Archelon

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Not to be confused with ARCHELON, the Sea Turtle Protection Society of Greece.

Archelon
Temporal range:
Ma
[1][2][3][4]
A suspended display
Mounted cast at the University of Manitoba
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Cryptodira
Family: †Protostegidae
Genus: †Archelon
Wieland, 1896
Species:
†A. ischyros
Binomial name
†Archelon ischyros
Wieland, 1896

Archelon is an extinct marine turtle from the Late Cretaceous, and is the largest turtle ever to have been documented, with the biggest specimen measuring 4.6 m (15 ft) from head to tail and 2.2–3.2 t (2.4–3.5 short tons) in body mass. It is known only from the Pierre Shale and has one species, A. ischyros. In the past, the genus also contained A. marshii and A. copei, though these have been reassigned to Protostega and Kansastega, respectively. The genus was named in 1895 by American paleontologist George Reber Wieland based on a skeleton from South Dakota, who placed it into the extinct family Protostegidae. The leatherback sea turtle (Dermochelys coriacea) was once thought to be its closest living relative, but now, Protostegidae is thought to be a completely separate lineage from any living sea turtle.

Archelon had a leathery

hesperornithiform seabirds, and mosasaurs
. It may have gone extinct due to the shrinking of the seaway, increased infant mortality rates (in the sea), higher instances of egg and hatchling predation (on land), and a rapidly cooling climate.

Taxonomy

Research history

Yale Peabody Museum

The

Late Campanian-age Pierre Shale of South Dakota along the Cheyenne River in Custer County by American paleontologist George Reber Wieland in 1895, and described by him the following year based on a mostly complete skeleton excluding the skull. He named it Archelon ischyros,[5] genus name from the Ancient Greek ἀρχη- (arkhe-) 'first/early',[6] χελώνη (chelone) 'turtle',[7] and species name from ἰσχυρός (ischyros) 'mighty' or 'powerful'.[8] Wieland placed it into the family Protostegidae, which included at the time the smaller Protostega and Protosphargis,[5] the latter being nowadays classified in a position closer to the family Cheloniidae.[9] A second specimen, a skull, was discovered in 1897 in the same region.[10]

Pencil drawing of the left-side view on the left and the top-side view on the right, with some fish in the background
1914 restoration by American paleontologist Samuel Wendell Williston

In 1900, Wieland described a second species, A. marshii, from remains collected in 1898 by American paleontologist

Natural History Museum Vienna.[15] In 2002, a fifth specimen, a partial skeleton, was discovered from the Pierre Shale of North Dakota along the Sheyenne River near Cooperstown.[3]

Evolution

A leatherback sea turtle leaving a beach, possibly after laying eggs.
The leatherback sea turtle was previously thought to be the closest living relative.

The

Chelonioidea (which includes all sea turtles). In this model, Archelon does not share a marine ancestor with any sea turtle.[16][17][18]

Protostegidae
Protostegidae
Archelon with some protostegids[19]
Protostega, namesake of Protostegidae

Description

The holotype measures 352 cm (11.5 ft) from head to tail, with the head measuring 60 cm (2 ft), the

tail 70 cm (2.3 ft).[5] The largest specimen, Brigitta, measures around 4.6 m (15 ft) from head to tail and 4.0 m (13 ft) from flipper to flipper,[3][15] and, in life, weighed around 2.2–3.2 t (2.4–3.5 short tons).[20][21] Skulls of Archelon measured at up to 100 cm (3.3 ft).[22]

A view of the right side of the skull, the beak is noticeably hooked
Archelon had a pronounced beak.

