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Basilosaurus

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Basilosaurus
Temporal range: Late Eocene 40–33.9 Ma [1]
B. cetoides, National Museum of Natural History, Washington DC
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Infraorder: Cetacea
Family: †Basilosauridae
Subfamily: †Basilosaurinae
Genus: †Basilosaurus
Harlan 1834
Species
Synonyms

Basilosaurus ("king lizard") is a genus of prehistoric cetacean that existed during the Late Eocene, 40 to 33.9 million years ago (mya). The first fossil of B. cetoides was discovered in the United States along the gulf coast[4][5][6][7][8] and Eastern United States[9], in the 1830’s and was initially believed to be some sort of reptile, hence the suffix -saurus[10], but it was later found to be a marine mammal. Fossils of B. isis have been found in Egypt, Jordan, and Morocco.[11][12][13]

Description

Basilosaurus skeleton compared to a Tylosaurus skeleton (at right) at Exhibition in the National Museum of Nature and Science, Tokyo, Japan
Size compared to a human
Restoration of a group

Measuring 18–20 m (59–66 ft),[14][15] Basilosaurus cetoides is one of the largest-known animals to exist from K–Pg extinction event 66 million years ago (mya) to around 15 million years ago when modern cetaceans began to reach enormous sizes.[16] At 15–18 m (49–59 ft),[17] B. isis is slightly smaller than B. cetoides.[15]

Cranium

Skull from the Field Museum of Natural History
Skeletal restoration by Gidley 1913

The dental formula for B. isis is 3.1.4.23.1.4.3. The upper and lower molars and second to fourth premolars are double-rooted and high-crowned.[18]

The head of Basilosaurus did not have room for a melon like modern toothed whales, and the brain was smaller in comparison, as well. They are not believed to have had the social capabilities of modern whales.

Fahlke et al. 2011 concluded that the skull of Basilosaurus is asymmetrical like in modern toothed whales, and not, as previously assumed, symmetrical like in baleen whales and artiodactyls closely related to cetaceans. In modern toothed whales, this asymmetry is associated with high-frequency sound production and echolocation, neither of which is thought to be present in Basilosaurus. This cranial torsion probably evolved in protocetids and basilosaurids together with directional underwater hearing and the sound-receiving apparatus in the mandible (the auditory fat pad and the pan bone (thin portion of mandible)).[19]

In the basilosaur skull, the inner and middle ear are enclosed by a dense tympanic bulla.[20] The synapomorphic cetacean air sinus system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses.[21] The periotic bone, which surrounds the inner ear, is partially isolated. The mandibular canal is large and laterally flanked by a thin bony wall, the pan bone or acoustic fenestra. These features enabled basilosaurs to hear directionally in water.[20]

The ear of basilosaurids is more derived than those in earlier archaeocetes, such as remingtonocetids and protocetids, in the acoustic isolation provided by the air-filled sinuses inserted between the ear and the skull. The basilosaurid ear did, however, have a large external auditory meatus, strongly reduced in modern cetaceans, but though this was probably functional, it can have been of little use under water.[22]

Hind limbs

B. isis hind limb

A 16-meter (52 ft) individual of B. isis had 35-centimeter-long (14 in) hind limbs with fused tarsals and only three digits. The limited size of the limb and the absence of an articulation with the sacral vertebrae, makes a locomotory function unlikely.[23] Analysis has shown that the reduced limbs could rapidly adduct between only two positions.[24]

Spine and movement

A complete Basilosaurus skeleton was found in 2015, and several attempts have been made to reconstruct the vertebral column from partial skeletons. Kellogg 1936 estimated a total of 58 vertebrae, based on two partial and nonoverlapping skeletons of B. cetoides from Alabama. More complete fossils uncovered in Egypt in the 1990s allowed a more accurate estimation: the vertebral column of B. isis has been reconstructed from three overlapping skeletons to a total of 70 vertebrae with a vertebral formula interpreted as seven cervical, 18 thoracic, 20 lumbar and sacral, and 25 caudal vertebrae. The vertebral formula of B. cetoides can be assumed to be the same.[25]

Basilosaurus has an anguilliform (eel-like) body shape because of the elongation of the centra of the thoracic through anterior caudal vertebrae. In life, these vertebrae were filled with marrow, and because of the enlarged size, made them buoyant. Basilosaurus probably swam predominantly in two dimensions at the sea surface, in contrast to the smaller Dorudon, which was likely a diving, three-dimensional swimmer.[26] The skeletal anatomy of the tail suggests that a small fluke was probably present, which would have aided only vertical motion.[27]

