Fragilicetus, Bisconti & Bosselaers, 2016

Bisconti, Michelangelo & Bosselaers, Mark, 2016, Fragilicetus velponi: a new mysticete genus and species and its implications for the origin of Balaenopteridae (Mammalia, Cetacea, Mysticeti), Zoological Journal of the Linnean Society 177 (2), pp. 450-474 : 451-452

FRAGILICETUS GEN. NOV.

Diagnosis: The diagnosis of Fragilicetus includes the presence of eschrichtiid-like and balaenopterid-like features in the same individual. Fragilicetus is distinguished from the other nonbalaenopterid mysticete families based on the presence, in the same individual, of a squamosal bulging into the temporal fossa; posterior projection of the posterolateral corner of the exoccipital; anterior placement of the posterior apex of the lambdoidal crest; squamosal cleft present and v-shaped (turning ventrally at its lateral end); abruptly depressed and flat supraorbital process of the frontal; anterior portion of temporal crest transversely elongated and forming a dorsal roof to the anterior portion of the temporal fossa; very short intertemporal region; infraorbital region of the frontal exposed dorsally between the ascending processes of the maxillae; anterior end of the parietal located more anteriorly than the posterior ends of the ascending process of the maxilla; descending suprameatal surface from the central portion of the periotic to the superior rim of the internal acoustic meatus; endocranial opening of the facial canal separated from the internal acoustic meatus by a thick crista transversa but not prolonged into a groove; triangular anterior process of the periotic; anterior process of the periotic and central portion of periotic on the same plane; groove for VII cranial nerve in posterior process reduced; anteroposteriorly short and flattened posterior process of the periotic.

Discussion: Fragilicetus velponi belongs to Balaenopteroidea (sensu Geisler & Sanders, 2003: Balaenopteroidea is defined as the clade including Eschrichtiidae and Balaenopteridae , their common ancestor, and all the descendants of this ancestor; see Deméré et al., 2005 for further details) because it has a long and definite ascending process of the maxilla, reduced interorbital region of the frontal, depressed supraorbital process of the frontal, and pars cochlearis of the periotic elongated along both the anteroposterior and transverse axes. The type of the depression of the supraorbital process of the frontal is different in Eschrichtiidae and Balaenopteridae ; although it is generally accepted that both families have a depressed supraorbital process of the frontal, the degree of development of the depression is in fact greater in Balaenopteridae . In Balaenopteridae the supraorbital process of the frontal is abruptly depressed over its whole longitudinal length but in the living Eschrichtius robustus , the depression is evident at the posterior portion of the supraorbital process whereas the more anterior portion is only slightly depressed. The depression of the supraorbital process of the frontal is more balaenopterid-like in the fossil Eschrichtioides gastaldii , in which the supraorbital process of the frontal is abruptly depressed in a manner that resembles very closely the condition observed in Balaenopteridae . Some Cetotheriidae Brandt, 1872 , such as Mixocetus elysius Kellogg, 1934b (our observations on Mi. elysius are based on the photographic illustrations provided by Kellogg, 1934b, in which the reconstructed portions are clearly evident), share the condition observed in Eschrichtius robustus but most of the cetotheriids and basal thalassotherians show a gently descending supraorbital process of the frontal. Hereafter, we use Cetotheriidae to mean Cetotheriidae s.s. of Bouetel & de Muizon (2006); see also El Adli, Deméré & Boessenecker (2014) and Bisconti (2014) for discussions on this taxon. Fragilicetus velponi differs from later-diverging Balaenopteridae in having a well-developed squamosal bulge, strongly developed exoccipital that projects posterolaterally, and in the lack of a posterior projection of the postglenoid process of the squamosal. Fragilicetus velponi differs from Eschrichtiidae in having a wide supraorbital process of the frontal, anteriorly compressed supraoccipital, and in the lack of dorsal tubercles on the supraoccipital. It is clearly different from Cetotheriidae in the following characters: (1) the posterior ends of its maxillae do not meet along the midline posteriorly and (2) the posterior ends of its maxillae are not superimposed onto the interorbital region of the frontal.

Etymology: Fragilis, Latin, fragile, in reference to the extreme fragility of the holotype skull. Cetus, Latin , whale.

Type species: Fragilicetus velponi sp. nov. This is currently the only included species.