Eoryctes melanus Thewissen and Gingerich, 1989

Wible, John R., 2022, Petrosal and cranial vascular system of the early Eocene palaeoryctid mammal Eoryctes melanus from northwestern Wyoming, USA, Acta Palaeontologica Polonica 67 (1), pp. 203-220 : 205-212

Eoryctes melanus Thewissen and Gingerich, 1989

Figs. 1–6 .

1989 Eoryctes melanus ; Thewissen and Gingerich 1989: 460, figs. 1–7.

2004 Eoryctes melanus Thewissen and Gingerich, 1989 ; Bloch et al. 2004: fig. 15.

2014 Eoryctes melanus Thewissen and Gingerich, 1989 ; Orihuela 2014: fig. 11.

Material.— Holotype, UM 68074 , CT scans of partial cranium (holotype also includes fragment of dentary, which was not examined); from NW/4, Sec. 1, T55N, R102W, Park County, Clarks Fork Basin , Wyoming, USA ( University of Michigan locality SC-133); early Wasatchian Land-Mammal Age, early Eocene.

Description.— Tympanic surface: In ventral view of UM 68074, the medial aspect of the petrosal contacts the basioccipital anteriorly and exoccipital posteriorly ( Fig. 1 : pe, bo, eo); as these parts of the occipital are not separated by sutures, a border between them cannot be defined. Given the disposition of the bones and foramina, it seems likely that the jugular foramen ( Fig. 1 : jf) is between only the exoccipital and petrosal. This is not unusual but the width of the exoccipital puts the jugular foramen in a lateral position, posterior to the pars cochlearis rather than posteromedial to it. The anterior aspect of the petrosal primarily abuts the epitympanic wing of the alisphenoid, but has a narrow contact medially with the basisphenoid and laterally with the squamosal ( Fig. 1 : ewas, bs, sq); as basi- and alisphenoids are not separated by a suture, their positions are identified as the author has done in extant mammals with the basisphenoid on the central stem of the basicranium and the alisphenoids as the lateral wings (e.g., Wible 2008, 2011). There are narrow gaps between the alisphenoid and petrosal that are not symmetrically arranged between the right and left sides, suggesting that they resulted from breakage and that a piriform fenestra is likely not present. Thewissen and Gingerich (1989: 464) reported a piriform fenestra and “the foramen for the entrance of the internal carotid artery into the braincase” in the sphenoid-petrosal suture. As shown below, the latter is surely not present, but the existence of a narrow piriform fenestra cannot be excluded. The lateral aspect of the petrosal contacts the squamosal ( Fig. 1 ) and the posterior aspect is not covered by other bones and forms the mastoid exposure on the occiput.

There is a foramen in the lateral aspect of the suture between the petrosal and alisphenoid ( Fig. 1 : fri) that is related to a branch of the stapedial artery described more fully below. A reviewer of an earlier version of this paper, Robert J. Asher, suggested this opening is a piriform fenestra based on a similar opening related to the same branch of the stapedial artery occurring in the extant tenrec Geogale aurita Milne Edwards and Grandidier, 1872 ( Asher 2001: fig. 6) and the extant lipotyphlan Solenodon paradoxus Brandt, 1833 ( Wible 2008: fig. 26; Fig. 7B : pf). Following MacPhee (1981), the piriform fenestra is a large, membrane-bound gap in the basicranium anterior to the petrosal that occurs in all fetal mammals; neighboring bones may fill in the membrane to partially or fully close the gap in adults or it may remain open. Being membrane bound, the piriform fenestra is in a common plane; however, the opening for the stapedial artery branch in UM 68074 is in a different plane ventral to that of the remainder of the suture between the alisphenoid and petrosal. This difference in depth is not apparent from the illustration ( Fig. 1 ), but the foramen in question is at the same level as the ventral surface of the promontorium of the left petrosal ( Fig. 1 : pe). Additionally, the foramen is in an oblique plane, angled anteroventrally, whereas a piriform fenestra is in a horizontal plane. In light of these planar differences, the foramen in question is not equated with the piriform fenestra.

Most of the features of the petrosal are described based on the more completely preserved left element; the right petrosal is damaged, revealing the basal coil of the cochlea Fig. 1 : cd). In a separate section are reconstructions of the major neurovascular structures that leave imprints on the petrosal. For descriptive purposes, the petrosal is divided into the anteroventromedial pars cochlearis for the cochlear duct and saccule and the posterodorsolateral pars canalicularis for the utricle and the semicircular canals.

