Cochimicetus convexus, Cedillo-Avila & González-Barba & Solis-Añorve, 2025

Cochimicetus convexus gen. et sp. nov.

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Etymology. Cochimi from the indigenous nomads from north of Baja California Sur, combined with cetus, the Latin word for “whale”. Convexus is a word in Latin that means convex form; it is illustrated in the anterior tip of the tympanic bulla, which differs from the pointed shape of other eomysticetids.

Holotype. MHN-UABCS-Sg6/71/208, with a slightly fragmented cranium and mandibles, was collected by Gerardo González Barba, Alberto Epelboim Jarovinsky, José Alberto David Aguilera, Ali González G., A. Ehecatl H. Cisneros, and F. Garza in 2011. This specimen is stored in the Paleontological Collection of Vertebrates at the Universidad Autónoma de Baja California Sur, Mexico.

Diagnosis. Cochimicetus convexus is classified as a Neoceti based on several derived characteristics: a partially open mesorostral groove; an anteroposteriorly elongated rostral portion of the maxilla; the presence of an anteorbital process of the maxilla; an anterodorsally inclined supraoccipital shield; and a narrow intertemporal region. It is further identified as a Mysticeti due to specific features, including the dorsoventrally thin posterolateral region of the maxilla on the rostrum; the apex of the zygomatic process of the squamosal closely opposed to the postorbital process of the frontal; and an external occipital crest restricted to the anterodorsal half of the supraoccipital shield. Cochimicetus convexus and eomysticetids differ from toothed mysticetes in their lack of large emergent teeth during adulthood, as well as in having a significantly elongated rostrum and a kinetic maxilla. Additionally, C. convexus differs from Maibalaena nesbittae in lacking frontal-parietal sutures that converge posteriorly, with the frontals penetrating between the parietals. The posterior border of the nasal is not aligned with the preorbital level. Furthermore, it diverges in the apex of its occipital shield, which represents the highest point of the skull, and in the triangular coronoid process of the mandible, which is anteroposteriorly longer than it is dorsoventrally tall. Cochimicetus convexus also differs from Sitsqwayk cornishorum in several ways: it does not have a dorsal surface of the nasals that forms a sagittal keel; it lacks an external occipital crest preserved as a blunt ridge that divides the anterior half of the occipital shield into two dorsolaterally sloping facets; there is no tympanic bulla in the lateral lobe extending posteriorly to the medial lobe, with the two areas separated by a notch; and it possesses a mandibular condyle and neck that are medially curved, giving the mandible a sinusoidal profile in dorsal view. Cochimicetus convexus is identified as a basally branching Eomysticetidae based on the following characteristics: the absence of a postorbital ridge; lack of a supramastoid crest along the entire zygomatic process of the squamosal; a zygomatic process of the squamosal with parallel medial and lateral margins; the development of secondary squamosal fossa; elongated nasals; the inclusion of a frontal with anteromedial projection; and an anteroposteriorly long and transversely narrow intertemporal region with an oval temporal fossa (longer anteroposteriorly than transversely wide). Cochimicetus convexus differs from all Eomysticetidae in the following combination of attributes: the tympanic bulla anterior tip is round in the dorsal or medial view and diverges from the pointed shape in other eomysticetids; it has a pronounced parietal squamosal suture; in the tympanic bulla, the orientation of the lateral furrow is anteroventrally orientated; the tympanic bulla position of the main ridge in the ventral view is the medial and lateral sides of the bulla; in the maxilla, nutrient foramina, and sulci are absent; the medial contact of the premaxilla along 2/3 of the rostrum is mostly separated from the rostrum and is medially fused from the anterior tip; the anteorbital notch is absent; and the external occipital ridge is absent.

Locality. The skull of Cochimicetus convexus was collected near Mount “Pilón”, situated between the towns of San Isidro and La Purísima (26°11'51.036"N, 112°3'28.800"W) in Baja California Sur, Mexico. This area is part of the San Gregorio Formation (late Oligocene, Chattian, 27.2–23.4 Ma), which consists of a sequence of interbedded phosphatic siliceous shale, diatomite, pelletoidal phosphatic sandstone, and rhyolite tuff. An age of 25 Ma was determined by Hausback (1984) using K– Ar radiometric methods on biotite from a tuff bed at Arroyo San Hilario. Additionally, four other tuffs in the San Gregorio Formation in the La Purísima area yielded K– Ar dates ranging from 27.2 to 22 Ma. On the other hand, evidence of diatomite indicates a late Oligocene –early Miocene age in the La Purísima area ( McLean et al., 1987). The specimen was found exposed at a depth of 25 m within a sequence interbedded with pelletoidal sandstone, siliceous shale, and phosphatic conglomer- GoogleMaps

CEDILLO-AVILA, GONZÁLEZ-BARBA, & SOLIS-AÑORVE: AN EOMYSTICETIDAE FROM MEXICO ate ( Figure 1E View FIGURE 1 ). Finally, Föllmi et al. (2019) obtained LA-ICP-MS zircon ages from the volcanic ash layers, which range between 27.84 and 21.21 Ma.

