Tlaxcallicetus sp.

Cisneros, Atzcalli Ehécatl Hernández, 2018, A new group of late Oligocene mysticetes from México, Palaeontologia Electronica (7 A) 21 (1), pp. 1-30 : 16-21

Tlaxcallicetus sp.

Figures 11-21

Diagnosis. Tlaxcallicetus sp. is different from others known Oligocene archaic chaeomysticetes (eomysticetids, Sitsqwayk cornishorum , Mauicetus parki , Whakakai waipata , and Horopeta umarere ) because it presents a long, thick, and posterolaterally directed J-shaped basioccipital crest. It also has an inverted trapezoidal foramen magnum opening, a periotic ovoid head with a transversally narrow anterior process similar to a lamina, and a prominent anterodorsal angle. Tlaxcallicetus sp. differs from Tlaxcallicetus guaycurae in the following autapomorphic features: a slender basioccipital crests (~ 22.5 mm width near its posterior end) directed posterolaterally in obtuse angle>90° and aligned to the medial part of the paroccipital process of the exoccipital. Moreover, the plain surface behind the foramen magnum suggests that it lacks an external occipital sulcus, and also lacks a dorsal condyloid fossa and a supracondylar septum. However, both specimens share general features, but the main observable autapomorphic feature is the presence of a periotic ovoid head with a transversely thin anterior process.

Material. MHN-UABCS_EcSj5/18/95, partial cranium with both broken left periotic and bulla, and fragment of left thyrohyal and fragmentary bones. Collected by G. Gonzalez-Barba, May 4, 1994.

Type locality, horizon and age. The same as Tlaxcallicetus guaycurae (see above).

Description

Skull. A fragment of the cranium is preserved as well as a piece of the hyoid bone (left thyrohyal). Tlaxcallicetus sp. partial cranium's size is comparable to Tlaxcallicetus guaycurae . The fragment displays a dorsal posterior part of the supraoccipital, a fragmentary posterior exoccipital piece, an incomplete ventral basioccipital as well as a fragment of the left ear. From a lateral view (left side), fractions of the parietal and the squamosal can be observed ( Figure 11 ). The supraoccipital piece is smooth with a flat to slightly convex surface, and presents a concave profile from a posterior stand-up point. The incomplete squamosal fossa in Tlaxcallicetus sp. is similar to the anteroposteriorly short and transversally broad squamosal fossa of Tlaxcallicetus guaycurae . The parietal/squamosal suture is the same as in the Tlaxcallicetus guaycurae , and the broken nuchal crest has a vertical profile in the rear. Posteriorly, the supraoccipital fuses with the exoccipital, and lacks a well-defined supracondylar septum. The left occipital condyle is relatively thick (37.5 mm). Under the occipital condyle, the exoccipital flows into the left basioccipital crest, and the jugular notch is more or less conserved. The basioccipital is mostly broken. Only a long and narrow left basioccipital crest with an eroded surface is preserved. Anteromedially, the crest forms the margin of the broken pterygoid sinus fossa. The apex of the basioccipital crest is bulbous and posterolaterally extends in an obtuse angle>90°. The pterygoid sinus fossa is deep but anteriorly incomplete. The foramen ovale is not preserved ( Figure 12 ), and the falciform process is broken.

