Paleontología Mexicana

3. Geological Setting

The fossil MU_EcSj1/29/142 was collected from the San Juan Member outcrops (El Cien Formation, late Oligocene; Fischer et al., 1995) at the locality Mesa del Tesoro (24.47366 N, -110.69646 W), which is a rich fossiliferous site 10 km north of San Juan de la Costa village in Bahía de La Paz, located ~50 km northwest from La Paz city, Baja California Sur, Mexico (Figure 1). The San Juan Member is the lower section of the El Cien Formation, which is composed of mudstones, siltstones, sandstones, conglomerates, coquinas, tuffs and phosphorites (Fischer et al., 1995). The strata show multiple transgressive and regressive sedimentary cycles, which end at the Oligocene/Miocene boundary (Fischer et al., 1995), and indicate coastal environments not deeper than 200 m suggesting shelf environments, coastal lagoons, estuaries and deltas (Schwennicke, 1994). The age of the San Juan Member has been estimated at ~28 to 25 Ma (Hausback, 1984; Applegate, 1986; Kim & Barron, 1986; Smith, 1991; Fischer et al., 1995; Drake et al., 2017; Galván-Escovedo et al., 2020).

In the San Juan de la Costa area, many cetacean fossils have been found within the numerous phosphatic beds from the San Juan Member’s middle section due to the mining activity that exploits the phosphatic beds and from palaeontological research (Hernández-Cisneros et al., 2017; Hernández-Cisneros, 2018). However, several records lack stratigraphic data because they were obtained from reworked material or are by-product mine spoils, e.g., MU_EcSj1/29/142. The eomysticetid, MU_EcSj1/29/142, likely belongs to an unknown phosphatic bed from the middle part of the stratigraphic section at the Mesa del Tesoro, perhaps close to the Humboldt bed, the main exploited phosphatic bed at the San Juan de la Costa zone (see Fischer et al., 1995; Hernández-Cisneros, 2018). Hence, due to the unknown stratigraphic data, we give an age between 25 to 28 Ma for MU_EcSj1/29/142 same as the estimated age for the San Juan Member at San Juan de la Costa area (Fischer et al., 1995; Drake et al., 2017; Galván-Escovedo et al., 2020).

4. Systematic Palaeontology

Order Artiodactyla, Owen (1848)

Suborder Whippomorpha, Waddell et al. (1999)

Infraorder Cetacea, Brisson (1762)

Pavorder Mysticeti, Gray (1864)

Section Chaeomysticeti, Mitchell (1989)

Family Eomysticetidae, Sanders and Barnes (2002b)

Genus Eomysticetus, Sanders and Barnes (2002b)

cf. Eomysticetus sp.

Figures 2–8.

Figure 2. Skull, MU_EcSj1/29/142 (cf. Eomysticetus sp.), dorsal view. (a) Reconstruction lines of the skull based on the preserved material. (b) Anatomical details. (c) Fragment of the left mandible, medial view.

Figure 3. Nasal region, MU_EcSj1/29/142 (cf. Eomysticetus sp.), skull, dorsal view.

Figure 4. Skull, MU_EcSj1/29/142 (cf. Eomysticetus sp.), ventral view and anatomical details.

4.1 Material

MHN-UABCS_EcSj1/29/142, partial skull, atlas and axis, third and fourth cervical vertebrae, eight incomplete thoracic vertebrae, approximately fourth fragmented lumbar vertebrae and several bone fragments. The specimen is housed in the palaeontological collection from the Universidad Autónoma de Baja California Sur. The specimen was collected by Gerardo González Barba, Lawrence G. Barnes and James Goedert, in November of 1999.

4.2 Locality

Mesa del Tesoro, San Juan de la Costa, La Paz, Baja California Sur, Mexico. (24.47366 N, -110.69646 W). San Juan Member, El Cien Formation.

4.3 Horizon

From an unknown phosphatic bed of the middle section at Mesa del Tesoro, around 35 to 60 m level in the stratigraphic column of the San Juan Member (see Hernández-Cisneros, 2018).

4.4 Age

Late Oligocene, ~28 to 25 Ma (Fischer et al., 1995; Drake et al., 2017; Galván-Escovedo et al., 2020).

