Corumictis wolsani, 2020

CORUMICTIS WOLSANI PATERSON , SP. NOV.

( FIGS 2–5; TABLE 1)

zoobank.org:act: A8B358F9-7EF3-46B8-8FF0- 3286CDBD8717

Type specimen: JODA 8167 , a nearly complete skull with the left P2-M1, right P3-M1, and alveoli for the left P1 and M2 .

Referred specimen: JODA 396, right dentary with m1, and p4 and m2 alveoli preserved.

Horizon and locality: JODA 396 – JDNM 9, Blue Basin, Grant County, Oregon, Unit F, Turtle Cove Member, John Day Formation; JODA 8167 – JDNM 35, Roundup Flat, Grant County, Oregon, Unit L, Kimberly Member, John Day Formation.

Age View in CoL : Early and Late Oligocene (Early and Late Arikareean, Ar1–Ar3), between Blue Basin Tuff dated 28.8 Mya and JD-BC-3 Tuff dated 23.9 ± 0.18 Mya ( Hunt & Stepleton, 2004; Albright et al., 2008).

Diagnosis: Mustelid of small size displaying a reduced and single-rooted M2 and lacking development of a tubular external auditory meatus and muscular tubercles on basioccipital.

Differential diagnosis: Differs from Corumictis julieni in having a larger skull size, the supraorbital processes are reduced in size and lateral extent, the paroccipital process is not as wide as long and is connected to the bulla by a ventral crest, the auditory bullae does not display the same lateral extent into a trough, the absence of muscular tubercles on basioccipital, less reduced mastoid, lack of tubular external auditory meatus, the presence of an M2, the presence of a hypoconid on m1 and the length of the m1 talonid.

Differs from Plesictis genettoides Pomel, 1846 in: smaller size, less prominent supraorbital processes, shorter postorbital region, wider and more divergent palate, P4 metastyle extended distally, incipient hypocone of M1 less distinct, dentary narrower and shallower, m1 trigonid more open with a relatively elongate paraconid, m1 metaconid smaller and more posteriorly placed, m1 talonid narrower and with an elongate blade-like hypoconid and indistinct entoconid.

Differs from Plesictis palustris Pomel, 1846 in: smaller size, less prominent paraoccipital processes, larger P4 protocone, larger M1 parastyle and larger and distally extended M1 metacone.

Differs from Plesictis branssatensis Viret, 1928 in: smaller size, lack of tubular external auditory meatus, P3 lacking bilobed posterior root, carnassial notch on P4 present.

Differs from Plesictis pygmaeus Schlosser, 1889 in: smaller size, more inflated auditory bullae, deep suprameatal fossa, P3 possessing lingual lobule, less prominent lingual shelf of P4, anteroposteriorly broader M1.

Etymology: The epithet is a patronym for Mieczysław Wolsan, who has extensively studied fossil musteloids and worked to reveal their evolutionary history.

Description: The type specimen (JODA 8167) represents a nearly complete cranium, which is missing the following: the anteriormost portion of the rostrum, most of the palatine bone, most of the zygomatic arches, the ectotympanic portion of the auditory bullae (except for the anterolateral portions on both sides) and the right occipital condyle. Teeth preserved include the left P2-M1 and right P3-M1, though the right P4 is lacking its distal portion, including the metastyle. Alveoli are present for the left P1 and M2. All preserved cranial sutures are closed, indicating the type specimen represents an adult individual (cite).

Skull: In the facial region of the skull ( Fig. 2A, B), the premaxillary bones are not preserved and only the posterior portions of the nasals are preserved.The lateral edges of the nasals are defined by prominent swellings, possibly representing subsidiary frontal sinuses. A shallow nasolabialis fossa is bound by an antorbital rim. The antorbital rim, while prominent, does not project significantly into the orbit as a distinct process. A lacrimal foramen is present just anteromedial to the infraorbital foramen, but the sutures of the lacrimal bone are well fused and not possible to interpret. No lacrimal fossa or fossa muscularis is present. The infraorbital foramen is large, round and upright. The more-complete left zygomatic arch has a fracture along what may be the maxillary–jugal suture. The jugal appears to rise well dorsally, but only the portion anterior to the supraorbital processes is present. No postorbital process is preserved, and it is unlikely the postorbital process was prominent. The postorbital constriction is short, but marked, before the braincase gently expands. Only fragments of the palate are preserved, and the posterior extent of the palatine is not discernible.