Basilosaurus reconstruction with a speculative dorsal ridge

Similarly sized thoracic, lumbar, sacral, and caudal vertebrae imply that it moved in an anguilliform fashion, but predominantly in the vertical plane. Paleontologist Philip D. Gingerich theorized that Basilosaurus may also have moved in a very odd, horizontal anguilliform fashion to some degree, something completely unknown in modern cetaceans. The vertebrae appear to have been hollow, and likely also fluid-filled. This would imply that Basilosaurus typically functioned in only two dimensions at the ocean surface, compared with the three-dimensional habits of most other cetaceans. Judging from the relatively weak axial musculature and the thick bones in the limbs, Basilosaurus is not believed to have been capable of sustained swimming or deep diving, or terrestrial locomotion.[28]

Paleobiology

Feeding

B. isis jaw muscles

The cheek teeth of Basilosaurus retain a complex morphology and functional occlusion. Heavy wear on the teeth reveals that food was first chewed then swallowed.[20] Scientists were able to estimate the bite force of Basilosaurus by analyzing the scarred skull bones of another species of prehistoric whale, Dorudon, and concluded they could bite with a force of 3,600 pounds per square inch (25,000 kPa).[29]

Analyses of the stomach contents of B. cetoides has shown that this species fed exclusively on fish and sharks, while bite marks on the skulls of juvenile Dorudon have been matched with the dentition of B. isis, suggesting a dietary difference between the two species, similar to that found in different populations of modern killer whales.[30] It was probably an active predator rather than a scavenger.[31] The discovery of juvenile Dorudon at Wadi Al Hitan bearing distinctive bite marks on their skulls indicates that B. isis would have aimed for the skulls of its victims to kill its prey, then subsequently tore its meals apart, based on the disarticulated remains of the Dorudon skeletons. The finding further cements theories that B. isis was an apex predator that may have hunted newborn and juvenile Dorudon at Wadi Al Hitan when mothers of the latter came to give birth.[32] The stomach contents of an elderly male includes the fish Pycnodus mokattamensis.[33]

Extinction

See also: Eocene–Oligocene extinction event
Gidley 1913 Basilosaurus cranium-neck reconstruction

Basilosaurus fossil record seems to end at about 33–35 mya.[34] Basilosaurus extinction coincides with the Eocene–Oligocene extinction event which happened 33.9 mya.[35] This extinction saw the death of all of basilosaurus group, the Archaeoceti, and has been attributed to a few factors including volcanic activity, meteor impacts and a sudden change in climate, the latter of which might have caused changes in the ocean by disrupting oceanic circulation.[36][37][38] Basilosaurus went extinct leaving no relatives.

Taxonomic history

Etymology

Caudal vertebra from Owen 1839

The two species of Basilosaurus are B. cetoides, whose remains were discovered in the United States, and B. isis, which was discovered in Egypt. B. cetoides is the type species for the genus.[39][40] During the early 19th century, B. cetoides fossils were so common (and sufficiently large) that they were regularly used as furniture in the American South.[41] Vertebrae were sent to the American Philosophical Society by a Judge Bry of Arkansas and Judge John Creagh of Clarke County, Alabama. Both fossils ended up in the hands of the anatomist Richard Harlan, who requested more examples from Creagh.[42][43] The first bones were unearthed when rain caused a hillside full of sea shells to slide. The bones were lying in a curved line "measuring upwards of four hundred feet in length, with intervals which were vacant." Many of these bones were used as andirons and destroyed; Bry saved the bones he could find, but was convinced more bones were still to be found on the location. Bry speculated that the bones must have belonged to a "sea monster" and supplied "a piece having the appearance of a tooth" to help determine which kind.[44]

Harlan identified the tooth as a wedge-shaped shell and instead focused on "a vertebra of enormous dimensions" which he assumed belonged to the order "Enalio-Sauri of Conybeare", "found only in the sub-cretaceous series."[45] He noted that some parts of the vertebra were similar to those of Plesiosaurus, but that they were completely different in proportions. Comparing his vertebra to those of large dinosaurs such as Megalosaurus and Iguanodon, Harlan concluded that his specimen was considerably larger—he estimated the animal to have been no less than 80–100 ft (24–30 m) long—and therefore suggested the name Basilosaurus, meaning "king lizard".[46]

Harlan brought his assembled specimens (including fragments of jaw and teeth, humerus, and rib fragments) to the UK where he presented them to anatomist Richard Owen. Owen concluded that the molar teeth were two-rooted, a dental morphology unknown in fishes and reptiles, and more complex and varied than in any known reptile, and therefore that the specimen must be a mammal. Owen correctly associated the teeth with cetaceans, but he thought it was an herbivorous animal, similar to sirenians.[47] Consequently, Owen proposed renaming the find Zeuglodon cetoides ("whale-like yoke teeth" in reference to the double-rooted teeth) and Harlan agreed.[48]