4.5 Diagnosis

The specimen, MU_EcSj1/29/142, is classified as an eomysticetid animal (Sanders & Barnes, 2002b; Boessenecker & Fordyce, 2016) by having: long and slender skull, an elongate and transversally narrow intertemporal region with exposures of the frontal and parietal, elongate nasal bones; narrow and elongated rostrum, oval and long temporal fossa. The specimen MU_EcSj1/29/142 share with Eomysticetus whitmorei Sanders & Barnes, 2002b: an anterolaterally concave margin of the occipital shield in dorsal view (an autapomorphy for Eomysticetus whitmorei; Boessenecker & Fordyce, 2016), a nasal lateral process and a sharp triangular supraoccipital. Also, the specimen, MU_EcSj1/29/142, has similar morphology with Eomysticetus carolinensis Sanders and Barnes, 2002b. However, the holotype of Eomysticetus carolinensis represents a possible juvenile ontogenetic state and a likely synonym regarding Eomysticetus whitmorei (Boessenecker and Fordyce, 2016). Thus, it is a challenge to suggest morphological affinities due to ontogenetic changes (Tsai & Fordyce, 2014; Boessenecker & Fordyce, 2015c). On the other hand, it is similar to Micromysticetus rothauseni Sanders & Barnes, 2002a, because it has a relatively short intertemporal region, broad and parallel premaxilla along nasals (Sanders & Barnes, 2002a; Boessenecker & Fordyce, 2016). Furthermore, the specimen, MU_EcSj1/29/142, is different from other eomysticetids by having a wide mesorostral groove, a broad premaxilla with an ascending process extended farther than the posterior ending of the nasals, long rectangular nasal bones with wedge posterior end, narial process, and a neural spine with an elliptical foramen and bifurcated end.

The following features separate the specimen MU_EcSj1/29/142 from the crown Mysticeti: transversely narrow intertemporal region, extremely long nasals (> 65% bizygomatic width), rigid nasals and premaxillae sutures, a dorsoventrally shallow palatal keel, and axis without vertebrarterial canal (for an extensive review of Eomysticetidae see Boessenecker & Fordyce, 2015a; 2016).

5. Description

5.1 Ontogenetic age and size

The specimen, MU_EcSj1/29/142, display well-developed and fused cranial sutures, well-fused vertebral epiphyses and neural arches, and a bifurcated neural spine in the axis indicating likely physical maturity of an adult animal (Perrin, 1975; Walsh & Berta, 2011; Boessenecker & Fordyce, 2015a). The body size is difficult to estimate due to the fragmented material but based on the preserved skull, we suggest that the specimen’s total body length may vary around 5 to 6 m long, similar to Tokarahia (Boessenecker & Fordyce, 2015b).

5.2 Skull

In dorsal view, the rostrum as preserved lacks most of the maxilla (Figures 2, 4, 6), but some fragments are attached to the premaxilla. On the other hand, the premaxilla is a distinctive feature of MU_EcSj1/29/142, mainly by the broad ascending process of the premaxilla readily distinct from other known eomysticetids. At the level of the narial fossa, the premaxilla is the wall of a wide mesorostral groove and inflected medially towards the nasal opening. In the anterior end of the nasal bones, the premaxilla is broad with a convex dorsal surface at the level of the nasal. It extends posteriorly to form the ascending process of the premaxilla, which lays on the medial part of the frontal. The posterior end of the ascending process extends far from the rear of the nasal bones, and the premaxilla/frontal suture is relatively tight (Figure 3). Premaxillae were wrapped laterally by the maxilla, but the contact and contribution of the maxilla are unclear.

In a lateral view, the rostrum is straight with not well preserved maxillary, and its lateral edge lay below the apex of the supraoccipital (Figure 7). However, the suture of the maxilla/palatine, plus the palatine sulcus and major palatine foramen, are visible. The nasal bones, like in other stem mysticetes, are longer and anterior to the antorbital notch. Their dorsal surface is flat, transversely narrow and rectangular. The anterolateral end of the nasals shows a more or less preserved short lateral process like a wedge (Figure 3), similar to Eomysticetus whitmorei. The nasal bones are joined tightly between the premaxillae, and the posterior suture is relatively tight with the frontal. The frontal is incomplete, the right supraorbital process conserves part of the orbital roof, the dorsal surface lacks supraorbital foramina, and the joint with the parietal has an irregular suture. The orbitotemporal crest is present in the posteromedial part of the frontal and near to the posterior edge. In the ventral view can be seen traces of the optical canal in the medial part of the frontal.