In ventral view, the pars cochlearis is dominated by the hemispherical promontorium ( Fig. 2A : pr). Following the method of Ekdale (2013), the cochlear duct is coiled 720°, equivalent to 2.0 coils. The surface topology of the promontorium reflects these two coils, with a shallow, curved sulcus on the medial aspect marking the separation between the dorsal basal coil and the ventral apical coil. On the posterolateral aspect of the promontorium is a large, round opening, the posterior carotid foramen ( Fig. 2A : pcf), leading into two canals of different sizes. The larger lateral canal, the stapedial canal ( Fig. 2A : sc), is slightly narrower than the posterior carotid foramen; it bulges dramatically from the promontorial surface and has a short, curved dorsolateral course. The smaller medial canal, the promontory canal ( Fig. 2C : pc), is best seen in the transparent petrosal as it produces only a slight bulge from the promontorial surface. The promontory canal is less than half the maximum dimension of the stapedial canal and curves anteromedially across the promontorium. The small posterior opening into the promontory canal is visible just inside the posterior carotid foramen in Fig. 2A .

As is typical in mammals, there are two large windows into the inner ear on the posterior and posterolateral aspects of the promontorium, the fenestra cochleae and fenestra vestibuli, respectively; these are hidden in ventral view. The location of the fenestra cochleae, the round window, is marked by a semilunar depression posterior to it, the cochlear fossula ( Fig. 2A : cf). Not visible in the figures is the fenestra vestibuli, the oval window for the footplate of the stapes, situated just dorsal to the terminus of the stapedial canal ( Fig. 2A : sc). The fenestra cochleae is oval, wider than tall, with a ratio of mediolateral width to dorsoventral height of 1.8; the margins of the fenestra vestibuli are not as evident but this opening is estimated to have a ratio of length to height minimally of 2.0. The broad column of bone that separates the two windows is the crista interfenestralis ( Fig. 2A : ci), which shows a complex morphology in Eoryctes melanus discussed with the pars canalicularis below.

A thin shelf arises from the medial and anterior aspects of the promontorium. The anterior component of this continuous shelf is identified as the epitympanic wing and the medial component as the rostral tympanic process ( Fig. 2A : ew, rtp). The epitympanic wing is horizontal and meets the epitympanic wing of the alisphenoid to roof the hypotympanic sinus of the alisphenoid; as noted above, there is breakage along the contact between the epitympanic wings of the petrosal and alisphenoid ( Fig. 1 ). Adjacent to the promontorium, the rostral tympanic process is horizontal but farther medially, it curves ventromedially, abutting a concave surface on the lateral aspect of the basioccipital, which represents a tympanic process of that bone. A sizeable crack delimits most of the rostral tympanic process from the remainder of the pars cochlearis on both the right and left petrosals ( Fig. 1 ). Thewissen and Gingerich (1989) raised the possibility that this crack might be a suture and that the bone medial to the crack is an independent entotympanic element. However, this is considered unlikely here as the cracks are not entirely symmetrical between the two sides and there is evidence of continuity between the rostral tympanic process and petrosal posterior to the crack on the left side and both anterior and posterior to the crack on the right side. The size of the rostral tympanic process is essentially the same on the left and right sides; however, the comparable structure on the left side of the referred specimen, UM 72623, appears to be slightly more extensive ( Thewissen and Gingerich 1989: fig. 4). The right side of UM 72623, has a large, irregular bone fragment ventral to the promontorium and separated from what is identified here as the rostral tympanic process, suggestive that E. melanus had at least a partial osseous floor to the tympanic cavity ( Thewissen and Gingerich 1989: fig. 4). As noted correctly by Thewissen and Gingerich (1989), it is uncertain whether this bone fragment is part of the petrosal or part of an entotympanic. UM 72623 also preserves fragments of the right ectotympanic, including the ring-shaped posterior crus that widens into a bony plate ( Thewissen and Gingerich 1989).

