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Zygorhiza kochii

Mammalia - Cetacea - Basilosauridae

Taxonomy
Basilosaurus kochii was named by Carus (1847) [Old number MNB 15324a-b]. Its type specimen is MB Ma 43248, a partial skull (posterior portion of skull), and it is a 3D body fossil. Its type locality is Clarksville, which is in a Priabonian coastal mixed carbonate-siliciclastic in the Yazoo Formation of Alabama. It is the type species of Basilosaurus, Zygorhiza.

It was recombined as Zygorhiza kochii by Kellogg (1936), Thurmond and Jones (1981), Fordyce (1982), Uhen (1998), Uhen (2002), van Vliet (2004), Uhen (2004), Bouetel and Muizon (2006), McLeod and Barnes (2008), Uhen (2008), Uhen (2009), Fitzgerald (2010), Marx (2011), Schouten (2011), Uhen (2013), Gingerich (2015), Velez-Juarbe et al. (2015), Uhen (2018).

Synonyms
Synonymy list
YearName and author
1847Basilosaurus kochii Carus p. 13 figs. pl. 2, Figs. 3-4
1849Zeuglodon hydrarchus Carus p. 385 figs. plates 39A, 39B
1851Zeuglodon brachyspondylus minor Müller pp. 69-80
1898Zeuglodon hydrarchus Trouessart p. 1010
1908Zygorhiza brachyspondylus True p. 13
1908Zygorhiza brachyspondylus minor True p. 13
1908Zeuglodon brachyspondylus minor True pp. 69-80
1914Zeuglodon brachyspondylum Abel pp. 203, 220
1914Zygorhiza minor Abel p. 220
1928Zygorhiza minor Kellogg pp. 40-41
1930Zygorhiza minor Hay p. 568
1936Zygorhiza kochii Kellogg p. 101
1981Zygorhiza kochii Thurmond and Jones p. 192
1982Zygorhiza kochii Fordyce p. 422
1998Zygorhiza kochii Uhen p. 38
2002Zygorhiza kochii Uhen p. 79
2004Zygorhiza kochii Uhen p. 12
2004Zygorhiza kochii van Vliet p. 143
2006Zygorhiza kochii Bouetel and Muizon p. 383
2008Zygorhiza kochii McLeod and Barnes p. 93
2008Zygorhiza kochii Uhen p. 562
2009Zygorhiza kochii Uhen p. 93
2010Zygorhiza kochii Fitzgerald p. 370 figs. Table 1
2011Zygorhiza kochii Marx p. 83 figs. Figure 2
2011Zygorhiza kochii Schouten p. 18
2013Zygorhiza kochii Uhen p. 11
2015Zygorhiza kochii Gingerich p. 164
2015Zygorhiza kochii Velez-Juarbe et al. p. 15 figs. Fig. 10
2018Zygorhiza kochii Uhen

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RankNameAuthor
kingdomAnimalia()
—Bilateria
—EubilateriaAx 1987
—Deuterostomia
phylumChordataHaeckel 1874
subphylumVertebrata
superclassGnathostomata
—Osteichthyes()
subclassSarcopterygii()
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
—Tetrapoda
—Reptiliomorpha
—Anthracosauria
subclassAmphibiosauriaKuhn 1967
—Cotylosauria()
—Amniota
subclassSynapsida
—Therapsida()
infraorderCynodontia()
—Mammaliamorpha
RankNameAuthor
—Mammaliaformes
classMammalia
—Cladotheria
—Zatheria
subclassTribosphenida()
subclassTheria
—Eutheria()
—Placentalia
—Boreoeutheria
—Laurasiatheria
—Scrotifera
—Euungulata
—Artiodactylamorpha
—Artiodactyla()
—Whippomorpha
orderCetacea
—Pelagiceti
familyBasilosauridae
genusZygorhiza
specieskochii()

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.

†Zygorhiza kochii Carus 1847
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Invalid names: Zeuglodon brachyspondylum Abel 1914 [synonym], Zeuglodon hydrarchus Carus 1849 [synonym], Zygorhiza brachyspondylus True 1908 [synonym], Zygorhiza minor True 1908 [synonym]
Diagnosis
ReferenceDiagnosis
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.
P. D. Gingerich 2015Zygorhiza kochii, the only species of the genus recognized at present, is distinctive in being the smallest archaeocete known from the Yazoo Formation of late Priabonian, late Eocene, age. Skeletally mature specimens have occipital condyles measuring 110–120 mm across their outer margins (Kellogg, 1936, p. 246–247), in contrast to contemporaneous Pontogeneus brachyspondylus (including Cynthiacetus maxwelli) measuring 142 mm (Uhen, 2005, p. 163) and Basilosaurus cetoides measuring 145 mm (Kellogg, 1936, p. 246–247). Z. kochii has posterior thoracic vertebrae (e.g., T11) measuring 69 mm in centrum length and 79 mm in transverse diameter (Kellogg, 1936, p. 143), in contrast to contemporaneous P. brachyspondylus (C. maxwelli) measuring 85+ mm in centrum length and 112 mm in transverse diameter (Uhen, 2005, p. 163), and B. cetoides measuring 302 mm in centrum length and 177 mm in transverse diameter (Kellogg, 1936, p. 246–247). Zygorhiza resembles contemporaneous Pontogeneus (including Cynthiacetus) in having unusually large vertebrarterial canals perforating the transverse processes of cervical vertebrae C3 and C4 (Kellogg, 1936, p. 133; Uhen, 2005, p. 158), which is a conspicuous difference from the smaller canals of slightly older Dorudon (Uhen, 2004, p. 69) and contemporaneous Basilosaurus (Kellogg, 1936, p. 39).