mosasaurs, evidenced by shark-bitten remains from and its rapid growth strategy to reach sizes up to 3.4 meters and reduce vulnerability. Oxygen fluctuations, particularly during periods of anoxia in deeper basin areas, influenced preservation and distribution, with many Protostega fossils occurring in oxygen-poor deposits of the Smoky Hill Chalk Member () that reflect episodic dysoxia. This dynamic highlights the seaway as a vibrant, fluctuating marine realm supporting a balanced trophic amid global oceanic changes.

References

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