In ventral view, the pars canalicularis is an uneven shelf lateral and posterior to the pars cochlearis. Preserved at the posterolateral corner of this shelf on the left side is a mediolaterally compressed, ventrally directed paroccipital process ( Figs. 1 , 2A : pp), the mastoid process of some authors (e.g., Butler 1956). The paroccipital process reaches ventrally to the same extent as the promontorium and is covered laterally by the mediolaterally compressed posttympanic process of the squamosal ( Fig. 1 : ptp). Thewissen and Gingerich (1989) noted that the ventral surface of the mastoid is broken in the holotype but based on the referred specimen reported the absence of a mastoid process. They did identify the structure called a paroccipital process on the left side of the holotype here. Extending anteromedially and decreasing in height from the paroccipital process in UM 68074 is the crista parotica ( Fig. 2A : cp); it ends opposite the fenestra vestibuli where it forms the lateral margin of the foramen transmitting the stapedial artery ( Fig. 2A : fsa). The medial aspect of the crista parotica has a distinct rounded bulge, which is identified here as the tympanohyal ( Fig. 2A : th). Medially, the tympanohyal abuts and is fused to the crista interfenestralis and part of the caudal tympanic process described below ( Fig. 2A : 3). Dorsal to this abutment is a conduit connecting the area of the fenestra vestibuli with the rear of the pars canalicularis. Posterior to the tympanohyal and medial to the paroccipital process is a rounded notch, the stylomastoid notch ( Fig. 2A : smn) for the facial nerve.

The principal features on the pars canalicularis posterior to the pars cochlearis are the fossa for the stapedius muscle ( Fig. 2A : sf) and the caudal tympanic process ( Fig. 2A : 1–3). The stapedius fossa is a deep, oval concavity, wider than long, with an area more than four times that of the fenestra vestibuli. The caudal tympanic process, following MacPhee (1981: figs. 2, 50a) and Wible and Shelley (2020: fig. 5), is divided into three sections: (i) medial section; (ii) lateral section, situated medial to the stapedius fossa; and (iii) lateral section, situated lateral to the stapedius fossa. UM 68074 has all three sections: (i) and (ii) are continuous and form a low ridge around the rear of the cochlear fossula; anterolaterally, (ii) merges with the crista interfenestralis and tympanohyal, (iii) remains separate from the other sections and is represented by a low, short ridge medial to the paroccipital process; it contributes to but does not complete a low wall behind the stapedius fossa.

The principal features on the pars canalicularis lateral to the pars cochlearis are the epitympanic recess ( Figs. 1 , 2A : er) and the tegmen tympani ( Fig. 2A : tt). The former makes a roof over the incudomallear articulation and the latter the roof over the middle ear rostral to the epitympanic recess ( Klaauw 1931). In UM 68074, the large epitympanic recess is round with its medial half on the petrosal and its lateral half on the squamosal. In the posterior wall of the epitympanic recess is a shallow depression entirely on the petrosal identified as the fossa incudis ( Fig. 2A : fi) for the crus breve (short process) of the incus. The tegmen tympani projects anteroventrally from the level of the epitympanic recess and at its anterior extent is roughly even with the ventral promontorial surface. At its widest, the tegmen tympani is just a little narrower than the promontorium. It has a longitudinal prominence marking the course of the stapedial artery and its rostral continuation, the ramus inferior. The anterior margin of the tegmen tympani forms the posterior border of the foramen for the ramus inferior ( Fig. 1 : fri), which is completed anteriorly by the alisphenoid. Where the tegmen tympani abuts the pars cochlearis is an oval depression, much longer than wide, interpreted as the fossa for the tensor tympani muscle ( Fig. 2A : ttf).

Endocranial surface: The holotype cranium is sagittally sectioned and the left endocranium is illustrated in Fig. 3A . The petrosal is situated in the posterolateral aspect of the endocranium. Its anterior border, which lies in the middle cranial fossa, is formed by the tegmen tympani in contact with the squamosal and alisphenoid, and the epitympanic wing underlying the alisphenoid. On the petrosal’s medial border, the rostral tympanic process has a point contact with the basisphenoid and an extensive one with the basioccipital. The posterior border is formed by the exoccipital ventrally and the supraoccipital dorsally ( Fig. 3A : so), but a suture does not distinguish the two. The petrosal’s dorsal border is between the pars canalicularis and parietal with a small incursion of squamosal contributing to the groove for the ramus superior ( Fig. 3B : grs).

