Basilosauridae

Basilosauridae is a family of extinct cetaceans that lived during the middle to late Eocene. Basilosaurids are known from all continents including Antarctica,[1] and are probably the first fully aquatic cetaceans.[2][3] The group is noted to be a paraphyletic assemblage of stem group whales[4] from which the monophyletic Neoceti are derived.[5]

## Contents

* 1 Characteristics
* 2 Systematics and Classification
* 3 Size
* 4 Taxonomy
* 5 See also
* 6 Notes
* 7 References

## Characteristics

Dorudon skeletal diagram. Cynthiacetus, mounted skeleton. Zygorhiza, mounted skeleton.

Basilosaurids ranged in size from 4 to 16 m (13 to 52 ft) and were fairly similar to modern cetaceans in overall body form and function.[6] Some genera tend to show signs of convergent evolution with mosasaurs by having long serpentine body shape, which suggests that this body plan seems to have been rather successful.[7] Basilosaurid forelimbs have broad and fan-shaped scapulae attached to a humerus, radius, and ulna which are flattened into a plane to which the elbow joint was restricted, effectively making pronation and supination impossible. Because of a shortage of forelimb fossils from other archaeocetes, it is not known if this arrangement is unique to basilosaurids, as some of the characteristics are also seen in Georgiacetus.[2]

As archaeocetes, basilosaurids lacked the telescoping skull of present whales. Their jaws were powerful,[8] with a dentition easily distinguishable from that of other archaeocetes: they lack upper third molars and the upper molars lack protocones, trigon basins, and lingual third roots. The cheek teeth have well-developed accessory denticles.

Unlike modern whales, basilosaurids possessed small hindlimbs with well defined femur, lower leg and feet. They were, however, very small and did not articulate with the vertebral column, which also lack true sacral vertebrae.[2] While they were unable to support body weight on land, they might have assisted as claspers during copulation.[9] Analysis of tail vertebrate from Basilosaurus and Dorudon indicate they possessed small flukes.[10]

## Systematics and Classification

In the past, basilosaurids were predominantly separated into two groups based on absolute body size and the presence of extreme elongation within the vertebral series. Basilosaurinae was proposed as a subfamily containing two genera: Basilosaurus and Basiloterus.[11] Alongside Basilosaurinae, Dorudontinae was proposed as a subfamily of smaller sized whales that included Dorudon and all other basilosaurids that didn't possess the extreme vertebral elongation seen in Basilosaurus and Basiloterus.[11]

Although Basilosauridae is historically recovered as a monophyletic group sister to Neoceti, more recent studies have shown that Basilosauridae may instead represent a paraphyletic grade that gave rise to modern whales.[5][12] The inner systematics of Basilosauridae as a whole are also under revision. Recent analyses have shown that large body size and vertebral elongation are traits that can evolve convergently among these whales, making these traits poor indicators of their true relationships.[5][12][13][14] As of the description of Tutcetus, all fully aquatic whales fall into a larger clade known as Pelagiceti, of which the poorly known cetacean Eocetus is the basalmost member. From there, there are three major clades that are recovered. One clade contains the diminutive basilosaurids Tutcetus, Chrysocetus, and Ocucajea. Another contains Basilosaurus and several taxa typically regarded as dorudontines, such as Dorudon, Zygorhiza, and the large Cynthiacetus. The final clade contains a monophyletic Pachycetinae sister to Neoceti, with Supayacetus as the most basal member of the Pachycetinae clade. The results of this analysis are shown below.[12]

