Anatomy and Relationships of a New Gray Whale from the Pliocene of Piedmont, Northwestern Italy

Abstract

A new fossil gray whale genus and species, Glaucobalaena inopinata, is established based on craniomandibular remains from the Pliocene Sabbie d’Asti Formation, Piedmont, northwestern Italy. The holotype (MGPT-PU 19512) consists of two cranial fragments corresponding to the posterolateral corners of the skull, including both partial periotics, and in the posterior portion of the right mandibular ramus preserving the condyle and angular process. The new taxon is characterized by gray whale (eschrichtiid) synapomorphies in the posterior portion of the mandible (dorsally raised mandibular condyle with articular surface faced dorsoposteriorly, well-developed and robust angular process of the mandible) and in the earbone (massive transverse elongation of the pars cochlearis, indistinct flange of the ventrolateral tuberosity, and triangular and short anterior process of the periotic). A CT scan of the cranial fragments allowed us to reconstruct tridimensional renderings of the periotic, revealing the dorsal morphology of this bone. A phylogenetic analysis confirmed the inclusion of Glaucobalaena inopinata within Eschrichtiidae (the family to whom gray whales are included) and showed that it is monophyletic with Gricetoides aurorae; our phylogenetic results show that Eschrichtioides gastaldii is the sister group of the genus Eschrichtius. Our work lends further support to the idea that Eschrichtiidae is a separate family of baleen whales, characterized by specialized ecomorphological characters evident in both skull and mandibular architecture.

1. Introduction

According to Bannister [1], the gray whale (Eschrichtius robustus) [2] is the only living genus and species in the family Eschrichtiidae [3]. It is represented by a medium-sized (sensu [4]) mysticete, whose total body length reaches a maximum of 15 m in female adults [5,6,7]. The distribution of this species encompasses the North Pacific from Baja California to the Japan Sea through the Aleutian Arc [5,6]. Up to the 18th century, a gray whale population was present in the North Atlantic [8] and was led to extinction by whaling [8,9,10]. Eschrichtius robustus is a bottom feeder able to excavate long grooves on the sea floor to put into the water groups of benthic invertebrates like amphipods, which it swallows after having expelled water and sediment from its mouth through its short baleen racks [11]. Sanderson and Wassersug [12] included the gray whale within their intermittent suction feeder category, especially due to the observation of suction occurrence in a calf that was artificially fed in a dedicated pool (e.g., [13]). Specific morphologies observed in the mandible of the gray whale that are thought to represent specific adaptations to this feeding style include: (1) dorsally-faced articular surface of the condyle, (2) wide, robust, and ventrally- and posteriorly-protruded angular process, (3) reduced-to-absent coronoid process, and (4) dorsoventral arc of the ramus [14,15,16]. Eventually, the apomorphic presence of a couple of strong tubercles on the supraoccipital in the extant E. robustus may be related to the need for robust attachment sites for powerful neck muscles used for lateral movements of the head during feeding.

The phylogenetic relationships of the gray whale are still controversial. A school of thought suggests that Eschrichtiidae is the sister group of Balaenopteridae and, together with the latter, forms the epifamily (sensu [17]) Balaenopteroidea, based on morphological characters and the fossil record; following this school of thought, Cetotheriidae is the sister group of Balaenopteroidea (e.g., [18] and literature therein). Other research, still based on morphology and the fossil record, suggests that Eschrichtiidae and Cetotheriidae are monophyletic, forming the superfamily-rank clade Cetotherioidea (e.g., [19]). A third approach suggests that Eschrichtiidae is nested within Balaenopteridae mostly on the basis of molecular data and total evidence analyses (e.g., [20,21] and literature therein). The impact of molecular information radically changed the topology of the relationships of Cetotheriidae, Balaenopteridae, and Eschrichtiidae, representing a serious challenge to the morphological-only view. Independent from the fact that molecular data may suffer from specific problems, like morphological data, a search for a consensus should represent a major goal for all the scientific research interested in resolving mysticete relationships.

The paleontological record, unfortunately, can give only limited help because of the scarcity of fossil specimens that can be assigned to the Eschrichtiidae family. Only one Miocene species (Archaeschrichtius ruggieroi from the Late Miocene of southern Italy [15]) and two Pliocene species (Eschrichtioides gastaldii from the Pliocene of northwest Italy and Gricetoides aurorae from the Pliocene of eastern United States [14,22]) have been described in the last decade, together with a handful of Pleistocene records assigned to Eschrichtius robustus and its closely related species Eschrichtius akishimaensis [23,24,25]. A series of North Atlantic specimens from the Pleistocene of Holland is currently under revision by one of our authors (MB) after being published in the 19th and early 20th century by Van Deinse and Junge [26]; it is anticipated that these Dutch specimens can be assigned to the extant E. robustus.