Archelon had a distinctly elongated and narrow head. It had a defined hooked beak which was probably covered in a sheath in life, reminiscent of the beaks of

birds of prey. However, in the back, the cutting edge of the beak is dull compared to such animals. Much of the length of the head derives from the elongated premaxillae–which is the front part of the beak in this animal–and maxillae. The jugal bones, the cheek bones, due to the elongate head, do not project as far as they do in other turtles. The nostrils are elongated and rest on the top of the skull, slightly posited forward, and are unusually horizontal compared to sea turtles. The jugal bones (cheekbones) are rounded as opposed to triangular in sea turtles. The articular bone, which formed the jaw joint, was probably heavily encased in cartilage. The jaw probably moved in a hammering motion.[10]

Foreflipper of Archelon

Five neck vertebrae were recovered from the holotype, and it probably had eight in total in life; they are X-shaped, procoelous–concave on the side towards the head and convex on the other–and their thick frame indicates strong neck muscles. Ten thoracic vertebrae were found, increasing in size until the sixth then rapidly decreasing, and they have little connection with the carapace. The three vertebrae of the sacrum are short and flat. It probably had eighteen tail vertebrae; the first eight to ten (probably in the same area as the carapace) had neural arches, whereas the remaining did not.[5] Its tail likely had a wide range of mobility, and the tail is thought to have been able to bend at nearly a 90° angle horizontally.[23]

The humeri in the upper arms are proportionally massive, and the radii and ulnae of the forearms are short and compact, indicating the animal had strong flippers in life. The flippers would have had a spread of between 490 and 610 cm (16 and 20 ft), though most likely the more conservative estimate.[24] Stretch marks on the limb bones indicate fast growth,[25] with similarities to the leatherback sea turtle, the fastest growing turtle known,[26] whose juveniles have an average growth rate of 8.5 cm (3.3 in) per year.[25]

Carapace

A bluish-gray turtle with some yellow-green spots on the neck and striations along the back edge of the flippers
Restoration

The carapace comprises on either side eight neuralia–the plates closest to the midline–and nine pleuralia–the plates that connect the midline to the ribs. The plates of the carapace are mostly uniform in dimensions, with the exception of the two pairs of plates corresponding to the eighth thoracic vertebra which are smaller than the others, and the pygal plate closest to the tail which is larger. Archelon has ten pairs of ribs, and, like the leatherback sea turtle but unlike other sea turtles, the first rib does not meet the first pleural. As in sea turtles, the first rib is noticeably shorter than the second, in this case, three quarters of the length. The second to fifth ribs project at a right angle from the midline, and, in the holotype, each measure 100 cm (3.3 ft) in length. A rib increases in thickness in the vertical direction

distally, as it gets farther from the midline, and the ribs are relatively larger and more well-developed than those of sea turtles. The second to fifth ribs, in the holotype, originate with a thickness of 2.5 cm (0.98 in) and terminate with around 4 to 5 cm (1.6 to 2.0 in) in thickness.[24][11]

The neuralia and pleuralia form highly irregular and finger-like sutures where they meet, and one plate may have lain over the other plate while the bone was still developing and malleable. The neuralia and pleuralia–the bony portions of the carapace–are particularly thin, and the ribs, especially the first rib, and the shoulder girdle are unusually heavy and may have had to carry extra stress to compensate, a condition seen in ancient ancestral turtles.[24][11] Archelon has osteosclerotic structures, where the bone is dense and heavy, which probably served as ballasts in life similar to the limb bones of whales and other open-ocean animals.[27]

The carapace, in life, probably featured a row of ridges along the midline over the

ichthyosaurs, and was probably also an adaptation to reduce overall weight.[25]

Plastron

North American Museum of Ancient Life

A turtle plastron, its underside, comprises (from head-most to tail-most) the epiplastron, the entoplastron, which is small and wedged in between the former and the hyoplastron; then is the hypoplastron and finally, the xiphiplastron. The plastron, as a whole, is thick,[23] and measures (in a specimen described in 1898) 210 cm (7 ft) in length.[11] Unlike the carapace, it features striations throughout.[28]

In protostegids, the epiplastron and entoplastron are fused together, forming a single unit called an "entepiplastron" or a "paraplastron." This entepiplastron is T-shaped, as opposed to the Y-shaped entoplastrons in other turtles. The top edge of the T rounds off, except at the center which features a small projection. The outward side is slightly convex and bends somewhat, away from the body. The two ends of the T flatten out, getting broader and thinner, as they get farther from the center.[28]