The parietals are widely exposed in the temporal region. The elongate dorsal surface of the intertemporal region is broken; the cross-section is semicircular to elliptic in shape. The parietal surface is slightly convex on the braincase, and the supraoccipital joint with the parietal forms the slightly concave edges of the nuchal crest. In lateral view, the parietal/squamosal suture down obliquely above the incomplete alisphenoid. The supraoccipital is triangular with a relatively concave surface and displays an eroded external occipital crest (Figures 2, 5). The broken surface of the occipital shield allows observing part of the cranial endocast. Ventrally, the cranium lacks details of the basioccipital, squamosal, pterygoid, sphenoid bones.

Figure 5. Cranium in posterior view with anatomical details, MU_EcSj1/29/142 (cf. Eomysticetus sp.).

Figure 6. Skull, MU_EcSj1/29/142 (cf. Eomysticetus sp.), anterodorsal view and anatomical details.

Figure 7. Skull, MU_EcSj1/29/142 (cf. Eomysticetus sp.), right lateral view and anatomical details.

5.3 Vertebrae

Around 16 vertebrae are partially preserved and require preparation. Only the atlas, axis, and thoracic vertebrae (?T5–T7) allow identifying morphological features (Figure 8). The atlas shape is similar to other vertebrae found in different eomysticetids (e.g., Yamatocetus, Eomysticetus, Tokarahia), with an approximately elliptical to rounded profile in anterior view. The transverse process is prominent and thick (akin to the present in Yamatocetus or Tokarahia). The hypapophysis and a rounded transverse foramen are present in the atlas. On the other hand, the axis contrasts widely with the known fossils mainly by having a bifurcated neural spine with defined lobules and a foramen (Figure 8d). These features are distinct to the medial sulcus and small tubercles present in the neural spine of Micromysticetus rothauseni (a youth animal, ChM PV4844; Sanders & Barnes, 2002a; Boessenecker & Fordyce, 2016) and Tokarahia sp., cf. Tokarahia lophocephalus (a likely subadult animal, OU 22081; Boessenecker & Fordyce, 2015b).

Figure 5. Cranium in posterior view with anatomical details, MU_EcSj1/29/142 (cf. Eomysticetus sp.).

Figure 6. Skull, MU_EcSj1/29/142 (cf. Eomysticetus sp.), anterodorsal view and anatomical details.

Figure 7. Skull, MU_EcSj1/29/142 (cf. Eomysticetus sp.), right lateral view and anatomical details.

The thoracic vertebrae ?T5–T7, as preserved (Figures 8e, 8f), have a dense and long transverse process, distinct to the short process present in the first four anterior thoracic vertebrae (e.g., Tohoraata and Waharoa; Boesseneker & Fordyce, 2015a, 2015c). The length of the incomplete centrum of the vertebra ?T7 (106 mm), the relatively high pedicle, the size and position of the fovea for the tuberculum and capitulum of the ribs, plus the lateral shape of the neural spine similar to the thoracic vertebrae 6–7 from Yamatocetus canaliculatus Okazaki, 2012, indicate a middle position in the thoracic vertebrae set (see Okazaki, 2012; Martínez-Cáceres et al., 2017).

Figure 8. Vertebrae and anatomical details, MU_EcSj1/29/142 (cf. Eomysticetus sp.). (a–b) Atlas (C1), anterior and dorsal view. (c) Atlas (C1), Axis (C2), and third cervical vertebra (C3) in left lateral view. (d) Axis (C2) in dorsal view. (e–f) Thoracic vertebrae, likely fifth to the seventh vertebra (?T5–T7), left lateral and dorsal view respectively.