Wadi El Hitan

Main article: Wadi El Hitan
Skeleton of B. isis at Wadi El Hitan

Wadi El Hitan, otherwise known as Whale Valley, is an Egyptian sandstone formation where many early-whale skeletons were discovered.[49] German botanist Georg August Schweinfurth discovered the first archaeocete whale in Egypt (Zeuglodon osiris, now Saghacetus osiris) in 1879. He visited the Qasr el Sagha Formation in 1884 and 1886 and missed the now famous Wadi El Hitan by a few kilometers. German paleontologist Wilhelm Barnim Dames described the material, including the type specimen of Z. osiris, a well-preserved dentary.[50]

Hugh Beadnell, head of the Geological Survey of Egypt 1896–1906,[51] named and described Zeuglodon isis in Andrews 1904 based on a partial mandible and several vertebrae from Wadi El Hitan in Egypt.[52] Andrews 1906[53] described a skull and some vertebrae of a smaller archaeocete and named it Prozeuglodon atrox, now known today as Dorudon atrox. Kellogg 1936 discovered deciduous teeth in this skull and it was then believed to be a juvenile [Pro]zeuglodon isis for decades before more complete fossils of mature Dorudon were discovered.[54][55][56]

In the 1980s, Elwyn L. Simons and Philip D. Gingerich started to excavate at Qasr el-Sagha and Wadi El Hitan with the hope of finding material that could match archaeocete fossils from Pakistan. Since then, over 500 archaeocete skeletons have been found at these two locations, of which most are B. isis or D. atrox, several of the latter carrying bite marks assumed to be from the former.[57] Gingerich, Smith & Simons 1990 described additional fossils including foot bones and speculated that the reduced hind limbs were used as copulatory guides.[58]

In 2016, a complete skeleton, the first-ever find for Basilosaurus, was uncovered in Wadi El Hitan, preserved with the remains of another whale (which was either a last meal or an unborn fetus) inside its ribcage. This specimen was later found to have eaten Dorudon and several species of fish.[59] The whale's skeleton also shows signs of scavenging or predation by large sharks, among them the otodontid Carcharocles sokolovi, an ancestor to the later C. megalodon.[60][61]

Nomina dubia

Albert Koch's "Hydrarchos" fossil skeleton from 1845
  • Zeuglodon wanklyni was described in 1876 based on a skull found in the Wanklyn's Barton Cliff in the United Kingdom. This single specimen, however, quickly disappeared and has since been declared a nomen nudum or referred to as Zygorhiza wanklyni.[62]
  • Zeuglodon vredense or vredensis was named in the 19th century based on a single, isolated tooth without any kind of accompanying description, and Kellogg 1936 therefore declared it a nomen nudum.[63][64]
  • Zeuglodon puschi[i] from Poland was named by Brandt 1873. Kellogg 1936 noted that the species is based on an incomplete vertebra of indeterminable position and, therefore, that the species is invalid.[65][66]
  • Zeuglodon brachyspondylus was described by Müller 1849 based on some vertebrae from Zeuglodon hydrarchus, better known as Dr Albert Koch's "Hydrarchos". Kellogg 1936, synonymized it with Pontogeneus priscus, which Uhen 2005 declared a nomen dubium.

Reassigned species

  • Basilosaurus drazindai was named by Gingerich et al. 1997 based on a single lumbar vertebra. It was later declared a nomen dubium by Uhen (2013), but Gingerich and Zouhri (in press) re-assigned it to Eocetus.[67][68]
  • Zeuglodon elliotsmithii, Z. sensitivius, Z. sensitivus, and Z. zitteli were synonymized and grouped under the genus Saghacetus by Gingerich 1992.
  • Zeuglodon paulsoni from Ukraine was named by Brandt 1873. It was synonymized with Platyosphys but is now considered nomen dubium. Gingerich and Zouhri (in press), however, maintain Platyosphys as valid.[68][69]

In popular culture

The species B. cetoides is the state fossil of Alabama,[15] and Mississippi named the prehistoric whale (B. cetoides and Zygorhiza kochii) its state fossil.[70][71]

In the novel Moby-Dick, Ishmael discusses Basilosaurus in chapter 104, "The Fossil Whale", describing it as 'one of the most extraordinary creatures which the mutations of the globe have blotted out of existence'.[72]