A thick, continuous ridge connects the hyoplastron, hypoplastron, and xiphiplastron. The hyoplastron features a large number of spines projecting around the circumference. The hyoplastron is slightly elliptical, and grows thinner as it gets farther from the center, before the spines erupt. The spines grow thick and narrow towards their middle portion. The seven to nine spines projecting towards the head are short and triangular. The six middle spines are long and thin. The last 19 spines are flat. There are no marks indicating contact with the entepiplastron. The hypoplastron is similar to the hyoplastron, except it has more spines, a total of 54.[28] The xiphiplastron is boomerang-shaped, a primitive characteristic in contrast to the straight ones seen in more modern turtles.[23]

Paleobiology

A shiny ammonite shell is a piece of shiny rock
Eutrephoceras dekayi may have been a food source.[23] Above from the Natural History Museum Nuremberg

Archelon was an

bivalves–some exceeding 120 cm (4 ft) in diameter–would have easily been able to sustain Archelon.[23] However, these were probably absent in the central Western Interior Seaway by the Early Campanian. Conversely, the beak may have been adapted for shearing flesh.[29] It might have been able to target larger fish and reptiles,[10] as well as, similar to the leatherback sea turtle, soft-bodied creatures such as squid and jellyfish.[3][20] However, it is possible the sharp beak was used only in combat against other Archelon. The nautilus Eutrephoceras dekayi was found in great number near an Archelon specimen, and may have been a potential food source.[23] Archelon may have also occasionally scavenged off the surface water.[29]

Natural History Museum Vienna[15]

Archelon probably had weaker arms, and thus less swimming power, than the leatherback sea turtle, and so did not frequent the open ocean as much, preferring shallower, calmer waters. This is indicated by the similarity of the humerus/arm and hand/arm ratios of it and cheloniids, which are known to have poor development of the limbs into flippers and a preference for shallow water.[30] Conversely, the large flipper-to-carapace ratio of protostegids and the similarly large flipper spread, like that of the predatory cheloniid loggerhead sea turtle (Caretta caretta), combined with a broad body, indicate they could have pursued active prey, though they probably could not have sustained high speeds.[31] Overall, it may have been a moderately-good swimmer, capable of open-ocean travel.[23]

Archelon, like other marine turtles, probably would have had to have come onshore to nest; like other turtles, it probably dug out a hollow in the sand, laid several dozens of eggs, and took no part in child rearing. The right lower flipper of the holotype is missing, and stunted growth of remaining flipper bone indicates this occurred early in life. It may have been the result of attempted predation by a bird while a hatchling and trying to escape to the sea, bitten off by some large predator such as a mosasaur or a Xiphactinus, or was crushed off by larger adults while herding on the shore. However, the latter is unlikely as juveniles probably did not frequent the coasts even during breeding season.[23] Brigitta is estimated to have lived to 100 years, and may have died while partially covered in mud brumating–a state of dormancy–on the ocean floor.[15][20] However, the longstanding belief that marine turtles brumate underwater like freshwater turtles may be incorrect given the high surfacing-frequency needed to prevent drowning.[32]

Paleoecology

A green turtle with white spots, underwater, swimming downwards
Restoration in environment

Archelon inhabited the shallow

ammonites–from the Pierre Shale Placenticeras placenta, Scaphites nodosus, Didymoceras, and Baculites ovatus–bivalves–such as the giant Inoceramus–[36] the squid-like belemnites,[33] and nautilus.[23]

As the seaway progressively migrated southward, it is possible Archelon was unable to migrate with it. The increasing threat of egg or hatchling predation by new marine or mammalian species may have led to the extinction of Archelon, and the disappearance of gigantic protostegids seems to have coincided with the increasing size of dermochelyids.

thermoregulatory capabilities.[38] Average water temperature may have decreased to 7 or 12 °C (45 or 54 °F) depending on estimated CO2 levels.[34] However, some Maastrichtian-age Kansas Pierre Shale fossils may have been eroded millions of years ago, and it is possible Archelon survived well into the Maastrichtian.[29]