Pelagiceti  |  |  |

Eocetus

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Chrysocetus

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Ocucajea

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Tutcetus rayanensis

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Zygorhiza

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Saghacetus

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Ancalecetus

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Basilosaurus

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Dorudon

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Pontogeneus peruvianus

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Supayacetus

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Pachycetus paulsonii

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Pachycetus wardii

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Antaecetus aithai

Neoceti

Certain enigmatic cetaceans from the Middle and Late Eocene may belong to Pelagiceti or the basilosaurid grade, but they are too fragmentary to have their relationships resolved. The poorly understood cetacean Eocetus is known from a heavily damaged skull and a handful of vertebrae, and the incomplete, poorly preserved nature of these remains have hindered efforts to find a definitive placement for the genus. Historically, it is regarded as a highly derived member of Protocetidae, but recent analyses have included it within Basilosauridae or Pelagiceti as a basal member.[12][15] The highly fragmentary "Platyosphys einori" from the Middle Eocene of Ukraine may also fall within Pelagiceti, but its known anatomy is very unique compared to other basilosaurids. The bones are from a juvenile animal, which was about 10 meters long at time of death. Not only is this size impressive for an Eocene whale, especially for a juvenile, but what is more remarkable is that it possessed osteoporotic ribs. Osteoporotic skeletal elements are a trait that is unknown in all other archaeocete whales, and only seen in the members of Neoceti. It is currently classified as Basilosauridae incertae sedis, but like Eocetus, it may represent a transitional form at the base of Pelagiceti, between the most advanced protocetids and the other members of the basilosaurid grade.[5]

## Size

File:Basilosaurus and Dorudon skeletons - Voss et al 2019.tif Basilosaurids have a diverse range of sizes. Tutcetus rayanensis, the smallest member, is about 2.51-2.55 meters (8 feet 3 inches - 8 feet 4 inches) long and weighs around 180.4-187.1 kilograms (398-412 pounds).[12] On the other hand, Basilosaurus cetoides is impressively long, reaching approximately 18 meters.[16] The largest known basilosaurid, Perucetus colossus, is believed to be even bigger, with a length of about 17–20 metres (56–66 ft) and possibly comparable to, if not larger than, the modern blue whale in terms of weight,[13] though other researchers argue that it was much lighter.[17][18]

## Taxonomy

* Family Basilosauridae
* Genus Perucetus
* Genus Tutcetus
* Genus Eocetus?
* Subfamily Basilosaurinae
* Genus Basilosaurus
* Genus Basiloterus
* Subfamily Dorudontinae
* Genus Ancalecetus
* Genus Chrysocetus
* Genus Cynthiacetus
* Genus Dorudon
* Genus Masracetus
* Genus Ocucajea
* Genus Saghacetus
* Genus Stromerius
* Genus Supayacetus
* Genus Zygorhiza
* Subfamily Pachycetinae
* Genus Pachycetus
* Genus Antaecetus