Description

Cochimicetus convexus is a relatively well-preserved skull with two fragmented mandibles fused to the cranium ( Figure 2 View FIGURE 2 ). Nevertheless, it has diagnostic elements. The total condylobasal length is 147 cm, and the bizygomatic width is 51 cm. The occipital shield is trefoil-shaped, without a crest, and has two tympanic bullae in situ; the left bulla was not included in the description.

Rostrum. In both dorsal and ventral views, the skull is fragmented in the rostral region, which complicates the visibility of the sutures, while the posterior region is well preserved ( Figures 3–4 View FIGURE 3 View FIGURE 4 ). In the ventral view, a palatal keel is formed by the vomer, and the medial margins of the maxilla are well developed in the rostral area. The maxilla is clearly visible from the ventral view, although the contact with the palatine is not very discernible due to the presence of fractures. Nevertheless, it can be inferred that the parietal suture exhibits an irregular shape in the anterior region of the basicranium.

In the anterior portion of the rostrum, it is difficult to identify the protrusion of the premaxilla, as this area is eroded in the ventral view. The vomer is exposed along the length of the rostrum, becoming more prominent in the anterior region. Posteriorly, the crest of the vomer is well defined, high ( Figure 3 View FIGURE 3 ), and narrow at the edges, with its most posterior point located anterior to the edge of the pterygoid of the hamulus.

Maxilla. In the dorsal view, the premaxilla extends beyond the maxilla. From the most anterior edge,

PALAEO- ELECTRONICA.ORG the maxilla is flat ( Figure 2 View FIGURE 2 ), narrow, and widens anterolaterally with a concave margin lateral. The contact between the frontals and the maxilla is unknown, but the left maxilla presents an exposure with the frontal supraorbital process; the ascending process of the maxilla is moderately long. In the ventral view, the maxilla’s both sides are very fragile and heavily eroded. The maxilla is narrow at its anterior edge and widens anterolaterally. Additionally, there is no visible foramina or sulci on its surface.

Premaxilla. In the dorsal view, the premaxilla is the anterior-most tip of the rostrum ( Figure 2 View FIGURE 2 ), with a length of 100.5 cm. The premaxilla has a sinuous form, is wide at the anterior and posterior tip, and is mediolaterally narrow around the nasal fossa. The premaxilla is very eroded ventrally.

Nasals. In the dorsal view, the nasals are flat and long ( Figure 3 View FIGURE 3 ), with parallel margins to the nasofrontal suture. In lateral and dorsal views, the nasal suture is visible and “m” shaped. The posterior bor- der of the nasals is at the same level as the premaxilla. It is unclear if a fusion with the maxilla exists and is narrower.

Sphenoids and pterygoid. In the ventral view ( Figures 4–5 View FIGURE 4 View FIGURE 5 ), the basisphenoid is damaged, and the alisphenoid is not exposed. However, the pterygoid has a semi triangular form and a deep sinus foramen. The best-preserved pterygoid is the left one, while the right pterygoid shows only fragments of the pterygoid hamulus. In general, the pterygoid is broad and long. It has a wide sinus fossa. The pterygoid hamulus is posteriorly directed and well developed in a finger shape.

Frontals. In the dorsal view, the premaxilla-nasofrontal and nasofrontal suture are corrugated. Only the left part is preserved ( Figures 3 View FIGURE 3 , 5 View FIGURE 5 ). The lateral view, it has a triangular shape, but the preorbital and postorbital processes are lost, and probably has no alveoli or foramina either. A slight remnant of a broken orbitotemporal crest, on the left edge of the anterior margin of the supraorbital process for the frontal, continues until the middle of the supraorbital process of the frontal.

Parietals. In the dorsal view, the parietals are long from the anterior margin of the occipital to the fronto-parietal suture ( Figures 3 View FIGURE 3 , 5 View FIGURE 5 ). The intertemporal region is constricted, and presumably, there is a thin, short sagittal crest, but poor preservation does not allow for certainty. The temporals are thick and concave, which is why they are oversized in the intertemporal region. The nuchal crest extends over the parietals but does not layer over the temporal fossa and is the most elevated point. The temporal fossa are larger than they are wider and have a notable subtemporal crest.

Squamosal. In the dorsal view, the posterior margin of the squamosal fossa extends laterally to the supramastoid crest; it does not extend to the tip of the zygomatic process due to the presence of the squamosal prominence ( Figures 3–4 View FIGURE 3 View FIGURE 4 ). The parietal-squamosal suture is prominent and elongated and ends in the subtemporal crest. In the lateral view, the suture of the squamosal and parietal descend to approximately the level of the posteriormost region of the pterygoid at its lateral border. The zygomatic process is well preserved (left side) and elongated, with a cylindrical shape (anteroposteriorly) slightly bent anterolaterally. The secondary fossa of the squamosal is at the end of the ridge formed by the squamosal prominence; it is circular and shallow ( Figure 4 View FIGURE 4 ).