6. Phylogenetic Analysis and Biogeographic Results

6.1 Phylogenetic results

Our phylogenetic result (Figure 9a) is similar to previous phylogenetic analyses (e.g., Boessenecker & Fordyce, 2016; Fordyce & Marx, 2018), which back up the monophyly of the group with branch support ~60 to 90%. The branch support is 72% and includes the known eomysticetid taxa Matapanui, Tokarahia, Waharoa, Micromysticetus, Yamatocetus, and Eomysticetus. We only illustrate the strict consensus tree under implied weights (Figure 9a; strict consensus tree recovered from 20 parsimonious trees, k =18.374024, 42.85138 steps, consistency index CI = 0.250, retention index RI = 0.736, rescaled consistency index RCI = 0.184, tree total length 1448). The following synapomorphic characters support the branch of Eomysticetidae: functional rostrum length more than one and a half times the bizygomatic width (1/2); oval temporal fossa (83/1); the anterior process of periotic in lateral view as two-bladed and L-shaped (146/2); anterior border of the proximal opening of the facial canal is continuous with the hiatus Fallopii and shaped like a fissure (178/2); medial lobe of tympanic bulla subequal in width to the lateral lobe or smaller (202/1); supraspinous fossa of scapula absent or nearly absent, with acromion process located near the anterior edge of the scapula (256/1). The relationship of MU_EcSj1/29/142 (cf. Eomysticetus sp.) with Eomysticetus whitmorei has branch support of 43% both share a common character, the concave lateral edge of the supraoccipital (112/2), perhaps an unequivocal feature for the genus Eomysticetus.

Figure 9. Phylogeny, area cladogram and eomysticetid distribution map. (a) Cladogram showing the phylogenetic relationship of MU_EcSj1/29/142 (cf. Eomysticetus sp.) between Eomysticetidae (range ages are from Bossenecker & Fordyce, 2016). (b) The general area cladogram shows the relationship of the areas where eomysticetids are distributed and indicates potential barriers and influence of the Central America corridor. (c) Map with the main eomysticetid species and its distribution, plus Cetotheriopsis lintianus, a likely eomysticetid (see Bossenecker & Fordyce, 2016). The black dashed arrow indicates the Central America Seaway corridor.

6.2 Biogeographic results

The geographic area relationships based on the phylogenetic results (Figures 9a, 9b) highlight the connection between the (eastern) North Pacific and the (western) North Atlantic concerning the South (western) Pacific Ocean (Figures 9a, 9b). The results illustrate the wide distribution of Eomysticetidae (Figure 9c; Boessenecker & Fordyce, 2017), perhaps linked to their zooplankton food sources (Clementz et al., 2014; Boessenecker & Fordyce, 2015c), but many distributional patterns among eomysticetids species still remain unclear. Furthermore, a relevant point is the presence of two different geographic barriers between the areas: 1) the warm equatorial waters in the Pacific Basin recognized as a crucial barrier regarding the cetacean distribution (Davies, 1963; Hernández-Cisneros & Velez Juarbe, 2021), and 2) the intermittent terrestrial barrier named as Gaarlandia in the Central American Seaway (Iturralde-Vinent & MacPhee, 1999; Iturralde-Vinent, 2006). Interestingly, the presence of the Gaarlandia barrier and the oscillation of oceanographic conditions (change on currents flows, temperature, and salinity) in the Central American Seaway (Iturralde-Vinent & MacPhee, 1999; von der Heydt & Dijkstra, 2006; Zhang et al., 2011; Tremblin et al., 2016) indicate what could have controlled the cetacean dispersal processes during the Oligocene, between the Atlantic and Pacific oceans. In fact, Gaarlandia was almost a complete terrestrial arch during the early Oligocene, and in the late Oligocene, the land arch was mostly an archipelago (Iturralde-Vinent & MacPhee, 1999). Both geological events influenced the distributional patterns of the surrounding marine fauna in the Central American Seaway region (Iturralde-Vinent & MacPhee, 1999; Iturralde-Vinent, 2006; Agnolin et al., 2019). Such a scenario indicates that the Pacific and Atlantic cetacean faunas were subject to vicariance events during the early Oligocene and dispersal events in the late Oligocene. Still, it is unclear if the fauna exchange was unidirectional, from one ocean to another or in both directions. In addition, the relationship of MU_EcSj1/29/142 (cf. Eomysticetus sp.) and Eomysticetus whitmorei proposes allopatric speciation events in this region (allopatric model I: vicariance; Wiley & Lieberman, 2011).

7. Discussion

The specimen MU_EcSj1/29/142 represents the first eomysticetid animal from the Pacific Coast of North America. However, the new fossil’s relevance lies in the biogeographical interpretation of the Eomysticetidae distributional patterns, which are a key to the Mysticeti evolutionary history. Hence, the biogeographical results indicate that eomysticetids are a good indicator of the environmental conditions that drove the origin of the “modern” baleen whales (Chaeomysticeti). The Eomysticetidae distribution and diversity show correspondence within multiple areas with key food sources for the crown Mysticeti evolution (Fordyce, 1977, 1980; Marx & Uhen, 2010; Clementz et al., 2014; Marx & Fordyce, 2015; Marx et al., 2019). Besides, the distributional patterns make it possible to identify likely speciation events between eomysticetids, such as allopatric speciation in the North Hemisphere, which may lead to explain how local environmental conditions and barriers drove cladogenesis of other chaeomysticetes during the Oligocene (e.g., Horopeta, Whakakai, Toipahautea; Tsai & Fordyce, 2015, 2016, 2018).