## See also

* Evolution of cetaceans
* Leviathan

## Notes

1. ↑ Buono, M.R.; Fernández, M.S.; Reguero, M.A.; Marenssi, S.A.; Santillana, S.N.; Mörs, T. (2016). "Eocene Basilosaurid Whales from the La Meseta Formation, Marambio (Seymour) Island, Antarctica". Ameghiniana 53 (3): 296–315. doi:10.5710/AMGH.02.02.2016.2922. Bibcode: 2016Amegh..53..296B.
2. ↑ 2.0 2.1 2.2 Uhen 2002
3. ↑ Buono M, Fordyce R.E., Marx F.G., Fernández M.S. & Reguero M. (2019). "Eocene Antarctica: a window into the earliest history of modern whales". Advances in Polar Science 30(3): p. 293-302. doi:10.13679/j.advps.2019.0005
4. ↑ Lloyd, G. T.; Slater, G. J. (2021). "A total-group phylogenetic metatree for Cetacea and the importance of fossil data in diversification analyses". Systematic Biology 70 (5): 922–939. doi:10.1093/sysbio/syab002. PMID 33507304.
5. ↑ 5.0 5.1 5.2 5.3 Davydenko, S.; Shevchenko, T.; Ryabokon, T.; Tretiakov, R.; Gol’din, P. (2021). "A Giant Eocene Whale from Ukraine Uncovers Early Cetacean Adaptations to the Fully Aquatic Life". Evolutionary Biology 48 (1): 67–80. doi:10.1007/s11692-020-09524-8. Bibcode: 2021EvBio..48...67D.
6. ↑ Uhen M. (2004). "Form, Function, and Anatomy of Dorudon Atrox (Mammalia, Cetacea): An Archaeocete from the Middle to Late Eocene of Egypt". Papers on Paleontology 34: p. 1-222
7. ↑ Oceans of Kansas: a Natural History of the Western Interior Sea. Indiana University Press. 2005. ISBN 9780253345479.
8. ↑ 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
9. ↑ Gingerich P.D., Smith B.H., Simons E.L. (1990). "Hind limbs of eocene basilosaurus: evidence of feet in whales". Science 249(4965): p. 154–157. doi:10.1126/science.249.4965.154
10. ↑ Philip D. Gingerich, Mohammed Sameh M. Antar und Iyad S. Zalmot: "Aegicetus gehennae, a new late Eocene protocetid (Cetacea, Archaeoceti) from Wadi Al Hitan, Egypt, and the transition to tail-powered swimming in whales". PLoS ONE 14(12): e0225391 doi:10.1371/journal.pone.0225391
11. ↑ 11.0 11.1 Basilosauridae in the Paleobiology Database. Retrieved July 2013.
12. ↑ 12.0 12.1 12.2 12.3 12.4 Antar, Mohammed S.; Gohar, Abdullah S.; El-Desouky, Heba; Seiffert, Erik R.; El-Sayed, Sanaa; Claxton, Alexander G.; Sallam, Hesham M. (10 August 2023). "A diminutive new basilosaurid whale reveals the trajectory of the cetacean life histories during the Eocene" (in en). Communications Biology 6 (1): 707. doi:10.1038/s42003-023-04986-w. PMID 37563270.
13. ↑ 13.0 13.1 Bianucci, Giovanni; Lambert, Olivier; Urbina, Mario; Merella, Marco; Collareta, Alberto; Bennion, Rebecca; Salas-Gismondi, Rodolfo; Benites-Palomino, Aldo et al. (August 2023). "A heavyweight early whale pushes the boundaries of vertebrate morphology" (in en). Nature 620 (7975): 824–829. doi:10.1038/s41586-023-06381-1. ISSN 1476-4687. PMID 37532931. Bibcode: 2023Natur.620..824B. https://www.nature.com/articles/s41586-023-06381-1.
14. ↑ Martínez-Cáceres, Manuel; Lambert, Olivier; Muizon, Christian De (31 March 2017). "The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru" (in en). Geodiversitas 39 (1): 7–163. doi:10.5252/g2017n1a1. ISSN 1280-9659. http://www.bioone.org/doi/10.5252/g2017n1a1.
15. ↑ Gingerich, Philip D.; Zouhri, Samir (2015-11-01). "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. ISSN 1464-343X. https://www.sciencedirect.com/science/article/pii/S1464343X1530039X.
16. ↑ Smith, Kathlyn M.; Hastings, Alexander K.; Bebej, Ryan M.; Uhen, Mark D. (March 2022). "Biogeographic, stratigraphic, and environmental distribution of Basilosaurus (Mammalia, Cetacea) in North America with a review of the late Eocene shoreline in the southeastern coastal plain" (in en). Journal of Paleontology 96 (2): 439–451. doi:10.1017/jpa.2021.90. Bibcode: 2022JPal...96..439S. https://doi.org/https://doi.org/10.1017/jpa.2021.90.
17. ↑ Motani, R.; Pyenson, N. D. (2024). "Downsizing a heavyweight: factors and methods that revise weight estimates of the giant fossil whale Perucetus colossus". PeerJ 12: e16978. doi:10.7717/peerj.16978. PMID 38436015.
18. ↑ Zimmer, Carl (29 February 2024). "Researchers Dispute Claim That Ancient Whale Was Heaviest Animal Ever - A new study argues that Perucetus, an ancient whale species, was certainly big, but not as big as today's blue whales.". The New York Times. Archived from the original on 29 February 2024. https://archive.today/20240229131654/https://www.nytimes.com/2024/02/29/science/giant-whale-perucetus.html. Retrieved 1 March 2024.

## References

* "An addition to the vertebrate fauna of the Miocene period, with a synopsis of the extinct Cetacea of the United States". Proceedings of the Academy of Natural Sciences of Philadelphia 19 (4): 138–57. 1868. OCLC 4909181381.
* Gingerich, Philip D (2007). "Stromerius nidensis, new archaeocete (Mammalia, Cetacea) from the Upper Eocene Qasr El-Sagha Formation, Fayum, Egypt". Contributions from the Museum of Paleontology 31 (13): 363–78. OCLC 214233870. http://deepblue.lib.umich.edu/bitstream/2027.42/57499/1/Vol 31 No 13 final 12-19-07.pdf.
* Uhen, Mark D (2002). "Basilosaurids". in Perrin, William R; Wiirsig, Bernd; Thewissen, J G M. Encyclopedia of Marine Mammals. Academic Press. pp. 78–81. ISBN 978-0-12-551340-1.
* Uhen, Mark D. (2013). "A review of North American Basilosauridae". Alabama Museum of Natural History Bulletin 31 (2): 1–45.

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