In the BBC documentary, Walking with Beasts, a female B. isis after mating struggles to stay alive in the changing world, trying to find food for her and her unborn calf. The Basilosaurus in the show is eating Durodon. Basilosaurus went on to be featured in another walking with TV special, Sea Monsters.[73][74]

In cryptozoology, Basilosaurus is sometimes used as an explanation for globsters and sea monster sightings such as the Sakhalin island carcass and the lake monster Ogopogo.[75][76]

See also

References

Notes

  1. ^ "Dir Abu Lifa (Eocene of Egypt)". PBDB. Mark Uhen.
  2. ^ "†Basilosaurus cetoides Owen 1839 (whale)". PBDB.
  3. ^ "†Basilosaurus isis Andrews 1904 (whale)". PBDB.
  4. ^ "Cedar Reach Rock (Eocene of the United States)". PBDB.
  5. ^ "Georgia Limestone Products Co. Quarry (Eocene of the United States)". PBDB.
  6. ^ "Crow Creek (Eocene of the United States)". PBDB.
  7. ^ "Montgomery Landing, Yazoo Fm. (Eocene of the United States)". PBDB.
  8. ^ "Florida Lime Company (Eocene of the United States)". PBDB.
  9. ^ "Notes on fossil whales from the Upper Eocene of Barton, Hampshire". Nature. 83 (2). 18 March 1971. doi:https://doi.org/10.1016/S0016-7878(72)80005-1. Retrieved 6/21/19. {{cite journal}}: Check |doi= value (help); Check date values in: |accessdate= (help); External link in |doi= (help)
  10. ^ Basilosaurus. marc zabludoff.
  11. ^ "El Argoub, south B1 (Eocene of Western Sahara)". PBDB.
  12. ^ "Qa' Faydat ad Dahikiya (Eocene of Jordan)". PBDB.
  13. ^ "Sandouk el-Borneta (Eocene of Egypt)". PBDB.
  14. ^ Houssaye, A; Tafforeau, P; de Muizon, C; Gingerich, PD (2011). "Evolution of Whales from Land to Sea". PLoS ONE. 10 (2): e0118409. doi:10.1371/journal.pone.0118409. PMC 4340927. PMID 25714394.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  15. ^ a b c Gingerich, Philip D. "Basilosaurus cetoides". Encyclopedia of Alabama. Retrieved 24 August 2013.
  16. ^ "Explore Our Collections: Basilosaurus". Smithonian, National Museum of Natural History. Retrieved 24 August 2013.
  17. ^ Gingerich, P. D. (2008). "Early Evolution of Whales: A Century of Research in Egypt" (PDF). In Fleagle, J. G.; Gilbert, C. C. (eds.). Elwyn Simons: A Search for Origins. Developments in Primatology: Progress and Prospects. pp. 107–124. ISBN 978-0-387-73895-6.
  18. ^ Fahlke 2012, p. 6
  19. ^ Fahlke et al. 2011
  20. ^ a b c Gingerich & Uhen 1998, p. 4
  21. ^ Racicot & Berta 2013, p. 50
  22. ^ Nummela et al. 2004, p. 776
  23. ^ Bejder & Hall 2002, p. 448
  24. ^ "Hind limbs of eocene basilosaurus: evidence of feet in whales". PUBMED. Gingerich PD, Smith BH, Simons EL.
  25. ^ Zalmout, Mustafa & Gingerich 2000, Discussion, p. 202
  26. ^ Gingerich 1998, pp. 433, 435
  27. ^ "Basilosaurus spp". College of Osteopathic Medicine. Robert Boessenecker and Jonathan Geisler.
  28. ^ "Basilosaurus isis". Hellenica World.
  29. ^ Snively, E.; Fahlke, J. M.; Welsh, R. C. (2015). "Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis". PLoS ONE. 10 (2): e0118380. doi:10.1371/journal.pone.0118380. PMC 4340796. PMID 25714832.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  30. ^ Fahlke 2012, p. 14
  31. ^ Snively, Eric; Fahlke, Julia M.; Welsh, Robert C. (25 February 2015). "Bone-Breaking Bite Force of Basilosaurus isis (Mammalia, Cetacea) from the Late Eocene of Egypt Estimated by Finite Element Analysis". PLOS ONE. 10 (2): e0118380. doi:10.1371/journal.pone.0118380. PMC 4340796. PMID 25714832.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  32. ^ https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0209021#ack