From vestigial supraorbital processes emerge the temporallines, whichmakeanimmediateposteromedial trajectory (about 25 degrees off the sagittal plane) toward the back of the skull. The temporal lines do not converge into a sagittal crest. From around the expected location of the frontal–parietal suture, the temporal lines travel nearly perpendicularly to the occiput, where they finally diverge into reduced lambdoidal crests. The median lambdoidal ridge, along the posterior surface of the skull, appears bulbous, lacking any semblance of a crest or keel.

The alisphenoid canal is absent. The orbital fissure and anterior lacerate foramen share a common opening. A thin flange separating the two protrudes only a short distance across this common opening and is only visible in the CT data, similar to the condition in Mephitis . The canal bounded by the anterior and posterior lacerate foramina is long and round, opening into the braincase posteriorly just before the anterior tip of the squamosal arises.

The presphenoid and basisphenoid are well visible in ventral view. They share a similar morphology, both being weakly concave with a faint median ridge. The pterygoid bone and hamuli are not well-preserved.

Only the medial portions of the glenoid fossae and associated postglenoid (=retroarticular) processes are preserved. This portion of the glenoid fossa is not immured anteriorly by a preglenoid process, but is enclosed posteriorly by a tall postglenoid process that somewhat overhangs the fossa in ventral view. The right postglenoid process is completely preserved and appears to terminate rather abruptly laterally, suggesting the lateral portion of glenoid fossa was not walled posteriorly.

Expectedly, the basicranial region of JODA 8167 excellently displays its arctoid affinities. The basisphenoid–basioccipital suture is completely fused. The presumed basisphenoid is narrow and concave. The presumed anterior portion of the basioccipital initially follows suit, and begins to widen posteriorly at the anterior wall of the bullae and continually broadens for the remainder of its length. At the level of the anterior wall of the promontory, the basioccipital develops a posteriorly widening, bulbous median convexity, surrounded by shallow concavities on each side appressing the bullar flanges. No muscular tubercles are present on JODA 8167.

The bullae are only partially preserved, but appear as if they would reach a similar relative size and extent to those of Corumictis julieni or Pseudobassaris . The caudal entotympanic is not preserved, but the lateral portion of both ectotympanic elements are present. The anteriormost extension of the ectotympanic just passes the posteriormost portion of the postglenoid process. An extended trough for the external auditory meatus is absent, as the ectotympanic remains round and inflated even in the portion surrounding the external auditory meatus. A deep suprameatal fossa is present on the dorsal wall of the external auditory meatus. The suprameatal fossa is narrow and U-shaped, with its lateral (external) wall hanging lower than its medial partition.

The posterior lacerate foramen (jugular foramen), appressed to the posterior portion of the promontory, is similarly sized to that of other early-diverging mustelids. Its position with respect to the posterior carotid foramen is unknown. The carotid canal travels abreast of the lateral wall of the basioccipital, vacating the bullar cavity via the anterior carotid foramen, at the anteromedial edge of the bulla.

As the bullae are incompletely preserved, much of the promontory is readily visible on the specimen. The ventral and posterior surfaces of the promontory are externally visible, and CT-data further reveal the internal features of this region ( Fig. 4). The round window (fenestra cochlea) is round and faces posteriorly and somewhat ventrolaterally. It is slightly larger than the oval window (fenestra vestibularis). The large fossa for the tensor tympani is well excavated into the dorsolateral portion of the promontory, at the coronal level of the cochlea. Lateral to the fossa for the tensor tympani, at about its anteroposterior midlength, a large, epitympanic recess arises. The epitympanic recess is well excavated dorsally and somewhat laterally into the temporal bone. An incipient crista arises, separating this cavity in roughly equal medial and lateral halves. The lateral half is somewhat ventrally located compared to the medial half. This lateral portion may be homologous to the epitympanic sinus of Segall (1943) and Decker & Wozecraft (1991). There are no ossicles preserved. A large, round cerebellar (=subarcuate) fossa begins just posterior to the openings for the facial nerves. The remainder of the bony labyrinth is described later in the virtual cranial endocast section.