The discovery of new fossil eschrichtiid species has the potential to provide a more solid morphological basis for gray whale relationships because new taxa may show different character combinations that are able to reinforce or to dismantle previous phylogenetic hypotheses. For this reason, the description of new paleontological materials referable to Eschrichtiidae should be welcome in the literature. Here we describe a new partial skull of an eschrichtiid from the Pliocene of Piedmont, northwestern Italy, that consists of two cranial fragments, including exoccipitals, squamosals, and parts of the ear bones, together with a posterior portion of the right mandibular ramus. We analyzed this specimen within a broad comparative context and by CT scanning the cranial fragments to extrapolate tridimensional renderings of the periotics. The relationships of the new specimen are then analyzed by including it within the large morphological dataset published by Bisconti et al. [18].

Based on the present work, we hypothesize: (a) the new specimen represents a new eschrichtiid species closely related to the monophyletic group formed by Eschrichtioides and Eschrichtius, (b) Eschrichtiidae is monophyletic, to the exclusion of all other mysticete taxa, (c) Eschrichtiidae is the sister group of Balaenopteridae, thus confirming the monophyly of Balaenopteroidea, and (d) Cetotheriidae is the sister group of Balaenopteroidea. The impact of this new eschrichtiid taxon in the mysticete fossil record of northern Italy is also briefly addressed.

2. Materials and Methods

2.1. Institutional Abbreviations

AMNH, American Museum of Natural History, New York, USA. IRSNB, Institute Royale des Sciences Naturelles de Belgique, Bruxelles, Belgium. MAUS, Museo dell’Ambiente, Università del Salento, Lecce, Italy. MCFEA, Museo Civico ‘Federico Eusebio’, Alba, Italy. MGPT-PU, Museo di Geologia e Paleontologia dell’Università di Torino, Torino, Italy. MPTA, Museo Paleontologico Territoriale dell’Astigiano, Ente di Gestione del Parco Paleontologico Astigiano, Asti, Italy. MuMAB, Museo Mare Antico e Biodiversità, Salsomaggiore Terme, Italy. NBC, Naturalis Biodiversity Center, Leiden, Holland. NMR, Het Natuurhistorisch Museum Rotterdam, Rotterdam, Holland. SAM-PQL, Iziko South African Museum Quaternary Palaeontology Langebaanweg, South Africa. USNM, United States National Museum, Smithsonian Institution, Washington, DC, USA.

2.2. Anatomy, Photography and Measurements

Anatomical terms are from refs. [27,28] (see also ref. [29]) for terms related to ear bone morphology. The specimen was photographed with a Nikon D750 DSLR (Nikon Corporation, Tokyo, Japan) with a Tamron 16–24 mm F/2.8 under the light provided by a Nikon SB500 speedlight flash mounted on the camera. Image processing was conducted in Adobe Photoshop 2024 with the DxO Nik Collection Viveza plug-in. Measurements were taken by Tacklife (Tacklife, Shenzhen, China) 300 mm and Sourcingmap D02 (Sourcingmap, Hong Kong, China) 150 mm digital calipers, both with an error margin to the nearest 0.01 mm.

2.3. Studied Specimen

The new eschrichtiid specimen was found during the revision of the paleocetological collection of the Museo di Geologia e Paleontologia of the Torino University. The whole collection is currently curated at MPTA. The specimen is identified with the number MGPT-PU 19512 and includes right and left posterolateral portions of the skull (comprising exoccipitals, squamosals, and parts of both periotics) and a posterior portion of the right mandibular ramus.

The original labels state that the specimen comes from an unknown locality in Piedmont and that the age is ‘Astian’. The ‘Astian’ stage is no longer in use, but it was used in the past in reference to a depositional environment characterized by sands rich in mollusks from the upper part of the Zanclean and the early Piacenzian.

These sands are known as the Sabbie d’Asti Formation, which is chronologically constrained between 3.8 and 3 Ma [30,31], and outcrops in these areas are evidenced in Figure 1. Sediment associated with the skull fragments was removed and analyzed under an optical microscope, confirming that the specimen was included within a matrix formed in the Sabbie d’Asti Formation.

2.4. CT Scan, Segmentation and Tridimensional Rendering

CT scans were conducted at the facility of the Radiodiagnostica Department of the ‘Cardinal Massaia’ Hospital in Asti using a GE Revolution EVO CT machine (GE Healthcare Italia, Milano, Italy). The CT dataset was imported into 3DSlicer 4.11, where we manually performed the segmentation process and generated the 3D models for the right periotic.

2.5. Phylogenetic Analysis

We compiled a matrix including 111 operational taxonomic units and 369 characters. The character states are mostly from Bisconti et al. [18] and are from morphological analyses of osteology and baleen. The character list with the description of character staes is provided in the Supplementary Materials (Table S1) together with the matrix (Table S2). Currently, this represents the largest and most inclusive morphological dataset used for inference of mysticete phylogenetics. We combined the morphological characters of two phylogenetically close species (Protocetus atavus and Georgiacetus vogtlensis) into a ‘Protocetidae’ outgroup. We then performed a phylogenetic analysis by using the four New Technology algorithms implemented in TNT [32] with default parameters. We assessed the morphological support at nodes by bootstrap (1000 replicates). We used TNT to calculate a strict consensus (Nelsen) tree based on the most parsimonious cladograms found by the analysis. Tree length (TL), Consistency (CI), and Retention (RI) indices were calculated by TNT; the Homoplasy Index (HI = 1 − CI) was calculated by hand. We calculated the Bremer support to the nodes of the strict consensus tree by TNT by searching for suboptimal trees with score up to one worse than best, and by searching with constraints three times (each with one replication, sectorial search, no ratchet, 10 drifting cycles, fusing results from the 1 replications) as implemented in TNT.