Periotic. Only a fragment of the left periotic is preserved including a piece of the pars cochlearis, the body of periotic, and the anterior process ( Figures 16-18 ). The head of the periotic has an ovoid shape, anteroposteriorly compressed and dorsoventrally deep. It rests on the periotic fossa and apparently was not firmly ankylosed to the squamosal. However, the linguiform anterodorsal angle is close to the squamosal wall through a thin lamina of bone. The anterior process is dorsoventrally deep (total estimate ~ 75 mm); more than twice the depth of the pars cochlearis in Whakakai waipata , Horopeta umarere , and Mauicetus parki . It constitutes a transversally thin lamina (between ~ 10.5 mm at the base and ~ 0.5 mm on the top). The anterior process has an ovoid outline with a vertical anterior keel. Both, the anterodorsal angle and the anteroventral angle are broken, and nearly all of the superior process is eroded. The anterior incisure marks the limit between the pars cochlearis and the anterior process, and is slightly vertical. Above of the anterior incisure a small hiatus Fallopii is present. From a lateral view, the periotic body's wall surface is smooth with some vertical small creases near the broad and indistinct anteroexternal sulcus. The pars cochlearis has a squarerectangular outline with a prominent and blunt anterointernal angle ( Figure 16.4 , 17.4 ). The suprameatal area is small and compressed halfway through the internal acoustic meatus, the aperture for vestibular aqueduct, and the medial posterior sulcus. The internal acoustic meatus is anterolaterally directed, oval, and deep, and its anterior margin is overhanging the facial canal. The spiral cribriform tract is a broad opening in the bottom of the internal acoustic meatus. On its lateral wall a stepped foramen singulare, as well as a medium-size facial canal are present. The former is isolated by a low transverse crest. On the posterior surface of the pars cochlearis, an oval aperture for vestibular aqueduct can be observed. Also, another medially minuscule aperture of the cochlear aqueduct is present. Below to the broken posterodorsal angle, a broad medial posterior sulcus is present. On the posterior margin of the pars cochlearis, an incomplete stylomastoid fossa is preserved as a depressed surface.

Bulla. Only the left bulla is preserved, and diagnostic details from the dorsal region are missing ( Figure 19-20 ). It is compressed dorsoventrally. In a dorsal view, the bulla has a rectangular-ovoid profile with a straight medial margin, and its anterior border obliquely truncated. The tympanic cavity extends anteroposteriorly. The involucrum is dorsoventrally thick posteriorly and becomes narrow anteriorly; with fine creases covering its middle part. Medially, the involucrum is posteriorly bulbous with a low anterior portion separated by a step. The involucral ridge extends anteroposteriorly closer to the ventral margin. From a ventral view, it meets the medial edge of the bulla. Dorsally, the base of the inner posterior pedicle is broken and located near the posterior edge. The outer lip is broken. From a ventral view, the outer posterior and inner posterior prominences are not well defined and are separated by an interprominental notch; a shallow median furrow extends from it.

Thyrohyoid. The proximal portion of the left thyrohyoid is preserved ( Figure 21 ). In its cross section, it is cylindrical and thick. The anterior end was attached (not fused) to the posterolateral face of the basihyoid. This was inferred based on the oval pitted surface of a possible cartilaginous joint.

Figures

FIGURE 11. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, dorsal view.

FIGURE 12. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, ventral view.

FIGURE 13. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, frontal view.

FIGURE 14. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, posterior view.

FIGURE 15. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, lateral view.

FIGURE 16. Left periotic with anatomical terms, Tlaxcallicetus sp. (MU EcSj5/18/95), medial (1), lateral (2), ventral (3), dorsal (4), posterior (5), and anterior (6) views.

FIGURE 17. Left periotic, Tlaxcallicetus sp. (MU EcSj5/18/95), medial (1), lateral (2), ventral (3), dorsal (4), posterior (5), and anterior (6) views.

FIGURE 18. Left periotic, Tlaxcallicetus sp. (MU EcSj5/18/95), closer view of internal acoustic meatus, dorsal view.

FIGURE 19. Left bulla with anatomical terms, Tlaxcallicetus sp. (MU EcSj5/18/95), ventral (1), dorsal (2), lateral (3), medial (4), posterior (5), and anterior (6) views.

FIGURE 20. Left bulla, Tlaxcallicetus sp. (MU EcSj5/18/95), ventral (1), dorsal (2), lateral (3), medial (4), posterior (5), and anterior (6) views.

FIGURE 21. Left thyrohyoid, Tlaxcallicetus sp. (MU EcSj5/18/95), dorsal (1), anterior (2), and posterior (3) views.