Ichthyoconodon

Ichthyoconodon Temporal range: 145–139 Ma PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Class:	Mammalia
Order:	Eutriconodonta
Family:	Triconodontidae
Subfamily:	Alticonodontinae
Genus:	†Ichthyoconodon Sigogneau-Russell, 1995
Type species
Ichthyoconodon jaworowskorum Sigogneau-Russell, 1995

Ichthyoconodon is an extinct genus of theriimorph mammal from the Lower Cretaceous of Morocco. Usually considered an eutriconodont, it is notable for its oceanic location and atypical shape, indicative of potentially unusual ecological niches; some researchers think that it may be the first marine mammal, or part of a bizarre clade of flying mammals.

Description

Ichthyoconodon is known primarily from isolated teeth from Anoual Syncline sediments of Morocco, in particular strata thought to date to the Berriasian. These teeth, mostly molars, possess many synapomorphies associated with triconodontid eutriconodonts, but its cusps are notoriously sharp for the group's standards and are rather specialised. They belong to animal in life possibly as large as a platypus.

Etymology

Ichthyoconodon essentially means "fish cone tooth", from the greek ιχθυς, "fish", κῶνος, "cone", and ὀδών, "tooth". The type species, I. jaworowskorum, was named "in honour of Zofia Kielan-Jaworowska and Zbigniew Jaworowski for their generous hospitality on many occasions".

Relationships

Ichthyoconodon has been found to be a eutriconodont, though the degree of speciation has been noted as being difficult to assess its status as such. Rose et al. found some skepticism about a eutriconodont identity, and some studies have suggested an identity not even as a mammal, but as a pterosaur, but most recent phylogenetic studies favour a eutriconodont identity.

The most recent phylogenetic studies favour a close relationship with Volaticotherium and Argentoconodon within the triconodontid clade Alticonodontinae.

Ecology

Ichthyoconodon's teeth are located in marine deposits, alongside taxa like hybodontid sharks, ornithocheirid pterosaurs, ray-finned fish and sea turtles, as well as several terrestrial taxa like theropods. Unlike other mammal teeth, including other contemporary teeth such as those of Hahnodon, which show some degree of degradation, Ichthyoconodon teeth are not significantly modified, ruling out the possibility that the animal was carried over from river deposits. This means that the mammal either died in situ or was only carried over for a short distance.

Because the teeth of Ichthyoconodon are rather sharp and convergent in some details to the teeth of piscivorous mammals like otters and seals, some researchers have suggested that it was a marine mammal. If this is the case, it is by far one of the two earliest known mammal species to have adapted to live in the sea, alongside the contemporary Dyskritodon amazighi. The Jurassic and Cretaceous docodonts like Castorocauda and Haldanodon, Early Cretaceous monotremes and the Late Cretaceous Didelphodon have also specialised for an aquatic lifestyle, but Ichthyoconodon and Dyskritodon amazighi would have been the only known Mesozoic mammals to have foraged in the sea.

Alternatively, Ichthyoconodon may have been volant, based on its relationship with the gliding Volaticotherium and the possibly also volant Argentoconodon. The presence of Argentoconodon in South America, Volaticotherium in Asia and Ichthyoconodon in North Africa in such a relatively close span of time suggests that there may have been a clade of Jurassic-Early Cretaceous flying triconodonts.

Either situation would have been exceptional for a Mesozoic mammal, further adding to the variety of ecological niches these animals occupied.