On the petrosal, the pars cochlearis is positioned ventromedially and the pars canalicularis dorsolaterally ( Fig. 3B : pco, pca). Each part is dominated by a large opening: the internal acoustic meatus on the former and the subarcuate fossa on the latter ( Fig. 3B : iam, saf). Both openings are oval, with the subarcuate fossa more than twice the area of the internal acoustic meatus. Details of the internal acoustic meatus are only preserved on the right petrosal ( Fig. 4 ). It has a shallowly recessed transverse crest ( Fig. 4 : tc), which delimits the foramen acusticum inferius and superius. The latter has the opening for the facial canal in front and the superior vestibular area behind; the former contains the spiral cribriform tract but additional structures could not be discerned. The subarcuate fossa is wider than deep and its aperture is constricted compared to the fossa proper. Anterior to the subarcuate fossa is a sharp but low crista petrosa ( Fig. 3B : crp), which does not extend ventrally onto the pars cochlearis as this area is gently rounded. Anterior to the crista petrosa is a concave area contributing to the posterior wall of the middle cranial fossa ( Fig. 3B : mcf), which also represents the roof of the tegmen tympani. In the anteromedial aspect of this concavity is a small opening with a groove emanating from it, the hiatus Fallopii ( Fig. 3B : hF). The aperture for the endolymphatic duct, the vestibular aqueduct, is preserved posterior to the subarcuate fossa ( Figs. 3B , 4 : va), while that for the perilymphatic duct, the cochlear canaliculus, is hidden in the illustrated views ( Fig. 3B : cc), deep within the jugular notch, recessed from the tympanic surface.

Several major arteries and veins leave impressions on the endocranial surface of the petrosal. On the dorsal surface of the tegmen tympani are large grooves for the primary branches of the stapedial artery, the ramus superior and ramus inferior ( Fig. 3B : grs, gri), the former groove slightly larger than the latter. The anterior (endocranial) opening of the promontory canal ( Fig. 3B : pc) is dorsal to the origin of the epitympanic wing. On the ventromedial surface of the pars cochlearis, at the origin of the rostral tympanic process, is a longitudinal sulcus for the inferior petrosal sinus ( Fig. 3B : sips). Lastly, posterior to the subarcuate fossa, on the pars canalicularis is the sulcus for the sigmoid sinus ( Fig. 3B : sss).

Reconstructions.— Facial nerve: The facial nerve, cranial nerve VII, courses through the substance of the petrosal in extant mammals ( Sisson 1910; Evans 1993; Strandring 2008). Thewissen and Gingerich (1989) noted that the course of the facial nerve in E. melanus is not exposed in the middle ear, but is enclosed in a canal. However, they did not provide details of the nerve’s course. Boyer and Georgi (2007) stated that the extensive crista parotica in palaeoryctids makes the facial canal appear deeper, but the nerve would still have been in contact with the tympanic cavity, although they did not clarify on what taxon or specimen this was based. The CT scans of UM 68074 help to clarify the facial nerve course as enclosed in canals, with the right petrosal preserving the anterior segment of the course and the left petrosal preserving the posterior segment; this information has been reconstructed onto the left petrosal ( Fig. 2C ).

The facial nerve enters the dorsal aspect of the internal acoustic meatus ( Fig. 3C : cn VII) where it enters the facial canal. After a short ventrolateral course, the facial canal expands into a larger space, the cavum supracochleare, which contains the geniculate ganglion of the facial nerve ( Fig. 2C : gg). Two nerves emerge from the geniculate ganglion, the larger is the continuation of the facial nerve and the smaller is the greater petrosal nerve ( Fig. 2C : fn, gpn). The latter runs forward in a canal to its exit from the petrosal via the hiatus Fallopii. From there, the greater petrosal nerve leaves the endocranium between the petrosal and alisphenoid, and runs forward in a groove on the epitympanic wing of the alisphenoid ( Fig. 1 : npc). A second groove is for the internal carotid nerve ( Fig. 5 : icn), which leaves the endocranium medial to the greater petrosal nerve, to form the nerve of the pterygoid canal ( Fig. 5 : npc). The nerve of the pterygoid canal crosses the epitympanic wing of the alisphenoid and enters a short canal in the alisphenoid that opens endocranially.

The larger nerve leaving the geniculate ganglion, the facial nerve, runs posterolaterally in a canal that ultimately opens anterolateral to the stapedius fossa, under cover of the crista parotica and paroccipital process ( Fig. 2C ). There is a window into this canal opposite the fenestra vestibuli where the stapedial artery enters the canal ( Fig. 2A : fsa). The nerve and artery share a short common canal before diverging, with the nerve running posterolaterally and the artery anteriorly (see Fig. 3B, C ). After exiting the facial canal near the stapedius fossa, the facial nerve turns ventrally in a shallow groove on the medial aspect of the paroccipital process, posterior to the tympanohyal ( Fig. 2C ). Its point of exit is the stylomastoid notch ( Fig. 2A : smn).

