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Zygorhiza

Mammalia - Artiodactyla - Basilosauridae

Synonymy list
YearName and author
1908Zygorhiza True p. 78
1914Zygorhiza Abel p. 220
1928Zygorhiza Kellogg p. 32 figs. Table 1
1930Zygorhiza Hay p. 568
1936Zygorhiza Kellogg p. 100
1945Zygorhiza Simpson p. 100
1954Zygorhiza Moustafa p. 87
1981Zygorhiza Thurmond and Jones p. 192
1982Zygorhiza Fordyce p. 422
1986Zygorhiza Hooker p. 342
1988Zygorhiza Carroll
1993Zygorhiza Benton p. 761
1997Zygorhiza Kohler and Fordyce p. 575
1997Zygorhiza McKenna and Bell p. 369
1998Zygorhiza Uhen p. 38
2002Zygorhiza Sepkoski, Jr.
2002Zygorhiza Uhen p. 79
2003Zygorhiza Geisler and Sanders p. 27
2004Zygorhiza Uhen p. 12
2006Zygorhiza Bouetel and Muizon p. 383
2008Zygorhiza McLeod and Barnes p. 93
2008Zygorhiza Uhen p. 562
2009Zygorhiza Fordyce and Roberts p. 553
2009Zygorhiza Uhen p. 93
2010Zygorhiza Fitzgerald p. 370 figs. Table 1
2011Zygorhiza Martinez-Cáceres and Muizon
2011Zygorhiza Schouten p. 18
2013Zygorhiza Uhen p. 11 figs. Figure 8
2015Zygorhiza Boessenecker and Fordyce figs. Table 1
2015Zygorhiza Gao and Ni p. 156 figs. Table 1
2015Zygorhiza Gingerich p. 164
2016Zygorhiza Marx et al. p. 101
2017Zygorhiza Berta p. 159
2017Zygorhiza Martínez-Cáceres et al. p. 11
2018Zygorhiza Uhen

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RankNameAuthor
kingdomAnimalia()
Triploblastica
Nephrozoa
Deuterostomia
phylumChordataHaeckel 1847
subphylumVertebrata
superclassGnathostomata
Osteichthyes()
Sarcopterygii
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
Tetrapoda()
Reptiliomorpha
Anthracosauria
Batrachosauria()
Cotylosauria()
RankNameAuthor
Amniota
Synapsida()
Therapsida()
infraorderCynodontia()
Epicynodontia
infraorderEucynodontia
Probainognathia
Mammaliamorpha
Mammaliaformes
classMammalia
orderArtiodactyla()
Cetacea()
Pelagiceti
familyBasilosauridae
genusZygorhiza

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

Diagnosis
ReferenceDiagnosis
M. D. Uhen 2008Zygorhiza is much smaller than both Basilosaurus and Cynthiacetus. Zygorhiza also lacks the elongate trunk vertebrae of Basilosaurus. Zygorhiza can be distinguished from Dorudon and Chrysocetus by the presence of accessory denticles on the cingula of the upper premolars (Köhler and Fordyce, 1997).
M. D. Uhen 2013Members of the species Zygorhiza kochii have strong vertical ornamentation on the enamel of the teeth, and well-developed cingula on the premolars with additional cuspules on these cingula (e.g. Kellogg, 1936; Köhler & Fordyce, 1997). These features are not shown in the type specimen, as it lacks teeth. See the discussion below regarding the replacement of the holotype with a neotype specimen, USNM 11962, which does show this di- agnostic feature clearly (Kellogg, 1936, plates 10-15).
Additional cranial features were investigated as pos- sible diagnostic characteristics of Zygorhiza kochii. All proved to be either too difficult to assess, or proved to be variable in nature. For instance, the periotic has proven to be a good source of diagnostic characters among taxa of Neoceti, but there are a very limited number of periotics associated with teeth and skulls for Zygorhiza kochii. Some small differences in the shape of the pars cochlearis and the anterior processes were observed, but it is impossible to know whether this is due to intraspecific variation or possible interspecific variation.
Also, the area of the cranial vertex was investigated, but proved to be highly variable. Among the five skulls at- tributed to Zygorhiza that preserve this area (RMM 2739, TM 8501, USNM 11962, 16638, and 16639), all have nota- ble differences in the conformation of the sutures in this region. In RMM 2739, the premaxillae meet posterior to the external nares, excluding the anterior nasals from the narial opening. In USNM 16638, the lateral margin of the nasal presents a “notch” into which the posteriormost end of the premaxilla is sutured. TM 8501, USNM 11962 and USNM 16638 all have a large anterior process of the fron- tal (narial process of the frontal). RMM 2739, has a small anterior processes of the frontal, while USNM 16639 has none. Finally, in USNM 11962, the posterior end of the me- dial margin of the maxilla makes contact with the anterior process of the frontal, while in all of the others (this can- not be observed in TM 8501) it contacts the lateral margin of the nasal. Since none of these features seem to correlat- ed to one another, for the time being, Zygorhiza kochii is considered to be a highly variable species in these respects.