Ichthyolestes

''Ichthyolestes'' ("fish thief") is an extinct genus of archaic cetacean that was endemic to the northern Indian subcontinent during the Lutetian stage. To date, this monotypic genus is only represented by ''Ichthyolestes pinfoldi''.

Like other members of the family Pakicetidae, which are considered the earliest and least specialized of the archaic cetaceans, ''Ichthyolestes'' represents an early quadrupedal phase of the land-to-sea transition which occurs in the cetacean lineage.

Discovery and classification

''Ichthyolestes pinfoldi'' was initially known only from teeth, which were found in the Eocene epoch near what is now Ganda Kas, Pakistan. Upon their discovery, ''I. pinfoldi'' was placed within the family Mesonychidae, a group of terrestrial mammals from which cetaceans were previously thought to have evolved. Due to morphological distinctions from other mesonychids, ''I. pinfoldi'' was later acknowledged and described as an archaeocete, making it the first pakicetid to be described.

Cranial and postcranial material of ''I. pinfoldi'' have also been found in the Kuldana Formation located in present-day Pakistan, from the early-middle Eocene, and from the Subathu sediments of Jammu and Kashmir, India.

Description

''Ichthyolestes'' is the smallest pakicetid, approximately 29% smaller than ''Pakicetus'', and has been considered "fox-sized". They retain many features typical of terrestrial Eocene artiodactyls, including long and gracile limb bones, a fused sacrum, small mandibular foramen, and no cranial telescoping. The body plan of ''Ichthyolestes'' is generally similar to ''Pakicetus'', but smaller and more gracile. Therefore, locomotion is also thought to be reliant on quadrupedal paddling.

Teeth

''Ichthyolestes'' exhibits heterodont and diphyodont dentition with cusped cheek teeth. The upper second molar has pointed cusps with a high narrow paracone and a lower connate metacone behind it. The molars are also relatively labio-lingually compressed with serrated crests along the labial cusps.

Cranial

''Ichthyolestes'' has dorsally oriented eyes and a narrow skull, which is smaller than the skulls of both ''Pakicetus'' and '' Nalacetus''. Like other pakicetids, ''Ichthyolestes'' lacks a supraorbital shield; however, there is some variation in supraorbital morphology between the three genera. The supraorbital region of ''Ichthyolestes'' and ''Pakicetus'' is cup-like and cradles the dorsal portion of the eye. ''Ichthyolestes'' also has a weaker incisure on the dorsal side of the supraorbital region compared to ''Pakicetus''.

All three pakicetid taxa largely retained the peripheral ear morphology of terrestrial mammals, but likely used bone conducting mechanisms when hearing underwater. The tympanic bulla also differs between pakicetid taxa. The absolute size of ''Ichthyolestes'' tympanic bulla is smaller than either ''Pakicetus'' or ''Nalacetus''; but when compared relative to their body size, the tympanic bulla of ''Ichthyolestes'' is proportionally larger than that of ''Pakicetus''.

The semicircular canal system of the vestibular apparatus in the inner ear, which is involved in neural control of locomotion, is similar in size to that of Eocene artiodactyls. They do not show the size reduction as seen in other pakicetids and Eocene cetaceans like ''Remingtonocetus'', ''Indocetus'' and ''Dorudon'', which have canal sizes within the upper range of modern cetaceans. This suggests that ''Ichthyolestes'' had not fully invaded aquatic ecosystems and still remained somewhat terrestrial.

Postcranial

Although ''Ichthyolestes'' is the smallest pakicetid, some features are larger or more robust than ''Nalacetus'', such as the astragalus, calcaneus and pelvis. ''Ichthyolestes'' also has proportionally longer lumbar and caudal vertebrae relative to its shorter limb segments.

''Ichthyolestes'' also has long digits and strong post-thoracic vertebrae. The sacrum is composed of four elongate vertebrae with complete fusion, except for the spinous processes of S2 to S4 which are columnar with smooth ventral faces. The lumbar vertebrae are narrower and shallower to ''Pakicetus'' and ''Nalacetus''. Although still relatively large compared to other related taxa, the atlas vertebrae of ''Ichthyolestes'' is smaller and more gracile than ''Pakicetus'' or ''Nalacetus'', and the neural canal is disproportionately large.

Terrestrial or semi-aquatic locomotion

The astragalus bone, which is present in the ankle of artiodactyls and archaic whales that retained feet, has similar dimensions in both Eocene artiodactyls and ''Ichthyolestes''. In addition to lending evidence to the theory of a direct relationship between artiodactyls and cetaceans, this knowledge has led to two competing hypotheses about the locomotion of ''Ichthyolestes'' and other pakicetids. The first states that ''Ichthyolestes'' and ''Pakicetus'' were terrestrial and cursorial, implying that aquatic locomotor adaptation occurred after the origin of Cetacea. The second states that ''Ichthyolestes'' and ''Pakicetus'' were already semi-aquatic and cetaceans originated from an earlier unknown Eocene artiodactyl, such as ''Elomeryx'' or ''Indohyus,'' implying that aquatic locomotor adaptations occurred before or during the origin of Cetacea.

Although the postcranial anatomy of ''Ichthyolestes'' is similar to that of Eocene artiodactyls and implies cursoriality, the assessment of bone morphology and microstructure indicate that they, and other pakicetids, were semi-aquatic like protocetids. Hypermineralization occurs in all regions of the skeleton; in particular, the long bones and ribs had small or absent marrow cavities due to the thick cortices which developed. Hypermineralization of load-bearing skeletal elements put ''Ichthyolestes'' at an increased risk of fractures during prolonged terrestrial loading and this risk increased with velocity, implying that terrestriality was limited. Additionally, the dense skeletons may have allowed bottom-walking or wading in shallow pools as it would counteract buoyancy created by inflated lungs and fur-trapped air. Therefore, the retention of an artiodactyl-like astragalus does not signify full terrestriality or cursoriality in ''Ichthyolestes''.

Paleoenvironment

Fossil findings indicate ''Ichthyolestes'' shared habitat with its relatives ''Pakicetus'' and ''Nalacetus''. Their fossils are usually found around river channel deposits, not marine deposits or fauna. The regions around northern Pakistan and northwest India, where fossils have been recovered, are thought to have been an arid environment with ephemeral streams and moderately developed floodplains. ''Ichthyolestes'', therefore, had an affinity for water and were either terrestrial or semi-aquatic. The fluvial facies of the lower Kuldana Formation represent shallow tropical riverine complexes and were likely habitats for the initial stages of transition into water.

See also

* ''Pakicetus''
* '' Nalacetus''
* Pakicetidae
* Evolution of cetaceans
* Eocene

References

Works cited

*

External Sources

Ichthyolestes
in the Paleobiology Database. Retrieved June 2013.

{{Taxonbar, from=Q1366647

Pakicetidae
Extinct animals of Pakistan
Extinct animals of India
Eocene mammals of Asia
Monotypic prehistoric cetacean genera
Fossil taxa described in 1958