See also

References

  1. S2CID 127523816
  2. doi:10.1139/e82-024
    .
  3. ^ a b c d e f Hoganson, J. W.; Woodward, B. (2004). "Skeleton of the Rare Giant Sea Turtle, Archelon, Recovered from the Cretaceous DeGrey Member of the Pierre Shale near Cooperstown, Griggs County, North Dakota" (PDF). North Dakota Geological Society Newsletter. 32 (1): 1–4. Archived from the original (PDF) on October 18, 2020. Retrieved December 23, 2018.
  4. ISBN 9780813724270
    .
  5. ^
    doi:10.2475/ajs.s4-2.12.399
    .
  6. ^ Liddell & Scott 1980, pp. 106–107.
  7. ^ Liddell & Scott 1980, p. 781.
  8. ^ Liddell & Scott 1980, p. 336.
  9. ^ Hirayama, R. (2005). "Systematic position of Protosphargis veronensis Capellini, an enigmatic sea turtle from the Late Cretaceous of Italy". Journal of Vertebrate Paleontology. 25 (3): 70A.
  10. ^
    doi:10.2475/ajs.s4-9.52.237
    .
  11. ^
    doi:10.2475/ajs.s4-27.158.101
    .
  12. ^ a b Zangerl, R. (1953). The Vertebrate Fauna of the Selma Formation of Alabama. Part III: The Turtles of the Family Protostegidae. Fieldiana: Geology Memoirs. Vol. 3. Chicago Natural History Museum. pp. 78–79, 128–130.
  13. ^
    doi:10.1080/02724634.1998.10011036
    .
  14. S2CID 261927100
    .
  15. ^ a b c d Derstler, K.; Leitch, A. D.; Larson, P. L.; Finsley, C.; Hill, L. (1993). "The World's Largest Turtles - The Vienna Archelon (4.6 m) and the Dallas Protostega (4.2 m), Upper Cretaceous of South Dakota and Texas". Journal of Vertebrate Paleontology. 13 (3): 33A.
  16. S2CID 85998318
    .
  17. S2CID 85295987
    .
  18. doi:10.5070/P9351037558
    .
  19. PMID 29666758
    .
  20. ^ a b c "The Archelon". Black Hills Institute of Geological Research, Inc. Archived from the original on March 12, 2016. Retrieved December 23, 2018.
  21. ISBN 9780691193809
    .
  22. ISBN 978-0-8493-8422-6
    .
  23. ^
    ISBN 978-1-104-52662-7
    .
  24. ^
    doi:10.2475/ajs.s4-15.87.211
    .
  25. ^ a b c Scheyer, T. M.; Sánchez-Villagra, M. R. (2007). "Carapace Bone Histology in the Giant Pleurodiran Turtle Stupendemys geographicus: Phylogeny and Function". Acta Palaeontologica Polonica. 52 (2): 137–154.
  26. JSTOR 1444768
    .
  27. doi:10.1111/bij.12265
    .
  28. ^
    doi:10.2475/ajs.s4-5.25.15
    .
  29. ^
    ISBN 978-0-253-02715-3
    .
  30. doi:10.7287/peerj.preprints.1039v1
    .
  31. S2CID 134854045
    .
  32. PMID 17148134
    .
  33. ^ a b c "Western Interior Seaway". The Virtual Fossil Museum. Retrieved December 25, 2018.
  34. ^
    doi:10.1130/G38311.1
    .
  35. doi:10.1016/0031-0182(90)90110-S
    .
  36. ^ Brinster, K. F. (1970). Molluscan Paleontology of the Pierre Shale (Upper Cretaceous), Bowman County, North Dakota (MS). University of North Dakota.
  37. ^ Weems, R. E. (1988). "Paleocene turtles from the Aquia and Brightseat Formations, with a Discussion of their Bearing on Sea Turtle Evolution and Phylogeny". Proceedings of the Biological Society of Washington. 101 (1): 144.
  38. PMID 29668704
    .

Cited text

Further reading

Wikispecies has information related to Archelon.
Wikimedia Commons has media related to Archelon.
  • Hay, O. P. 1908. The fossil turtles of North America. Carnegie Institution of Washington, Publication No. 75, 568 pp, 113 pl.
  • Wieland, G. R. 1902. Notes on the Cretaceous turtles, Toxochelys and Archelon, with a classification of the marine Testudinata. American Journal of Science, Series 4, 14:95-108, 2 text-figs.

External links

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