3. Results

3.1. Systematic Paleontology

Mammalia Linnaeus, 1758 [33]

Cetacea Brisson, 1762 [34]

Pelagiceti Uhen, 2008 [35]

Mysticeti Flower, 1864 [36]

Chaeomysticeti Mitchell, 1989 [37]

Balaenomorpha Geisler & Sanders, 2003 [38]

Thalassotherii Bisconti, Lambert, Bosselaers, 2013 [17]

Balaenopteroidea Gray, 1868 [39]

Eschrichtiidae Ellerman & Morrison-Scott, 1951 [3]

Glaucabalaena n. gen.

Etymology. Glaucus is a Latin term for grey. Balaena is a Latin term for whale.

Diagnosis. Glaucabalaena is included within Eschrichtiidae because it shows the following combination of characters: transversely elongated pars cochlearis of periotic, short anterior process of the periotic with irregular outline, robust and well-developed angular process of the mandible, which protrudes posteriorly and ventrally, dorsally-faced articular surface of mandibular condyle located on the top of a short and mainly cylindrical rising. It differs from Eschrichtioides because the rising of the mandibular condyle is lower, the angular process is larger, and its total body length is estimated to be about 30% shorter than the length of E. gastaldii. It differs from Eschrichtius because the anteroposterior length of its mandibular condyle is half the length of that of the extant gray whale. It differs from Gricetoides because it shows a deep and long facial sulcus in the posterior process and because it shows a well-developed and protruding posterolateral corner of the anterior process of the periotic.

Type species. The type and only included species is Glaucabalaena inopinata n. sp.

Glaucabalaena inopinata n. sp.

Etymology. The adjective inopinata (from inopinatus, Latin, unexpected) is due to the unexpected finding of a second gray whale species in the Pliocene of Piedmont.

Holotype. MGPT-PU 19915, currently curated at MPTA. The specimen consists of two fragments of the posterolateral corners of the skull (including parts of both squamosals, as well as exoccipital and periotics) and a proximal fragment of a mandibular ramus.

Diagnosis. Same as for genus.

Locality and horizon. The specimen is identified by a label where no indication of the locality of the discovery is provided. On the label, the only information is that the specimen is from the ‘Astian’ of Piedmont. As discussed, the ‘Astian’ is no longer in use and corresponds to the depositional environment of the Sabbie d’Asti Formation, which was chronologically constrained between 3.2 and 3.0 Ma by Ferrero and Pavia [31]. Associated sediment with the MGPT-PU 19515 skull confirms that the matrix enveloping the specimen was formed within the Sabbie d’Asti Formation outcropping in the areas shown in Figure 1. The estimated age of the specimen is that of the Sabbie d’Asti Formation, which is 3.2–3.0 Ma.

3.2. Description

Description. There is a limited amount of erosion on the ventral border of the angular process of the mandible. The mandibular ramus is broken a few mm anterior to the condyle and the breakage seems recent, given that it shows sharp borders. The internal texture of the bone is visible in the naturally occurring transverse section of the mandibular ramus due to the removal of superficial bone in the angular process of the mandible. Both periotics are present but the pars cochlearis is horizontally severed in both cases, revealing the internal canals.

Skull portions. Both the right and left posterolateral portions of the skull are preserved (Figure 2 and Figure 3), measurements for which are provided in

Table 1. Measurements in millimeters of portions of the cranial fragments.

Character	Measurement in mm
Width of exoccipital	149
Height of cranial fragment	236
Anteroposterior diameter of pterygoid fossa	89
Transverse diameter of pterygoid fossa	58
Length of external acoustic meatus	86

. The preserved portion of the exoccipital is robust and shows a rounded posterolateral corner in the dorsal view. Its superior surface is flat-to-slightly convex. Part of the posterior border of the right exoccipital and of the right squamosal are abraded in the vicinity of the attachment site for the posterior process of the periotic and the external acoustic meatus; as a result, the right paroccipital process is not preserved (Figure 2). Most of the posteroventral surface of the left exoccipital is abraded, showing the internal texture of the bone; as a result, the paroccipital process is not preserved (Figure 4).

In the posterior view, the posterior process of the periotic is visible on both sides, showing a good degree of preservation and allowing observation of the facial sulcus. Slightly ventrally, a large external acoustic meatus is observed, which shows a concave bony surface projecting medially and slightly anteriorly from the lateral surface of the cranial fragments.