A moderately developed basioccipital embayment opens laterally ( Fig. 3). This embayment (presumably housing the inferior petrosal sinus) begins as a dorsal excavation of the basioccipital, just anterior to the oval window. It continues posteriorly with well-defined ventral and dorsal floors of the basioccipital, and terminates as a ventral excavation just anterior to the posterior lacerate foramen. This basioccipital embayment, while likely larger than in any known living musteloid ( Hunt & Barnes, 1994), is smaller than that of Gustafsonia , an early amphicyonid ( Tomiya & Tseng, 2016) and the early stem pinnipeds Puijila darwini Rybczynski et al., 2009 (Paterson et al., pers. obs.) and Enaliarctos emlongi Berta, 1991 ( Hunt & Barnes, 1994; Cullen et al., 2014). Modern mustelids display only a gentle indentation, occupying merely the dorsalmost portion of the lateral wall of the basioccipital ( Hunt & Barnes, 1994). As there is no dorsolateral flange of the basioccipital enclosing the embayment (as observed in Simocyon : Wang, 1997), it is unlikely that the inferior petrosal sinus housed a double loop (the ‘ursid loop’) of the internal carotid artery.

A small mastoid process protrudes laterally and somewhat posteriorly from the braincase, so that no sulci are present dorsad to the mastoid process. The mastoid process and the squamosal sulcus do not communicate with each other via a sulcus, like they do in some pinnipedimorphs. The paroccipital process is not closely associated with the mastoid process. The paroccipital process is thin, rod-like and is directed posteriorly and somewhat ventrally. A small hypoglossal foramen lies mediad to the paroccipital process, anterolateral to the large occipital condyles.

Virtual cranial endocast and endocranial space: A virtual cranial endocast ( Fig. 5) was reconstructed from the CT data in Amira v.5. The brain appears demonstrably primitive among mustelids, similar to the brains of other Oligocene Caniformes (excepting Potamotherium ), including that of SG3210 ( Corumictis julieni ) ( Piveteau, 1951). The brain is elongate and anteriorly narrow. Accordingly, the cruciate sulcus is located well-anteriorly. The cruciate sulcus is not particularly prominent and diverges from the midline at an angle approaching 45 degrees. The longtitudinal fissure is particularly well-marked. At about the level of the anterior margin of the bullae, the lateral gyri begin to further diverge from each other, contributing to notably rounded left and right posterior margins. The lateral sulcus runs on a similar plane to the midline, diverging both anteriorly and posteriorly. The suprasylvian sulcus is crescent-shaped, as its anterior portion curves somewhat inferiorly and its posterior portion gently descends the braincase. The lateral and suprasylvian sulci are the most prominent sulci, but are still only moderately deep. A sylvian sulcus is discernible, although not particularly prominent. The pseudosylvian sulcus is absent. In general form, the brain appears reminiscent of Plesictis branssatensis ( Radinsky, 1971) and Mustelictis piveteaui Lange, 1969 ( Lange, 1970).

The turbinates, although poorly preserved, appear simple. The margins of the ethmoidal fossae are obscured due to a large fracture in this area, so only the posteriormost portions of the olfactory bulbs were segmented out. The olfactory bulbs do not appear particularly large, although they are well-separated from the frontal pole of the brain.

The bony labyrinth is relatively indistinct compared to that of extant mustelids and procyonids ( Grohé et al., 2016). The cochlea is neither enlarged nor reduced, and its coils are only moderately defined. Unlike semi-aquatic musteloids, the cochlea is not anteriorly directed. None of the semicircular canals deviate from a circular shape, including the anterior canal, which is less ellipsoid than that of semi-aquatic musteloids ( Grohé et al., 2016). The lateral and posterior canals share an angle that is slightly acute, although straighter than that of semi-aquatic musteloids. The vestibule is somewhat laterally compressed, although less so than that of extinct mephitids ( Grohé et al., 2016).

The calibre of the choana can only be inferred from the partially preserved perpendicular portion of the palatine bone and the sphenoethmoidal plate on the left side of the specimen, as the vomer is not present. The choana was likely wide and ovular with a rounded lateral wall.

The optic foramina open about 0.4 mm anteriorly to the common fossa for the anterior lacerate foramen and foramen rotundum. At their outset, the optic canals are separated by a prominent ventral ridge, and do not come into direct contact with each other before fading into the intercranial space.

T h e t e n t o r i u m i s n o t p a r t i c u l a r l y l a r g e. I n coronal aspect, it arises near the anterior extent of the bony labyrinth, as a narrow projection of bone before expanding into a mediolaterally broad, diamond-shaped ventral projection. The wings of this projection diverge posteriorly, eventually meeting the occiput to enclose the large transverse sinuses.

