| Ichthyolestes | |
|---|---|
 | |
| Ichthyolestes life restoration, pencil drawing. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Infraorder: | Cetacea |
| Family: | †Pakicetidae |
| Genus: | †Ichthyolestes Dehm & Oettingen-Spielberg 1958 |
| Type species | |
| †Ichthyolestes pinfoldi | |
Ichthyolestes ("fish thief") is an extinct genus ofarchaic cetacean that was endemic to the northernIndian subcontinent during theLutetian stage.[1] To date, thismonotypic genus is only represented byIchthyolestes pinfoldi.[1]
Like other members of the familyPakicetidae, which are considered the earliest and least specialized of the archaic cetaceans,[2]Ichthyolestes represents an early quadrupedal phase of the land-to-sea transition which occurs in the cetacean lineage.[3][4]
Ichthyolestes pinfoldi was initially known only from teeth, which were found in theEocene epoch near what is now Ganda Kas,Pakistan.[1][5] Upon their discovery,I. pinfoldi was placed within the familyMesonychidae,[1][3] a group of terrestrial mammals from which cetaceans were previously thought to have evolved.[6] Due to morphological distinctions from other mesonychids,I. pinfoldi was later acknowledged and described as anarchaeocete,[7][8] making it the first pakicetid to be described.[4]
Cranial and postcranial material ofI. pinfoldi have also been found in theKuldana Formation located in present-day Pakistan, from the early-middle Eocene,[8][6][9][2] and from theSubathu sediments of Jammu and Kashmir,India.[10]
Ichthyolestes is the smallest pakicetid, approximately 29% smaller thanPakicetus, and has been considered “fox-sized.”[9] They retain many features typical of terrestrial Eoceneartiodactyls, including long and gracilelimb bones, a fusedsacrum, smallmandibular foramen, and no cranial telescoping.[9][11] The body plan ofIchthyolestes is generally similar toPakicetus, but smaller and more gracile.[2][4] Therefore, locomotion is also thought to be reliant on quadrupedal paddling.[4]
Ichthyolestes exhibitsheterodont anddiphyodont dentition withcusped cheek teeth.[4] The upper second molar has pointed cusps with a high narrowparacone and a lower connatemetacone behind it.[8][10] The molars are also relatively labio-lingually compressed with serrated crests along the labial cusps.[8][10]
Ichthyolestes has dorsally oriented eyes and a narrow skull, which is smaller than the skulls of bothPakicetus andNalacetus.[3] Like other pakicetids,Ichthyolestes lacks asupraorbital shield; however, there is some variation in supraorbitalmorphology between the three genera. The supraorbital region ofIchthyolestes andPakicetus 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 toPakicetus.[3]
All three pakicetid taxa largely retained the peripheral ear morphology of terrestrial mammals,[11] but likely used bone conducting mechanisms when hearing underwater. Thetympanic bulla also differs between pakicetid taxa.[3] The absolute size ofIchthyolestes tympanic bulla is smaller than eitherPakicetus orNalacetus; but when compared relative to their body size, the tympanic bulla ofIchthyolestes is proportionally larger than that ofPakicetus.[3]
Thesemicircular canal system of thevestibular apparatus in the inner ear, which is involved in neural control of locomotion, is similar in size to that of Eocene artiodactyls.[12] They do not show the size reduction as seen in other pakicetids and Eocene cetaceans likeRemingtonocetus,Indocetus, andDorudon, which have canal sizes within the upper range of modern cetaceans.[12] This suggests thatIchthyolestes had not fully invaded aquatic ecosystems and still remained somewhat terrestrial.[12]
AlthoughIchthyolestes is the smallest pakicetid, some features are larger or more robust thanNalacetus, such as theastragalus,calcaneus, andpelvis.Ichthyolestes also has proportionally longerlumbar andcaudal vertebrae relative to its shorter limb segments.[2]
Ichthyolestes also has long digits and strong post-thoracic vertebrae.[2] Thesacrum is composed of four elongate vertebrae with complete fusion, except for thespinous processes of S2 to S4 which are columnar with smooth ventral faces.[2] The lumbar vertebrae are narrower and shallower toPakicetus andNalacetus.[2] Although still relatively large compared to other related taxa, the atlas vertebrae ofIchthyolestes is smaller and more gracile thanPakicetus orNalacetus, and theneural canal is disproportionately large.[2]
Theastragalus bone, which is present in the ankle ofartiodactyls and archaic whales that retained feet, has similar dimensions in both Eocene artiodactyls andIchthyolestes.[5] In addition to lending evidence to the theory of a direct relationship between artiodactyls and cetaceans,[11] this knowledge has led to two competing hypotheses about the locomotion ofIchthyolestes and otherpakicetids. The first states thatIchthyolestes andPakicetus were terrestrial andcursorial, implying that aquatic locomotor adaptation occurred after the origin ofCetacea.[9][13] The second states thatIchthyolestes andPakicetus were already semi-aquatic and cetaceans originated from an earlier unknown Eocene artiodactyl, such asElomeryx[14][15] orIndohyus,[16][17] implying that aquatic locomotor adaptations occurred before or during the origin of Cetacea.[5]
Although the postcranial anatomy ofIchthyolestes is similar to that of Eocene artiodactyls and implies cursoriality,[2][5] the assessment ofbonemorphology andmicrostructure indicate that they, and other pakicetids, were semi-aquatic likeprotocetids.[2] Hypermineralization occurs in all regions of the skeleton; in particular, thelong bones andribs had small or absent marrow cavities due to the thick cortices which developed.[2] Hypermineralization of load-bearing skeletal elements putIchthyolestes at an increased risk of fractures during prolonged terrestrial loading and this risk increased withvelocity, implying that terrestriality was limited.[2] Additionally, the dense skeletons may have allowed bottom-walking or wading in shallow pools as it would counteract buoyancy created by inflatedlungs andfur-trapped air.[2] Therefore, the retention of an artiodactyl-like astragalus does not signify full terrestriality or cursoriality inIchthyolestes.[2][5]
Fossil findings indicateIchthyolestes sharedhabitat with its relativesPakicetus andNalacetus.[18] Their fossils are usually found around river channel deposits, not marine deposits or fauna.[4] The regions around northern Pakistan and northwest India, where fossils have been recovered, are thought to have been anarid environment with ephemeral streams and moderately developedfloodplains.[19]Ichthyolestes, therefore, had an affinity for water and were either terrestrial or semi-aquatic.[4] 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.[5][2]
Ichthyolestes in thePaleobiology Database. Retrieved June 2013.