Laterally, part of the squamosal is preserved. The lateral surface of the squamosal is slightly laterally convex. The zygomatic process of the squamosal is not preserved but the postglenoid process is. This process is massive in the lateral view and triangular in the posterior view. Its external outline is mostly rounded, and it protrudes ventrally and posteriorly, suggesting that the glenoid fossa of the squamosal was not highly concave like in modern balaenopterids, but, rather, that it was mainly flat or slightly concave like in the modern gray whale Eschrichtius robustus and in the fossil Eschrichtioides gastaldii.

The falciform process is a rectangular structure projecting anteroventrally and located anteroventrally from a wide pseudoval foramen entirely included within the squamosal. A fissure departs from the anteroventral border of the pseudoval foramen and projects ventrally.

Ventrally, the subtemporal crest separates the posterior wall of the temporal fossa from a scarcely concave subtemporal fossa.

Periotic. Both periotics are preserved in part and are articulated with both cranial fragments (Figure 4 and Figure 5). Periotic measurements are provided in

Table 2. Measurements in millimeters of periotic characters.

Character	Measurement in mm
Length of right posterior process	113
Maximum width of right posterior process	30
Maximum anteroposterior diameter of right pars cochlearis (as preserved)	31
Maximum transverse diameter of right pars cochlearis (as preserved)	41
Maximum anteroposterior diameter of left pars cochlearis (as preserved)	43
Maximum transverse diameter of left pars cochlearis (as preserved)	54.5
Maximum transverse diameter of left facial canal	7.9
Maximum length of left anterior process of the periotic	101.1
Maximum width of left anterior process of the periotic	22.8

. The posterior process is elongated and shows a deep facial sulcus running along most of its length; it is exposed in the lateral surface of the skull and appears as a small triangle. The maximum diameter of the facial sulcus is 32 mm. The anterior border of the facial sulcus is a flat surface corresponding to the posterior face of the posterior process. The posterior process is at a right angle with respect to the body of the periotics. The stylomastoid fossa is shallow, flat, and roofed by a flange projected from the body of the periotic (Figure 4).

On both sides, the pars cochlearis is horizontally severed, showing the internal arrangement of the periotic canals. The pars cochlearis is elongated along the transverse axis and clearly protrudes ventrally from the anterior process. A large groove corresponding to the internal acoustic meatus is observed in both periotics; it is olive shaped as it opens medially, then it widens more laterally, and finally it narrows at its lateral end.

The endocranial opening of the facial canal is separated from the internal acoustic meatus by a crista transversa that is 4 mm in anteroposterior diameter.

Posterior to the internal acoustic meatus, a transverse groove runs from the endocranial surface of the pars cochlearis to its lateral surface. We interpret this groove as part of the vestibular aqueduct, whose endocranial opening is the endolymphatic foramen still present in the periotics. The endocranial opening of the facial canal runs anteriorly and laterally to the internal acoustic meatus and shows a wide curvature when observed in ventral view; it opens laterally to the pars cochlearis. Sectioned turns of the cochlea are evident in the natural sections of the pars cochlearis together with the canal connecting the fenestra cochleae and the perilymphatic foramen (Figure 5). A narrow canal is observed in the pars cochlearis running medially and anteriorly, which opens in the anterior wall of the pars cochlearis. This is interpreted as an accessory facial opening; similar openings are observed in the periotics of some extant species like Eubalaena glacialis and some balaenopterids.

Anterior to the pars cochlearis, a raised, curved structure runs parallel to the anterior wall of the pars cochlearis itself; this is interpreted as the attachment site for the tensor tympani muscle, which is not shaped as a groove, but, rather, as a shallow excavation at the basis of the raised structure (Figure 5). The anterior process is relatively short and shows an irregular outline. It is triangular with a concave lateral border. The posterolateral corner of the anterior process is inflated, protrudes laterally, and appears more robust than the flange of the ventrolateral tuberosity. The anterior pedicle of the tympanic bulla is broken and cannot be discerned with certainty. The flange of the ventrolateral tuberosity is indistinct. Both anterior and posterior pedicles for the articulation with the tympanic bulla are broken. The posterior pedicle is located slightly posteriorly to the stylomastoid fossa on the proximal-most portion of the posterior process of the periotic and can be observed in both right and left periotics. The anterior pedicle can be observed only in the left periotic; it is located close to the protrusion of the posterolateral corner of the anterior process, and it is triangular and small-sized (Figure 5). In the CT scan-generated slides, the periotics are well distinct from the remainder of the skull (Figure 6). An evident space is interposed between the dorsal surface of the periotic and the articular surface of the periotic fossa in the squamosal. This made the segmentation relatively easy.

The resulting tridimensional rendering of the right periotic is shown in Figure 7 and Figure 8. From the rendering, it is evident that the suprameatal area is low and there is no superior process in this periotic; moreover, the flange of the ventrolateral tuberosity is dorsally flat and its surface is continuous with the dorsal surface of the body of the periotic. The posterior process is prismatic, with three main ridges: a dorsal ridge separating the anterior and the posterior faces, an anteroventral ridge separating the anterior and the ventral face, and a posteroventral ridge between posterior and ventral faces. The ventral face corresponds to the facial sulcus that is anteroposteriorly constrained between the anterior and posterior faces. A well-developed ventral crest corresponding to the anterior wall of the pars cochlearis is observed, which forms the anterior border of the endocranial opening of the facial canal whose dorsal outline is preserved. The internal acoustic meatus is represented by the dorsal border and appears smaller than the internal opening of the facial canal.