The septal process of the nasal bone and the nasal part of the frontal bone contribute to a flange that projects ventrally from the roof of the skull, stretching nearly two thirds of the way to the palate at its deepest. From this projection arises a well-preserved cribiform plate. Tracking it coronally, its position generally follows that of the supraorbital processes.

The frontal sinuses arise just as the cribiform plate expands and recedes into the surrounding skull bones posteriorly. There is a pair on each side, the larger of which is more dorsomedially positioned and wellrounded in coronal aspect. The same-sided sinuses are appressed to each other, but far away from the sinuses of the opposite side, straying towards the outskirts of the intracranial region and nearly intruding upon the lateral projections of the supraorbital processes.

Dentition: Measurements of the dentition of JODA 8167 are presented in Table 1. The P1 is not preserved. However, an alveolus, similar in size to the anterior alveolus of the P2, is visible in HRxCT. It is unknown if P1 was single- or double-rooted. Posteriorly, and somewhat laterally, in close proximity, is a double-rooted P2, although lacking the anterior and occlusal portions. The P3 is somewhat obliquely oriented compared to the tooth row axis in ventral view, and displays a tall primary cusp, incipient development of an anterior accessory cusp and strong lingual cingulum. A narrow lobule protrudes lingually from the main body of the tooth, just distal to the primary cusp, in line with the most lingual of the three roots. Like the P3, the P4 is obliquely oriented, with its distal portion deflected laterally. The P4 displays a small parastyle, a high paracone and a low metastyle. The paracone and metastyle are separated by a carnassial notch, producing a shearing blade. The protocone is cusp-like and located lingually and slightly anteriorly compared to the paracone. The P4 lacks an internal shelf and a hypocone.

Mesiobuccally, the M1 exhibits a strong, buccally protruding parastyle, causing the buccal border of the M1 to appear notched. Immediately lingual to the parastyle, and separated from it by a slight notch, there is a tall paracone that nearly reaches the height of the parastyle. The cusp-like metacone is lower than the paracone and sits distal to it, extending significantly more distally than the rest of the M1. The lingual half of the M1 is heavily constricted anteroposteriorly. The protocone is cusp-like, reaching the height of the metacone and occupies the majority of the M1’s lingual half. The protocone root of the M1 penetrates the dorsal surface of the maxilla ( Fig. 2B).

A preprotocrista is present, stretching from the anterior paraconal cingulum to the protocone, although it abruptly terminates in a marked incision before actually reaching the protocone. Anterior to the preprotocrista, a weak cingulum begins, wrapping around the lingual side of the tooth and becoming more prominent posteriorly. This cingulum swells lingually and slightly posteriorly to the protocone, in the expected location of the hypocone. This incipient cingular hypocone is much lower than the protocone and connects to it via a weak crest. There is no postprotocrista. The anterior and posterior borders of the lingual side are nearly parallel, but converge slightly as they approach the lingual border.

The crown of M2 is absent, but an alveolus for a single root is present just posterolingual to the metacone of the M1. The root of the M2 is present on each side and penetrates the dorsal surface of the maxilla. The alveolus is small and the M2 was likely vestigial.

Referred dentary: Despite coming from different stratigraphic levels, the m 1 in the referred dentary readily occludes with the P4 and M1 of the holotype skull, suggesting they are from the same taxon. The elongate trigonid of the m1 fits the metastyle shear facet of the P4 and the blade-like m1 hypoconid matches the M1 protocone/preprotocrista/paracone notch.

The dentary consists of a portion of the horizontal ramus, with only the m1 present ( Fig. 6). The p4 is absent, but represented by a pair of alveoli. The trigonid of the m1 is significantly longer and higher than the talonid and houses three main cusps. A sharp paraconid reaches its apex mesiolingually. The protoconid is incomplete, but appears wide and almost certainly reached a greater height than the paraconid. A small, but sharp metaconid shares a distal border with the protoconid. There is no entoconid and the talonid is dominated by an elongate and prominent hypoconid. The lingual border of the talonid is relatively straight in dorsal view and rises gently dorsally towards the posterior end, displaying a slight lingual notch ( Wang et al., 2004). A notch is better developed on the labial border of the talonid, replete with an elongate and nearly cusp-like hypoconid. The m2 is absent, but preserves two alveoli, indicating it was double-rooted.

The dentary is slender. At its deepest point, the body of the mandible is barely taller than the apex of the preserved paraconid of the m1. Anterior to the m1, the body of the mandible slightly tapers and curves dorsally. The anteriormost portion of the masseteric fossa is present, and appears shallow.