On the other hand, the new record of MU_EcSj1/29/142 as cf. Eomysticetus sp. on the North American west coast indicates a close relationship between the North Pacific and North Atlantic cetacean faunas during the Oligocene. Moreover, the studies of the Oligocene cetacean fauna in the most southern record of North America (Baja California Sur) are still in the first steps, but it is known that the record has representative groups of stem Neoceti and relict taxa such as Kekenodontidae (Hernández-Cisneros & Tsai, 2016; Hernández-Cisneros et al., 2017), which are likely related with the North Atlantic faunas. Consequently, these potential records will allow knowing details on turnover fauna processes and distributional patterns of the cetaceans during the Eocene–Oligocene transition and Oligocene in the North Hemisphere, plus the role of the Central American Seaway corridor and the Gaarlandia barrier in these processes (Iturralde-Vinent & MacPhee, 1999; Leigh et al., 2014).

Lastly, it is noteworthy that the widespread eomysticetid distribution lacks an accurate explanation regarding their ancestral area. The current evidence of the perhaps oldest eomysticetids (e.g., Yamatocetus; Marx & Fordyce, 2015) suggests that their common ancestor during the late Eocene likely inhabited the North Hemisphere before colonizing the Southern Hemisphere oceans. However, this idea needs to be supported with more evidences. In addition, the fossil record in Mexico indicates having new eomysticetid species, and propose occurrences of Eomysticetus whitmorei (MHN-UABCS_EcTm9/82/254 and MHN-UABCS_EcSj1/29/276; Hernández-Cisneros, 2012; Hernández-Cisneros et al., 2017) and ?Yamatocetus species (MHN-UABCS_Sg6/71/208; Hernández-Cisneros et al., 2017; Cedillo-Avila, 2018). Another record perhaps implicates an early Miocene surviving eomysticetid from Baja California Sur, but its geochronological dating is still uncertain (specimen MRAHBCS 002, Figure 9a; AEHC et al., pers. comm.).

8. Concluding remarks

Eomysticetidae fossil record indicates distributional patterns that might help to explain the baleen whales evolution in the oceans. Besides, the fossil record of North America has much to add about eomysticetids and other baleen whales. On the other hand, the Oligocene cetacean fossil record from Baja California Sur will contribute significatively to the Neoceti evolutionary history, which is until now the closes point to the extinct Central American Seaway with Oligocene cetacean record in North America. In addition, it represents a transitional climate point (tropical to temperate) with a rich fossil biota. Lastly, the biogeographical approach offers insight into the cetacean evolution from spatial and geographical scales. It allows establishing a frame to identify environmental variables, biological or geological events, and others processes that might influence the cetacean evolutionary history. The biogeographical approach applied to the cetacean palaeontology is young, but we hope to see more maps illustrating the intricate cetacean evolution.

Acknowledgements

We want to thank anonymous reviewers, as well as the editors, for their assistance and comments. We are also grateful to Cruz-Silva, J.A., for his comments during the preparation of this paper. We appreciate the editorial support of Sandra Ramos Amézquita. The projects SIP 20131259 and 20211371 (CICIMAR-IPN) led by EHNS supported the present work. We want to thank G. González-Barba for allowing access to the MU_EcSj1/29/142 fossil. Our appreciation goes to many people who helped us in different ways. Part of this study was made during AEHC’s MSc thesis work at Centro Interdisciplinario de Ciencias Marinas-Instituto Politécnico Nacional (CICIMAR-IPN), supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT), scholarship 290143 (2012–2014). AEHC was funded by CONACYT/Becas Mexico, Doctoral Fellowship; Grant No. 513362/290143 (2017-2021), and Programa de Beca Institucional de Posgrado-IPN (2021–2022).

Author Contributions

AEHC conceived the study; AEHC and EHNS analysed the data and wrote the paper. Both authors approved the final version of this manuscript and agree to be held accountable for the content therein.

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