Dorudon

''Dorudon'' ("spear-tooth") is a genus of extinct basilosaurid ancient whales that lived alongside ''Basilosaurus'' 41.03 to 33.9 million years ago in the Eocene. It was a small whale, with ''D. atrox'' measuring long and weighing . ''Dorudon'' lived in warm seas around the world and fed on small fish and mollusks. Fossils have been found along the former shorelines of the Tethys Sea in present-day Egypt and Pakistan, as well as in the United States, New Zealand and Western Sahara.

Taxonomic history

described ''Dorudon serratus'' based on a fragmentary maxilla and a few teeth found in South Carolina. He concluded that the teeth must have belonged to a mammal since they were two-rooted, that they must have been teeth from a juvenile since they were hollow, and also noted their similarity to the teeth then described for ''Zeuglodon'' (''Basilosaurus''). When exploring the type locality, Gibbes discovered a lower jaw and twelve caudal vertebrae, which he felt obliged to assign to ''Zeuglodon'' together with his original material. Gibbes concluded that ''Dorudon'' were juvenile ''Zeuglodon'' and consequently withdrew his new genus. He did, however, allow Louis Agassiz at Harvard to examine his specimens, and the Swiss professor replied that these were neither teeth of a juvenile nor those of ''Zeuglodon'', but of a separate genus just as Gibbes had first proposed.

described ''Prozeuglodon atrox'' (="Proto-''Basilosaurus''") based on a nearly complete skull, a dentary and three associated vertebrae presented to him by the Geological Museum of Cairo. , however, realized that Andrews' specimen was a juvenile, and, he assumed, the same species as ''Zeuglodon isis'', described by Andrews 1906. Kellogg also realized that the generic name ''Zeuglodon'' was invalid and therefore recombined it ''Prozeuglodon isis''. Since then many specimens have been referred to ''Prozeuglodon atrox'', including virtually every part of the skeleton, and it has become obvious that it is a separate genus, not a juvenile "Proto-''Zeuglodon''". Kellogg placed several of the species of ''Zeuglodon'' described from Egypt in the early 20th century (including ''Z. osiris'', ''Z. zitteli'', ''Z. elliotsmithii'' and ''Z. sensitivius'') in the genus ''Dorudon''. synonymized these four species and grouped them as '' Saghacetus osiris''.

The current taxonomic status of ''Dorudon'' is based on 's revision of ''Dorudon'' and detailed description of ''D. atrox''. Before this, the taxonomy of ''Dorudon'' was in disarray and based on a limited set of specimens.

''D. atrox'' is known from Egypt, ''D. serratus'' from Georgia and South Carolina in the United States. The type species ''D. serratus'' was, and still is, based solely on two partial maxillae with a few teeth, cranial fragments, and a dozen vertebrae with some additional material, collected but not described by Gibbes, and referred to the type species. Before Uhen 2004, ''D. atrox'' was based solely on Andrews holotype skull, lower jaw, and the vertebrae he referred to it, but is now the best-known archaeocete species.

The two species of ''Dorudon'' differ from other members of Dorudontinae mainly in size: they are considerably larger than '' Saghacetus'' and slightly larger than '' Zygorhiza'', but also differ from both these genera in dental and/or cranial morphology. The limited known material for ''D. serratus'' makes it difficult to compare the two species of ''Dorudon''. placed ''D. atrox'' in the same genus as ''D. serratus'' because of similarities in size and morphology, but kept them as separate species because of differences in dental morphology. Even though ''D. serratus'' is the type species, the description of ''Dorudon'' is largely based on ''D. atrox'' because of its completeness. The cranial morphology of ''D. atrox'' makes it distinct from all other archaeocetes.

Description

''Dorudon'' was a medium-sized whale, with ''D. atrox'' reaching in length and in body mass. Dorudontines were originally believed to be juvenile individuals of ''Basilosaurus'' as their fossils are similar but smaller. They have since been shown to be a different genus with the discovery of ''Dorudon'' juveniles. Although they look very much like modern whales, basilosaurines and dorudontines lacked the melon organ that would allow their descendants to use echolocation as effectively as modern whales. Like other basilosaurids, their nostrils were midway from the snout to the top of the head.

Dentition

The dental formula for ''Dorudon atrox'' is .

Typical for cetaceans, the upper incisors are aligned with the cheek teeth, and, except the small I¹, separated by large diastemata containing pits into which the lower incisors fit. The upper incisors are simple conical teeth with a single root, lacking accessory denticles, and difficult to distinguish from lower incisors. The upper incisors are missing in most specimens and are only known from two specimens. The upper canine is a little larger than the upper incisors, and, like them, directed slightly buccally and mesially.

P¹, only preserved in a single specimen, is the only single-rooted upper premolar. Apparently, P¹ is conical, smaller than the remaining premolars and lacks accessory denticles. P² is the largest upper tooth and the first in the upper row with large accessory denticles. Like the more posterior premolars, it is buccolingually compressed and double-rooted. It has a dominant central protocone flanked by denticles that decrease in size mesially and distally, resulting in a tooth with a triangular profile. P³ is similar to but slightly smaller than P², except that it has a projection on the lingual side which is the remnant of a third root. In P⁴, smaller than P^2–3, the larger distal root is formed by the fusion of two roots.

The upper molars extend onto the zygomatic arch and are considerably smaller than their neighbouring premolars. Like P⁴, their distal root is wider than the mesial and formed by the fusion of two roots. The profiles of the molars are more rounded than those of the premolars.

Similar to the upper incisors, the lower incisors are simple conical teeth curved distally and aligned with the cheek teeth. I₁, the smallest tooth, is sitting on the anteriormost portion of the dentary, with its alveolus left open towards the mandibular symphysis and located as close to the alveolus of I₂ as it can. I₂, I₃ and C₁ are very similar, considerably larger than I₁.

The lower premolars are double-rooted, buccolingually compressed teeth, except the deciduous P₁ which is single-rooted. P₃ is the second-largest cheek tooth, P₄ the largest; both are very similar, dominated by the central cusp.

In the lower molars, the accessory denticles on the mesial edges are replaced by a deep groove called the reentrant groove. The apical cusp is the primitive protoconid. M₂ and M₃ are morphologically very similar. M₃ is sitting high on the ascending mandibular ramus.

Skull

According to study of its endocranium, the encephalisation of ''D. atrox'' was no greater than that of terrestrial mesonychians.

Paleoecology

''Dorudon'' calves may have fallen prey to hungry ''Basilosaurus'', as shown by unhealed bite marks on the skulls of some juvenile ''Dorudon''.

See also

*Evolution of cetaceans

References

Notes

Sources

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* Haines, Tim & Chambers, Paul. (2006) The Complete Guide to Prehistoric Life. Canada: Firefly Books Ltd.
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External links

BBC Wildfacts: Dorudon

{{Taxonbar, from=Q131137

Eocene mammals of Africa
Basilosauridae
Eocene mammals of North America
Prehistoric cetacean genera
Fossil taxa described in 1845