Tlaxcallicetus, Cisneros, 2018

Cisneros, Atzcalli Ehécatl Hernández, 2018, A new group of late Oligocene mysticetes from México, Palaeontologia Electronica (7 A) 21 (1), pp. 1-30 : 6-8

Tlaxcallicetus , new genus

( Figures 3-9 )

zoobank.org/ 620CDBE7-4B64-4815-A38F-F1CEA9A76958

Type species. Tlaxcallicetus guaycurae , new species.

Included species. Tlaxcallicetus guaycurae ( MU EcSj5/06/31) new species, late Oligocene, Baja California Sur, Mexico .

Diagnosis. Group of stem mysticetes characterized by a massive cranium. Broad supraoccipital with a concave surface and triangular profile ( Figure 3 ); opening of the foramen magnum with an inverted trapezoidal outline; as well a thick zygomatic process, dorsoventrally and mediolaterally ( Figure 6). The posterior end of the nuchal crest is elevated in vertical as in basilosaurids ( Figure 7 ). Tlaxcallicetus shows the following autapomorphic features: i) a periotic with a long and flattened anteroposteriorly compound posterior process (derived feature); ii) a head of the periotic (pars cochlearis, body of the periotic and anterior process) anteroposteriorly compressed and dorsoventrally expanded with ovoid form, in lateral and medial views; iii) an anterior process transversally thin like a lamina with an anterodorsal angle prominent as a linguiform outline, which is approximately more than twice the dorsoventral depth size of the pars cochlearis; iv) presence of a basioccipital crest anteroposteriorly longest and thick with a similar profile as “J”; v) a prominent exoccipital directed posteriorly, and dorsoventrally thick (~ 50 mm at posterior end) with a semicircular outline in dorsal view; vi) and presence of a non-prominent postglenoid process.

Tlaxcallicetus mainly differentiates from other described Oligocene stem Mysticeti ( Aetiocetidae , Mammalodontidae , Eomysticetidae , and Sitsqwayk , Mauicetus , Horopeta , and Whakakai ) in a periotic bone with a compound posterior process (posterior processes of bulla and periotic fused) and the ovoid head of the periotic. Tlaxcallicetus is interpreted as a chaeomysticete based on the next derived features: a compound posterior process of the periotic; transversally widened intertemporal constriction; and a postglenoid process of the squamosal posteriorly directed in lateral view ( Hernández-Cisneros et al., 2017). Thus, Tlaxcallicetus is readily distinct from toothed forms ( Llanocetidae , Mammalodontidae , Aetiocetidae ) but remnant teeth as in eomysticetids is unknown (see Boessenecker and Fordyce, 2015a). It is largely different from Eomysticetidae : in a zygomatic process of squamosal with an inflated and rounded squamosal prominence, as well as a posteriorly divergent basioccipital crest; from Sitsqwayk cornishorum , in an opening of the foramen magnum with an inverted trapezoidal outline, presence of a thick basioccipital crest that is laterally directed near to or at right angle to the long axis of the skull; from Mauicetus parki , in a deeply concave and broad supraoccipital, a posteriorly divergent basioccipital crest, a periotic with a vertical anterior keel and a reduced suprameatal area; from Horopeta umarere and Whakakai waipata , in a deeply concave and broad supraoccipital, an internal acoustic meatus as a single aperture, and a reduced suprameatal area. Tlaxcallicetus shows several plesiomorphic features: a vertical profile of the nuchal crest at the posterior part, a developed superior process of the periotic, an internal acoustic meatus as a single aperture and a presence of a low transverse crest, a straight medial profile of pars cochlearis, and a presence of fovea epitubaria. The latter features make Tlaxcallicetus different from the crown Mysticeti ( Balaenidae , Cetotheriidae , and Balaenopteroidea, sensu Marx and Fordyce [2015]).

Etymology. From tlaxcal-li (Nahuatl), which means tortilla (‘cornbread’), a Mexican iconic food, and refers to Tlaxcala ‘the place of tortillas’. From cetus (Greek) that enunciates any large sea creature. Tlaxcal-li alludes to the transversally thin anterior process of the periotic.

General Description

Size and ontogenetic age. Body length was estimated following Pyenson and Sponberg (2011) equations for stem Mysticeti and stem Balaenopteridae based on the dimension of the bizygomatic width. This latter measure was taken using the preserved dimension from the midline at the condyle to the margin of the preserved zygomatic process in Tlaxcallicetus guaycurae ( MU EcSj5/06/31). Its estimated total length is between 6.4 and 7.7 m, whereas Tlaxcallicetus sp. ( MU EcSj5/18/95) is fragmented but comparable in size to Tlaxcallicetus guaycurae under incomplete standard dimensions ( Hernández-Cisneros et al., 2017). Both specimens are considered as adults because they present well-developed cranial sutures, although they are not completely fused. Juvenile status is discarded because they display a complete occipital ossification ( Walsh and Berta, 2011), and occipital condyles without pitted or porous surface exhibiting no associated cartilage ( Boessenecker and Fordyce, 2015b).

Cranial topography. Tlaxcallicetus guaycurae ( MU EcSj5/06/31) has the most complete cranium (sensu Mead and Fordyce, 2009) but lacks rostrum, frontal, intertemporal region, and postcranial elements. Specimen Tlaxcallicetus sp. ( MU EcSj5/ 18/95) includes a broken cranium with partial portions of the posterior middle part of the supraoccipital, left parietal and squamosal, occipital condyles, basioccipital, a fragment of the left thyrohyal bone and broken left ear bones (bulla + periotic). Both specimens show damage due to mining activity, and diagenetic alteration is apparently absent. The main morphological link between Tlaxcallicetus guaycurae and Tlaxcallicetus sp. is the periotic bone, which is phenetically similar in the ovoid shape of the head of the periotic. Nonetheless, Tlaxcallicetus sp. is classified as an undetermined species due to its preservation degree. Although it shows different features (see below) from Tlaxcallicetus guaycurae , these are not enough to establish a new species taxon or to classify as one species.

In addition, the posterior process of the periotic in Tlaxcallicetus sp. is not preserved. However, a new specimen (in field yet) recently discovered in mesa El Tesoro locality shares a similar morphology to Tlaxcallicetus sp. A preliminary identification suggests affinity to Tlaxcallicetus sp. and shows a similar periotic ( Figures 2.5 , 10 ). The similar ear bone morphology suggests a close relationship between both specimens described here. Note that the ear bones are considered a good diagnostic feature to identify Cetacea ( Oishi and Hasegawa, 1994) , and the periotic bones are useful for phylogenetic analysis as well as for recognizing groups ( Geisler and Luo, 1996; Steeman, 2010).

Figures

FIGURE 2. General stratigraphy and observations at mesa El Tesoro; 1) view of the strata over the mined phosphatic Humboldt bed; 2-3) position of the gray phosphatic sandstone over the Humboldt bed; 4) fossils Tlaxcallicetus cf. guaycurae in situ in the gray phosphatic sandstone and 5) ventral view, left ear region of Tlaxcallicetus sp., field observation at mesa El Tesoro.

FIGURE 3. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, dorsal view.

FIGURE 4. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, ventral view.

FIGURE 5. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, frontal view.

FIGURE 7. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, lateral view.

FIGURE 10. Ear region and left periotic comparison between Tlaxcallicetus species, ventral view. Tlaxcallicetus sp. (MU EcSj5/18/95) (1-2), Tlaxcallicetus guaycurae (MU EcSj5/06/31) (3-4), and specimen found at mesa El Tesoro, Tlaxcallicetus sp., field observation (5-6).