  33. ^ Voss, Manja; Antar, Mohammed Sameh M.; Zalmout, Iyad S.; Gingerich, Philip D. (2019). "Stomach contents of the archaeocete Basilosaurus isis: Apex predator in oceans of the late Eocene". PLOS ONE. 14 (1): e0209021. doi:10.1371/journal.pone.0209021.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  34. ^ "†Basilosaurus Harlan 1834 (whale)". PBDB.
  35. ^ Ivany, Linda C.; Patterson, William P.; Lohmann, Kyger C. (2000). "Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary". Nature. 407 (6806): 887–890. doi:10.1038/35038044. PMID 11057663.
  36. ^ "Basilosaurus cetoides". Encyclopedia of Alabama. Philip D. Gingerich.
  37. ^ "Russia's Popigai Meteor Crash Linked to Mass Extinction". Yahoo news. Becky Oskin, Senior Writer.
  38. ^ Molina, Eustoquio; Gonzalvo, Concepción; Ortiz, Silvia; Cruz, Luis E. (2006-02-28). "Foraminiferal turnover across the Eocene–Oligocene transition at Fuente Caldera, southern Spain: No cause–effect relationship between meteorite impacts and extinctions". Marine Micropaleontology. 58 (4): 270–286. Bibcode:2006MarMP..58..270M. doi:10.1016/j.marmicro.2005.11.006.
  39. ^ Zalmout, Mustafa & Gingerich 2000
  40. ^ "Basilosaurus". BBC Nature. Retrieved 24 August 2013.
  41. ^ Zimmer 1998, p. 141
  42. ^ Switek, Brian (September 21, 2008). "The Legacy of the Basilosaurus". ScienceBlogs. Retrieved 14 September 2013.
  43. ^ Brian Switek (December 2010). "How Did Whales Evolve?". Smithsonian.
  44. ^ Harlan 1834, p. 400
  45. ^ Harlan 1834, p. 401
  46. ^ Harlan 1834, pp. 402–403
  47. ^ Owen 1839, pp. 72–73
  48. ^ Owen 1839, p. 75
  49. ^ "Wadi Al-Hitan". World Heritage Site. Retrieved 17 August 2017.
  50. ^ Gingerich 2007, pp. 110–112
  51. ^ Gingerich 2007, p. 113
  52. ^ Andrews 1904, pp. 214–215
  53. ^ Andrews 1906, pp. 255
  54. ^ Kellogg 1936, p. 81
  55. ^ Gingerich 2007, p. 114
  56. ^ Uhen 2004, p. 11
  57. ^ Gingerich 2007, pp. 117–119
  58. ^ Gingerich, Smith & Simons 1990, Abstract
  59. ^ https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0209021#ack
  60. ^ Griffiths, Sarah (2015-06-04). "Egyptian fossils reveal a whale inside a whale, eaten by a shark | Daily Mail Online". Dailymail.co.uk. Retrieved 2018-06-05.
  61. ^ https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0209021#ack
  62. ^ Basilosauridae in the Paleobiology Database: Taxonomic history. Retrieved August 2013.
  63. ^ Kellogg 1936, p. 264
  64. ^ Zeuglodon vredense (nomen nudum) in the Paleobiology Database. Retrieved August 2013.
  65. ^ Kellogg 1936, p. 263
  66. ^ Zeuglodon puschii (nomen dubium) in the Paleobiology Database. Retrieved August 2013.
  67. ^ Basilosaurus drazindai in the Paleobiology Database. Retrieved August 2013.
  68. ^ a b Gingerich, Philip D.; Zouhri, Samir (2015). "New fauna of archaeocete whales (Mammalia, Cetacea) from the Bartonian middle Eocene of southern Morocco". Journal of African Earth Sciences. 111: 273–286. doi:10.1016/j.jafrearsci.2015.08.006.
  69. ^ Gol'din & Zvonok 2013, Abstract
  70. ^ "State Symbols". State of Mississippi. Archived from the original on 22 August 2013. Retrieved 24 August 2013. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  71. ^ Fossil whale: State Fossil of Mississippi (PDF), Mississippi Department of Environmental Quality, 1991, retrieved 2019-05-09
  72. ^ Melville, H. (1988). Moby-Dick, or The Whale (Northwestern-Newberry Edition of the Writings of Herman Melville 6 ed.). Evanston, Ill.: Northwestern U. Press.
  73. ^ "Sea monster facts". BBC.
  74. ^ "Walking with Beasts". BBC. Retrieved 9 December 2018.
  75. ^ "Exploring Canadian Monsters: British Columbia". Mysterious Universe. Jason Offutt. Retrieved March 25, 2018.
  76. ^ "A Russian sea monster carcass is claimed to be that of an ancient 'archaeocete' whale". Science Blogs tetrapodzoology. tetrapodzoology.

Sources

External links

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