Cynthiacetus Uhen, 2005

Martínez-Cáceres, Manuel, Lambert, Olivier & Muizon, Christian de, 2017, The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru, Geodiversitas 39 (1), pp. 7-163 : 15-16

Genus Cynthiacetus Uhen, 2005

TYPE SPECIES. — Cynthiacetus maxwelli Uhen, 2005 by original designation.

INCLUDED SPECIES. — Cynthiacetus maxwelli Uhen, 2005 ; C. peruvianus Martínez-Cáceres & Muizon, 2011 .

ETYMOLOGY. — Cynthia - refers to the name of the quarry in which the holotype of the type species was discovered, the Cynthia Clay Pit. - cetus derives from the Greek ΚῆΤΟς, meaning whale.

EMENDED DIAGNOSIS. — Large basilosaurid lacking the vertebral elongation observed in Basilosaurus ; skull slightly shorter than Basilosaurus and significantly longer than most other dorudontines. Cynthiacetus differs from all other basilosaurid genera in having: 1) nasals which taper strongly anteriorly, even more than in Basilosaurus and Dorudon ; 2) an atlas with a high, massive and dome-shaped neural arch; 3) significantly larger vertebrarterial foramina on the cervical vertebrae (C3-C7 series); and 4) absence or reduction of the anteroventral expansion of the parapophysis in the transverse processes of middle cervical vertebrae (C3-C5). As compared to Basilosaurus , Cynthiacetus differs principally in having a proportionally more elongated rostrum; less ornamented labial and lingual surface of premolars and molars; two large infraorbital foramina rather than one; larger neural canal in thoracic, lumbar and caudal vertebrae; proportionally shorter humerus, radius and ulna; anteriorly convex anterior border of the scapula; proportionally larger femur; and absence of vertebral elongation on thoracic, lumbar and caudal vertebrae.

Cynthiacetus is considerably larger than Saghacetus and differs mainly in having a wider occipital shield, with less pronounced transverse compression of the nuchal crests; two prominent occipital tuberosities; nasal which extends posterior to the maxilla; large anterior process of the frontal separating both posterior ends of the nasals; more inflated braincase; a proportionally shorter i1; a more densely wrinkled enamel on both lingual and labial surfaces of teeth; and shorter posterior thoracics and anterior lumbars.

Dorudon and Cynthiacetus are very similar in overall shape. Except for the size and other characters cited above, Cynthiacetus differs from Dorudon principally in having a proportionally smaller forelimb and a slightly lower occipital shield bearing two tuberosities instead of one.

As compared to Zygorhiza , Cynthiacetus has nuchal crests being posterodorsally oriented instead of being anterolaterally oriented; posterior end of the nasal being posterior to the maxilla; larger and wider anterior process of the frontal separating the posterior end of the nasals at midline; less ornamentation on the enamel; and less transversely expanded braincase.

DISCUSSION

Before 2005, the only large (c. 10 m long) “dorudontine” was Pontogeneus brachyspondylus (see above and Müller 1849; Kellogg 1936; Slijper 1936). Uhen (2005) and Gingerich (2007) increased to three the number of large dorudontine genera (cf. above). Recently, Gingerich (2015) proposed to synonymize Cynthiacetus maxwelli with Pontogeneus brachyspondylus . However, Pontogeneus is regarded here as a Basilosauridae incertae sedis since the type specimen of P. brachyspondylus is a single cervical vertebra that cannot be distinguished from both Cynthiacetus and Masracetus (but see discussion in Gingerich 2015 for an alternative conclusion). A more accurate diagnosis was given for Cynthiacetus by Martínez-Cáceres & Muizon (2011) who pointed out various autapomorphies of the genus. However, this diagnosis does not differentiate Cynthiacetus from Masracetus (which was not included in the phylogenetic analysis at this time). They are regarded here as different genera but this should be confirmed by a detailed comparison with a new undescribed specimen referred to Masracetus ( Gingerich 2007) . If Cynthiacetus and Masracetus are synonymized, Masracetus should be considered as a junior synonym and Cynthiacetus would become the only large dorudontine valid genus.