The perilymphatic foramen is located at the same height as the internal acoustic meatus. An elongated and conical medial crest is observed in the tridimensional rendering, corresponding to part of the transverse elongation of the pars cochlearis. Tabbed in the 3D rendering, the anterior process appears less triangular, and both the posterolateral corner and the flange of the ventrolateral tuberosity confirm the observations made above based on the visual inspection of the bone (Figure 7 and Figure 8).

Mandible. The preserved part includes the mandibular condyle, the angular process, and part of the portion comprised between the condyle and the coronoid process (Figure 9). Mandibular measurements are provided in

Table 3. Measurements in millimeters of mandibular characters.

Character	Measurement in mm
Maximum anteroposterior diameter of mandibular condyle	64
Maximum transverse diameter of mandibular condyle	91
Maximum height of posterior portion of mandibular ramus	152
Maximum anteroposterior length of mandibular fragment	142
Maximum height of angular process	72
Maximum height of anterior border of mandibular fragment	80
Maximum width of anterior border of mandibular fragment	33.5

.

When the mandibular ramus is oriented horizontally, as in the conventional view described by Bisconti et al. [4], the articular surface of the condyle is located on the top of an approximately cylindrical rising, protruding posterodorsally and connecting the articular surface to the body of the mandibular ramus. In the lateral view, the articular surface is flat-to-slightly dorsally convex. In the posterior view, the condyle shows a round outline protruding laterally and medially from the mandibular body.

In the lateral view, the angular process is well-developed. It shows a ventrally rounded outline and protrudes both ventrally and posteriorly. Its posterior-most apex is located more posteriorly than the condyle. The lateral surface of the angular process is slightly concave, showing that the attachment sites for the masseter muscle was present as a shallow and elliptical fossa. Medially, the angular process is inflated and shows a flat surface. The condyle and angular process are separated by a shallow groove that can be observed both laterally and medially. Posteriorly, this groove is shaped as a shallow and dorsoventrally higher concavity. Anterior to the condyle, the mandibular body narrows transversely, and its transverse outline is almost rectangular in the anterior view.

3.3. Comparisons

Glaucabalaena inopinata shows the following typical anatomical characters of Eschrichtiidae: (1) posterodorsally directed and raised mandibular condyle with articular surface of condyle faced dorsally; (2) large and protruding angular process of mandible; (3) short anterior process of the periotic with irregular outline; and (4) indistinct flange of ventrolateral tuberosity. All these characters are observed in the extant Eschrichtius robustus and in the Pliocene Eschrichtioides gastaldii and Gricetoides aurorae. In particular, the external outline of the posterior portion of the mandibular ramus is almost indistinguishable from that of Eschrichtioides gastaldii, and only slight differences are observed that allow the separation of E. gastaldii from Glaucabalaena inopinata for this element. These differences include a shorter rising of the mandibular condyle in G. inopinata and a rounder outline of the angular process (Figure 9B).

The details of the periotic support the inclusion of Glaucabalaena inopinata within Eschrichtiidae based on the peculiar reduction and flatness of the flange of the ventrolateral tuberosity, which is shared with Eschrichtius robustus, and based on the overall shape and the irregular outline of the anterior process (Figure 7). The proportional thickness of the crista transversa with respect to the anteroposterior diameters of both the internal acoustic meatus and the endocranial opening of the facial canal is consistent with the morphological pattern observed in the extant Eschrichtius robustus. The noteworthy transverse elongation of the pars cochlearis, which is longer than the anterior process, closely resembles the same portions of the periotics of the extant gray whale.

The roofed and shallow stylomastoid fossa shows a morphological pattern shared with the extant E. robustus. The evident subdivision of the body of the periotic from the anterior process through a transverse groove and the presence of a well-defined posterolateral corner of the anterior process are also shared by G. inopinata and E. robustus, even if, in the former, the lateroposterior corner of the anterior process is both more protruding and rounder. The presence of a deep facial sulcus in the ventral surface of the posterior process of the periotic of Glaucabalaena inopinata is a major difference that distinguishes this new taxon from both Eschrichtius robustus and Gricetoides aurorae (Figure 4, Figure 7 and Figure 8). In both these latter taxa, the facial sulcus is a shallow concavity scarcely evident by observation of the skulls in ventral view, but in Glaucabalaena inopinata it is a well-developed, deep, and long groove running along the ventral surface of the posterior process of the periotic. The periotic of Eschrichtioides gastaldii has never been prepared so it is difficult to assess all its morphological characters; however, part of the right periotic is exposed in the holotype and shows a slightly deeper facial sulcus than that observed in E. robustus and G. aurorae, suggesting a more similar morphology to that of Glaucabalaena inopinata. In the right periotic of Eschrichtioides gastaldii, the facial sulcus exposed on the ventral surface of the posterior process of the periotic is even more evident, confirming what was mentioned before.

