Basiloterus

''Basiloterus'' is an extinct genus of late-Eocene archaeocete whale from the Drazinda Formation in southwestern Punjab, Pakistan and possibly also the Barton Group (originally Barton Beds) of England. Known from two isolated lumbar vertebrae, the elongated nature of these elements has been taken as possible evidence that ''Basiloterus'' was a close relative of the better-known ''Basilosaurus''. This was also the reasoning behind its name, which roughly translates to "another king". However, publications since then not only lead to some major changes of the internal relationships within Basilosauridae but have also called into question how diagnostic elongated vertebrae are for members of this group, as other early whales have developed similar anatomy independently. Though the identity of ''Basiloterus'' as a basilosaurid is generally maintained, its exact position within more recent interpretations of the family is unclear.

History and naming

The fossil remains of ''Basiloterus'' were discovered in 1996 in the green shales that compose the middle parts of the Drazinda Formation in Pakistan. The genus was described in 1997 by Gingerich ''et al.'' alongside the species ''Basilosaurus drazindai'' on the basis of two lumbar vertebrae thought to represent a single individual. Gingerich and colleagues tentatively refer a vertebral centrum from the Barton Group of England to this genus as well on the basis of its age, size and general morphology.

The scientific name derives from the Ancient Greek "basileus" and "heteros" or "oteros", in combination translating to "other king" or "another king". The name was explained as being chosen as a companion name to ''Basilosaurus'', whose name translates to "king reptile". The species name of ''Basiloterus husseini'' honors Dr. S. Taseer Hussain, a researcher who warked at the Howard University in Washington, D.C. and sponsored the first expedition to the Drazinda Formation.

Description

''Basiloterus'' differs from species of ''Basilosaurus'' in that the centra of the lumbar vertebrae were less elongated relative to their height compared to those of ''Basilosaurus''. The crosssections of the vertebrae are more circular than in ''Pachycetus'', but the front and back surfaces are still not perfectly round. Instead, the anterior epiphyseal surface, the part of the centrum that faces the vertebra before it, is described as mildly saddle-shaped, whereas the surface that faces backwards is more oval and flattened. The actual epiphysis are, however, missing in both specimens. The preserved parts of the vertebrae measure in length, accounting for the missing epiphysis this means would suggest that the vertebrae measured up to long if complete. The vertebrae narrow towards the middle from all sides, giving them a dumbbell shape as described by Gol'din and Zvonok, which sets them apart from those of ''Basilotritus''. The lower surface of the vertebrae is concave and does not feature a keel, instead displaying three vascular foramina that are located within the concavity.

The transverse processes, which project from the sides of the centrum, are described as being oriented anteroventrally. This downturned state is thought to allow for an increased arc of movement of the iliocostalis muscle. The transverse processes are less elongated than those of '' Basilotritus''.

The laminae at the base of the neural spine is thin and leads into the spinous process while also enclosing the moderately large neural canal in junction with the pedicles. The pedicles connect to the laminae at an angle and are described as being situated closer to the front of the vertebra, something considered to be typical for basilosaurids by Gingerich. The metapophysis, which are robust pyramidal structures that project from the pedicles, are described as being oriented much more vertically than those of ''Basilosaurus''. They extend beyond the front-most edge of the centrum, but do not flare out much. The spinous process extends to a height of above the vertebral centrum, all the while being directed at a 10-degree angle forward. The base of the neural spine is long from the front to the back but is only thick. The anterior to posterior length of the neural spine is another feature that differentiates ''Basiloterus'' from ''Basilotritus'', as this element is less elongated. The tip of the spinous process does not expand, suggesting only weak ligaments and muscle attachments.

According to Gingerich and colleagues, ''Basiloterus'' was smaller than ''Basilosaurus'', and Gol'din & Zvonok state that the vertebrae are of similar size to those of ''Basilotritus''. The size of the vertebrae has also been compared to ''Pachycetus robustus'', with Gingerich determining that those of ''Basiloterus'' were around 25% larger.

Phylogeny

''Gingerich'' and colleagues considered ''Basiloterus'' to be a close relative of ''Basilosaurus'' based on the size and shape of the lumbar vertebrae as well as the fact that the two genera were contemporary. Later, Mark D. Uhen argued that this first identification may not be as certain as initially believed. Although Uhen agrees that elongated vertebrae are a distinguishing feature of basilosaurines within the family Basilosauridae, he also points out that other groups evolved this trait independently. According to Uhen, this means that ''Basiloterus'' and ''Basilosaurus drazindai'' could also be protocetids similar to '' Eocetus'', which also developed elongated vertebrae. While few papers examined or even acknowledge ''Basiloterus'' following the type description, those that do feature it continue to regard it as a basilosaurid.

The phylogenetic tree provided by Uhen in 2009's ''Encyclopedia of Marine Mammals'' suggests a paraphyletic Basilosauridae leading up to modern cetaceans, although some more recent studies have proposed monophyletic solutions for Basilosauridae as well. Despite his reservations towards the identity of ''Basiloterus'' as a basilosaurine, the tree still depicts it as the immediate sister taxon to ''Basilosaurus'' as proposed by Gingerich and colleagues.

The elongation of the vertebrae as a convergent trait is again noted by Gol'din and Zvonok, who propose that it had been independently gained by ''Basiloterus'', ''Basilotritus'' and ''Basilosaurus''. They argue that this independent acquisition of elongated lumbars renders it an unreliable character to separate the subfamilies Basilosaurinae and Dorudontinae, further supported by the close relationship they recover between ''Basilosaurus'' and ''Dorudon''. In their results, ''Dorudon'' is the closest relative of ''Basilosaurus'' in place of ''Basiloterus'', which is not included in their results. Similar results also appear in later papers such as the description of the small-bodied basilosaurid ''Tutcetus'', which also argues against the concept of a distinct Basilosaurinae and Dorudontinae and does not discuss ''Basiloterus''. The description of ''Cynthiacetus peruvianus'' also fails to recover two distinct subfamilies and simply refers to ''Basiloterus'' as a basilosaurid of uncertain relation to the other taxa within the family.

Paleobiology

Although not found in the same area, ''Basiloterus'' likely coexisted with ''Basilosaurus drazindai'' as they both come from the same green shales of the Drazinda Formation, which also yielded the remains of the giant softshell turtle ''Drazinderetes''. The Drazinda Formation consists of shallow marine sediments.

See also

* Evolution of cetaceans

References

{{Taxonbar, from1=Q18580345, from2=Q1064339

Basilosauridae
Monotypic prehistoric cetacean genera
Bartonian genera
Eocene mammals of Asia
Fossils of Pakistan
Eocene mammals of Europe
Fossils of England
Fossil taxa described in 1997