In the ventral view ( Figures 5–6 View FIGURE 5 View FIGURE 6 ), the disruption of the mandible does not allow us to observe the total length of the glenoid fossa; thus, it cannot be measured. The glenoid fossa is subtly concave, and the postglenoid process is vertical with a superior semicircular margin. It is concave in the lateral view, and from the superior tip, it becomes thick towards the bottom.

Basioccipital and stylohyoid. In the ventral view, the basioccipital is broad, compared to the basisphenoid, and the basioccipital has a fracture in the middle, while the suture of the basioccipital can be seen laterally with a straight shape ( Figures 4 View FIGURE 4 , 7 View FIGURE 7 ). The left basioccipital crest is exposed with a slightly oval and robust shape, with a straight lateral border and a convex medial border. The right basioccipital crest is inconspicuous because the stylohyoid, together with the tympanic bulla, has pushed it to the lateral border. Above these are the stylohyoid bones. No other bones of the hyoid apparatus were found, but the position of the stylohyoid is considered natural. Stylohyoids are long and robust with a length of about 20 cm and a width of 3.5 cm measured medially. The stylohyoids are located on the basioccipital.

Supraoccipital and exoccipital. In the dorsal view, the supraoccipital is triangular and lower than the nuchal crests; it has no external ridge. The occipital shield is trefoil-shaped due to concave tubercles (in the posterior view), and it has no ridge dividing it into two halves; it is about 11.5 cm wide ( Figures 4 View FIGURE 4 , 7 View FIGURE 7 ). The occipital condyles are rounded but eroded, but in the dorsal view, they protrude from the skull, although not very far (14.7 cm). The foramen magnum is rounded 5.8 cm. In the posterior view, the exoccipital is small compared to the squamosal (in the posterior view), with a width of 38 cm. In the ventral view, the paroccipital is well exposed on the right side, and it lies well anterior to the occipital condyles and has a ventral direction. It is thin at its margin.

Tympanic bulla. The tympanic bulla was found in situ ( Figures 5–6 View FIGURE 5 View FIGURE 6 ) and was subsequently removed with great care for a more thorough analysis. In the dorsal view, the tympanic bulla has is pear-shaped, with a rounded anterior tip that widens posteriorly. In the posterior view, the elliptical foramen is not visible due to erosion in that region ( Figure 7 View FIGURE 7 ). The lateral lobe extends further posteriorly than the medial lobe, with both lobes separated by a notch. Along the outer margin of the medial lobe, the ventromedial ridge is prominent, extending almost 75% of the total length. A median furrow is incised in the posterior part of the lateral lobe and continues straight to the middle section of the tympanic bulla.

In the ventral view, the anterior and posterior pedicles are indicated by small fragments. The tympanic bulla appears elongated in the lateral view, with a rounded anterior tip. The anterior and posterior parts are somewhat proportional, but the posterior part is more pronounced, featuring a welldefined lateral furrow. The sigmoid process is located further posteriorly, curving slightly within the lateral furrow and extending much further back within the bulla. The overlaps in the posterior portion of the lateral lobe are also rounded.

In the medial view, the involucrum is narrow at the anterior end, and the dorsal margin is mostly smooth, with creases along its edge. At the most

CEDILLO-AVILA, GONZÁLEZ-BARBA, & SOLIS-AÑORVE: AN EOMYSTICETIDAE FROM MEXICO posterior part, the dorsal margin exhibits a “step” that transitions into a bulbous section. The sigmoid process is positioned immediately posterior to the outer posterior pedicle, while the inner posterior pedicle is closely compacted with the median furrow. Importantly, the tympanic bulla was carefully extracted using an air script, leaving remnants still visible in the skull. As observed in the lateral view and the medial furrow in the posterior view, both lobes share similar widths, as well as the shapes of the inner lip/tip and posterior pedicle.

Mandibles. Both mandibles are preserved in situ, although the left mandible, which is approximately 139 cm long, is better preserved. It has several fractures along its body ( Figure 3 View FIGURE 3 ).

In general, the mandible in the dorsal view has a slight medially directed curvature, and the curvature is mainly observed in the middle and distal regions of the mandible. The mandible in the medial or lateral view narrows at the anterior region and widens in the posterior region. In the ventral view, the mandibular foramen is observed to be wide; it is wider anteroposteriorly than it is high. The mandibular condyle is visible in the posterior or medial view, and its posterior edge is rounded and broad. The neck of the mandible is concave, forming the coronoid process at the top. The coronoid process is eroded in the upper region, but it is possible to trace the convex and rounded shape of the edge of the coronoid process, which shows no posterior deviation. In the dorsal view, the coronoid process has a medial displacement. In the lateral view, a mental foramen is visible in the anterior part of the mandible (see Figures S1–S View FIGURE 1 2 View FIGURE 2 ).