The posteroventrally-protruded postglenoid process of the squamosal is shared with the extant Eschrichtius robustus and the Pliocene species and represents a typical character of the extant and fossil gray whales. The peculiar orientation of the postglenoid process contributes to the relative flatness of the glenoid fossa of the squamosal, through which a fibrocartilageous tissue envelopes the mandibular condyle, connecting the mandibular ramus to the skull. The peculiar craniomandibular joint of the living gray whale was well illustrated in refs. [40,41]. What we observe in Glaucobalaena inopinata is a morphological pattern of the craniomandibular joint in line with what was observed in the living gray whale Eschrichtius robustus and in the Pliocene Eschrichtioides gastaldii.

Damage at the posteroventral border of the exoccipital in the relevant portions of both Gricetoides aurorae and Glaucabalaena inopinata prevents a detailed comparison of the paroccipital process against the peculiar morphology observed in the extant Eschrichtius robustus (Figure 10). In Eschrichtioides gastaldii the concavity of the paroccipital process is less developed and evident than that observed in Eschrichtius robustus.

3.4. Phylogenetic Results

The FUSE algorithm implemented in TNT found two equally parsimonious cladograms, whose strict consensus is shown in Figure 11. The cladograms are 2216 steps in length and their CI is 0.228, RI is 0.727, and HI is 0.772. All other algorithms found less parsimonious cladograms.

The present results show that Eschrichtiidae is monophyletic, includes Eschrichtius, Eschrichtioides, Gricetoides, Archaeschrichtius, and Glaucabalaena, and is the sister group of Balaenopteridae, thus confirming the monophyly of the epifamily Balaenopteroidea as defined by Bisconti et al. [19]. In the strict consensus cladogram, Glaucabalaena and Gricetoides form a monophyletic group to the exclusion of Archaeschrichtius. Eschrichtioides and Eschrichtius form another monophyletic group. Our results confirm the inclusion of Eschrichtius akishimaensis within the genus Eschrichtius.

In the strict consensus of Figure 11, some discrepancies are present with respect to the result published in ref. [18], especially regarding the intra-family relationships of Balaenopteridae and, in particular, with respect to the relative position of Plesiobalaenoptera quarantellii, ‘Balaenoptera’ bertae, and ‘Megaptera’ hubachi. It is likely that these discrepancies are due to the addition of Glaucabalaena, Gricetoides, and a number of toothed mysticetes that implied new assessments of character transformations by the software. In particular, the incompleteness of many of the newly added taxa may provide the software with additional problems, implying changes in the results. The bootstrap supporting values show high support for mysticete clades like Mysticeti, Eomysticetidae, Balaenoidea, and some balaenopterid clades, and low support for a high number of other clades. This problem is well known and is attributed to the high level of homoplasy present in the dataset. The Bremer support values are moderately high for a small number of clades, including Mysticeti, Balaenomorpha and Thalassotherii. Most of the other clades have Bremer support values lower than 3 and only a few have values included between 3 and 5. Following from the bootstrap and Bremer results, we conclude that the dataset used here is particularly sensitive to minimal morphological transformations and may be prone to generate different results based on the inclusion of new taxa characterized by new character combinations. This suggests that multiple events of homoplasious transformations occurred within the Mysticeti lineage.

4. Discussion

4.1. The Fossil Record of Gray Whales

The extant gray whale Eschrichtius robustus has been variously interpreted as closely related to right and bowhead whales [42,43], monophyletic with Cetotheriidae [19,44], a sister group of Balaenopteridae (e.g., [18,45,46,47,48,49]), and part of Balaenopteridae (e.g., [20,21,50,51,52]). Molecular works, particularly, support the inclusion of Eschrichtius robustus within Balaenopteridae, suggesting Eschrichtiidae should be dismissed. Even though it is clear that strong morphological differences allow a clear separation of E. robustus from all living and fossil balaenopterids, in molecular and total evidence analyses this species falls within a monophyletic Balaenopteridae, suggesting it should be considered another balaenopterid species.

However, as morphological characters clearly distinguish E. robustus from balaenopterids, questions are raised as to how to interpret morphological and molecular characters in phylogenetic analyses of mysticetes.

In the Mysticeti clade, extinct taxa largely outnumber the extant species from which genes can be sampled and compared [18]; therefore, genetic-based works are able to sample only a small fraction of the whole mysticete diversity. This may represent a potential flaw, still only partially understood (see refs. [53,54] for different views on this point), that is able to influence molecular-based phylogenetic results. Currently, DNA sequences can only be scored for extant species, increasing the number of question marks in total evidence analysis because such sequences cannot be scored in the fossil taxa that form most of the diversity of mysticetes.

