Megaconus

Megaconus Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Life restoration of Megaconus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	†Allotheria
Genus:	†Megaconus Zhou et al., 2013
Type species
†Megaconus mammaliaformis Zhou et al., 2013

Megaconus is an extinct

Megaconus is one of the few early mammaliaforms known from a complete skeleton. The skeleton includes both the jaw bones and the teeth, which are the most informative features because they allow for comparisons with other mammaliaforms known only from dental features. Megaconus has a dentition similar to those of rodents, with large incisors at the front of the jaws and broad molars in the back. One distinguishing feature of Megaconus is a pair of enlarged premolar teeth in the lower jaw. The teeth of Megaconus have many cusps, allowing them to interlock tightly when the jaws are closed. If Megaconus is a non-mammalian mammaliaform, it is one of the most basal mammaliaforms to possess such complex teeth.^[1]

The

The genus name Megaconus means "large cusp", coming from the Latin mega ("large") and the Greek conus ("cusp"), in reference to the pair of large premolars in its lower jaw. The species name mammaliaformis references its ancestral mammalian features.^[1]

Relationships

Megaconus is part of the clade Haramiyida, which is otherwise known almost exclusively from teeth. The relationships of haramiyidans to other early mammals is contested.

One idea is that haramiyidans are close relatives of Multituberculata, the most diverse group of Mesozoic mammals, and that both are part of the larger clade

A second hypothesis holds that haramiyidans originated before the appearance of crown group Mammalia as more basal members of the larger clade Mammaliaformes and that multituberculates are deeply nested within the crown group, splitting up Allotheria. This phylogeny was supported by the phylogenetic analysis that included Megaconus. Features of Megaconus that link it with basal mammaliaforms include a groove in the lower jaw holding the middle ear bones (in multituberculates and other true mammals, these bones are completely separated from the lower jaw) and a calcaneum or ankle bone that lacks an enlarged heel. Below is a cladogram modified from the analysis:^[1]

A 2022 study found it to be a basal allotherian instead, due to lacking apomorphies characteristic of euharamiyidans.^[3] Furthermore, a 2020 study found it to fall outside Mammaliaformes entirely, as the sister taxon to Tritylodontidae.^[4]

References