Morphology-based phylogenetic analyses suggest that mysticetes can be subdivided into several well-defined groups, especially in the thalassotherian clade: basal thalassotherian taxa (a group still needing a formal name), Cetotheriidae, Eschrichtiidae, and Balaenopteridae (see, [45,46,47,48,49,50,51,52,53,54,55,56]). The fossil record of gray whales does not seem to show transitional stages from early-diverging mysticetes to gray whales. All species assigned to the gray whale group show morphological characters typical of this group, including (if preserved): a couple of protruding tubercles present on the supraoccipital, dorsally-faced articular surface of the mandibular condyle, well-developed and high angular condyle of the mandible, strongly reduced coronoid process of the mandible, dorsoventrally-arched mandibular ramus, anterior process of periotic shorter than transverse elongation of pars cochlearis, and indistinct flange of ventrolateral tuberosity of the periotic. Based on different combinations of these characters, Archaeschrichtius, Gricetoides, Eschrichtioides, Glaucabalaena, and Eschrichtius form a morphologically distinct group that is found to be monophyletic thanks to the phylogenetic analysis of the present paper.

Additional fossil and subfossil materials have to be added to this group, including the Eschrichtius specimens from the Pleistocene of North Sea, those from the western part of the North Atlantic, and those from Taiwan (e.g., [9,24]). These specimens were not included in our phylogenetic analysis and, in literature, are considered more closely related to the extant Eschrichtius robustus than to the other extinct eschrichtiid species; based on this, we support their inclusion within this same genus.

Archaeschrichtius ruggieroi was assigned to Eschrichtiidae in ref. [15] despite its fragmentary preservation. The holotype of this species (MAUS 230) is represented by a partial mandibular ramus lacking the posterior-most portion. However, this species shows the presence of a well-developed dorsoventral arc in the mandibular ramus and a complex morphology of the coronoid process of the mandible, in which two different crests are observed, resembling, in topological terms, those observed in both Eschrichtioides gastaldii and Eschrichtius robustus. Unfortunately, the phylogenetic results of the present paper are unable to establish whether Archaeschrichtius may be more primitive than the other eschrichtiid taxa, probably due to the incompleteness of this specimen and the lack of the mandible in Gricetoides.

Eschrichtioides gastaldii was previously interpreted as a balaenopterid by Portis [57], who assigned it to the genus Balaenoptera. More recent re-evaluations of the holotype showed the presence of eschrichtiid unique apomorphic characters, including orientation and shape of the mandibular condyle, relative development of the angular process of the mandible, reduction of the coronoid process of the mandible, and the dorsoventral arc in the mandible itself. Additionally, a couple of tubercles were observed in the supraoccipital of this Pliocene species, confirming its inclusion in Eschrichtiidae [14,49,58]. Eschrichtioides gastaldii differs from the extant Eschrichtius robustus due to a flat maxillary and premaxillary portion of the rostrum and a wider supraoccipital with a broad and round anterior border. The holotype of E. gastaldii (MGPT-PU 13802) is considered an adult individual based on the fusion of all the cranial sutures; its total body length was estimated to be around 9.5 m by Slater et al. [21], which is about 60% of the total body length of an adult living gray whale. Based on the phylogenetic results of the present paper, Eschrichtioides gastaldii is the sister group of Eschrichtius.

4.2. Gray Whale Affinities

Gricetoides aurorae is represented by a partial skull with periotic and tympanic bulla that was described by Whitmore and Kaltenbach [22] and illustrated in high-definition pictures in the Morphobank project 776 (Project DOI: 10.7934/P776, http://dx.doi.org/10.7934/P776 (accessed on 30 August 2024) associated to ref. [59]). In the periotic, the pars cochlearis is transversely elongated and anteroposteriorly shortened, the flange of the ventrolateral tuberosity is indistinct, the posterolateral corner of the anterior process of the periotic is not distinguished, and the length of the anterior process is shorter than the transverse length of the pars cochlearis.

In the holotype of this species (USNM 182921), the facial sulcus running through the posterior process of the periotic is wide and resembles that of modern balaenopterids. In this sense, it contrasts with that of Glaucabalaena inopinata, which shows a narrow facial sulcus running along a flat ventral surface of the posterior process of the periotic. In a periotic associated to Gricetoides, the facial sulcus is a wide and shallow groove running along the ventral surface of the posterior process of the periotic and shows a balaenopterid-like morphology. In this associated periotic, the pars cochlearis is probably incomplete medially, so that the transverse elongation cannot be observed [22].

The arrangement of the endocranial foramina of Gricetoides aurorae observed in the associated periotic is consistent with that seen in Eschrichtius robustus and Glaucobalaena inopinata. Unfortunately, a dissection of the periotic from the holotype skull was not attempted and we do not know the arrangement of the endocranial foramina in this specimen. Finally, Glaucabalaena inopinata shows a set of characters highly consistent with its inclusion within Eschrichtiidae. The peculiar morphology of both the periotic (shape and outline of anterior process, relationships of anterior process and elongated pars cochlearis, proportions in the morphological characteristics of the endocranial foramina of the pars cochlearis and the crista transversa) and mandible (shape, orientation, and size of the mandibular condyle and of the angular process) provide support to the proposed eschrichtiid affinities of this new species. In conclusion, even though Glaucabalaena and Gricetoides form a monophyletic group, enough morphological differences are observed to allow their assignments to different genera.

4.3. Phylogenetic Relationships of Gray Whales

The monophyletic group formed by Eschrichtius, Eschrichtioides, Gricetoides, Archaeschrichtius, and Glaucabalaena is based on a number of synapomorphies that do not show intrafamily homoplasious distribution. All the taxa in which the mandible is known (i.e., Eschrichtioides, Glaucobalaena, and Eschrichtius) show a well-developed angular process, dorsally-placed mandibular condyle, reduction of coronoid process, coronoid process with accessory and parallel crest (satellite process of ref. [17]), and dorsoventral arc of the mandibular ramus. Archaeschrichtius differs from other eschrichtiids in having higher coronoid and satellite processes and in having an elongated postcoronoid crest with a wide postcoronoid fossa.

All the taxa in which the periotic is preserved (i.e., Gricetoides, Glaucabalaena, Eschrichtius, and, in part, Eschrichtioides) show a transversely elongated and anteroposteriorly narrow pars cochlearis, pars cochlearis longer than anterior process, indistinct flange of the ventrolateral tuberosity, moderately thick (4+ mm) crista transversa, and obvious and deep facial sulcus in the posterior process.

A posteriorly protruded angular process of the mandible and a deep facial sulcus in the posterior process of the periotic together with a posterior protrusion of the posterolateral corner of the exoccipital in dorsal and ventral views suggest some affinity between Eschrichtiidae and Cetotheriidae. In the dorsal view, the eschrichtiid skull resembles that of Cetotheriidae in the shape of the anterior border of the supraoccipital and in the morphology of the parietal exposure at the cranial vertex, which shows temporal crests shaped like externally concave edges [14,60]. In past phylogenetic analysis, Eschrichtiidae was found to be monophyletic with Cetotheriidae to the exclusion of Balaenopteridae [19], but this pattern was not confirmed by further works. The tympanoperiotic complex of eschrichtiids, however, shares characters with Balaenopteridae, including a triangular anterior process of the periotic, highly elongated pars cochlearis along the transverse axis, and presence of an inflated lateral lobe and of a ventral keel in the tympanic bulla [28]; this work.

The character combination exhibited by both extant and fossil gray whales is unique among mysticetes. When coded for a morphology-based phylogenetic analysis, such a combination supports a sister group relationship of Eschrichtiidae and Balaenopteridae, confirming the monophyly of the epifamily (sensu [17]) Balaenopteroidea (e.g., [18,40,45,49,61]). The coexistence of both balaenopterid and cetotheriid characters in the craniomandibular anatomy of extant and extinct gray whale species suggests that this group may be considered basal to balaenopterids and that it should be considered when trying to understand the earliest morphological transformations leading to the balaenopterid morphotype.

Bisconti and Bosselaers [62] provided a transformational approach to understand the early evolution of Balaenopteridae by showing which cranial character states changed in the earliest phases of balaenopterid evolution. They observed that changes in the orientation of the supraorbital processes of the frontal and elongation of the ascending processes of the maxilla occurred primitively in balaenopterid evolution; second, the reduction of the posterolateral projection of the exoccipital occurred together with a subsequent transverse expansion of the intertemporal constriction.

Based on our analysis, we suggest that balaenopterids inherited a periotic with a triangular anterior process and a massively elongated pars cochlearis along the transverse axis together with a tympanic bulla with a well-developed lateral lobe from a common ancestor shared with eschrichtiids. This hypothesis is supported by the present phylogenetic analysis and by several other phylogenetic results published by different authors over the last twenty years (e.g., [18,40,45,49,61,63]).

5. Conclusions

We established Glaucabalaena inopinata gen. et sp. nov. based on craniomandibular remains from the Pliocene of Piedmont, northwestern Italy. Characters of the periotic (shape of the anterior process, transverse elongation of the pars cochlearis, deep facial sulcus in the posterior process) and of the mandible (dorsally faced and raised mandibular condyle, well-developed and robust angular process) support the inclusion of G. inopinata within Eschrichtiidae. A phylogenetic analysis including 369 characters scored for 111 taxa supported both the monophyly of Eschrichtiidae and the inclusion of G. inopinata within it.

The Pliocene of Piedmont revealed the presence of two different gray whale taxa, representing 66% of the global pre-Pleistocene record of Eschrichtiidae. Together with the late Miocene Archaeschrichtius ruggieroi, the Mediterranean fossil record of gray whales is surprising. Only one other Pliocene species is known from outside the Mediterranean (i.e., Gricetoides aurorae from Lee Creek Mine, eastern United States). Understanding why the gray whale fossil record is so scarce on the global scale is a difficult task that was previously discussed in literature [64]. Understanding why the Mediterranean record of gray whale evolution is so abundant in sedimentary contexts characterized by subtropical-to-tropical climates with respect to the rest of the world is another question that is difficult to currently answer and certainly deserves further research.