Cranial vessels: Thewissen and Gingerich (1989) made a latex endocast from the cranial cavity of UM 68074, which preserved considerable detail about the cranial vascular system. Between the endocast and the grooves, canals, and foramina on the skull exterior, these authors described much of what is reconstructed in the current report, often using different terminology. There are only a few instances where our interpretations differ. Thewissen and Gingerich (1989) did not illustrate the cranial vascular system as is done here to help the reader visualize what is a relatively complex pattern.

The main arteries are reconstructed on the left petrosal of UM 68074 in ventral view in Fig. 2B . It shows a branching pattern in the middle ear similar to that in early eutherians, such as Early Cretaceous Prokennalestes trofimovi Kielan-Jaworowska and Dashzeveg, 1989 ( Wible et al. 2001), with the internal carotid (“ica” in Fig. 2B ) in a transpromontorial position and its primary branch, the stapedial artery, dividing into the ramus superior and ramus inferior ( Fig. 2B : sa, rs, ri; see also Fig. 7B ). Thewissen and Gingerich (1989) noted correctly that the course of these vessels through the middle ear is shielded by bone, the exception being the stapedial artery as it crosses the fenestra vestibuli. They provided measurements of the bony tubes for the internal carotid prior to the origin of the stapedial artery (approximately 1.0 mm), for the internal carotid after the origin of the stapedial artery in the promontory canal (0.6 mm), and for the stapedial artery (0.8 mm). The CT scans studied here allow measurements of the interior of these bony tubes on the promontorium: the posterior carotid foramen is 0.93 × 0.77 mm (area = 0.71 mm 2); the promontory canal is 0.24 × 0.20 mm (area = 0.05 mm 2); and the stapedial canal is 0.51 × 0. 49 mm (area = 0.25 mm 2). Therefore, the stapedial canal is five times the area of the promontory canal at their origins. The difference in the size of the arteries within their respective canals is even greater given that the promontory canal typically has two occupants, the internal carotid artery and nerve, although there are odd instances in extant mammals where there is only one occupant, the internal carotid nerve ( Conroy and Wible 1978).

The promontory canal opens endocranially at the anterior aspect of the pars cochlearis, dorsal to the epitympanic wing Fig. 3B ). The internal carotid artery curves anteromedially in a faint groove on the dorsal surface of the epitympanic wing ( Fig. 3C : ica) and then onto the basisphenoid. Although a carotid groove on the basisphenoid is lacking, the trajectory of the vessel on the petrosal puts the artery wholly lateral to the shallow hypophyseal fossa ( Fig. 3A : hyf). Given the relatively small size of the internal carotid artery at the promontory canal, the vertebral artery was likely the most significant contributor of blood to the brain in E. melanus .

Beyond the fenestra vestibuli, the stapedial artery enters the foramen ( Fig. 2A : fsa) into the common canal for the artery and the facial nerve (see above). The stapedial artery turns anteriorly and divides into its end branches, the ramus superior and ramus inferior ( Fig. 3C : rs, ri), which are no longer in a canal but in open grooves on the endocranial surface ( Fig. 3B : grs, gri). Thewissen and Gingerich (1989) reported the ramus inferior to be the larger of the two, but they are subequal based on the CT scans.

After traversing the roof of the tegmen tympani, the ramus inferior ( Fig. 5 ) leaves the endocranium at a foramen between the petrosal and alisphenoid ( Fig. 1 : fri) and runs forward in a groove on the alisphenoid ( Fig. 1 : gri). Thewissen and Gingerich (1989) identified this groove as the Glaserian fissure, which conducts the chorda tympani and sometimes the ramus inferior from the middle ear in extant mammals ( Klaauw 1931). However, a separate Glaserian fissure is identified here for the chorda tympani on the posteromedial surface of the entoglenoid process of the right squamosal in UM 68074 ( Fig. 1 : arrowhead), a pattern remarkably like that in the extant Solenodon paradoxus ( Wible 2008) . At the foramen ovale ( Fig. 1 : fo), the ramus inferior divides into the ramus mandibularis and ramus infraorbitalis ( Fig. 5 : rm, rio). Reconstruction of the former is based on the prevalent pattern in extant mammals for the origin of the artery accompanying the mandibular nerve, the third division of the trigeminal ( Fig. 5 : V 3), to the lower jaw and of the latter on the presence of the alisphenoid canal ( Fig. 1 : asc; the alar canal of Thewissen and Gingerich 1989), which transmits the ramus infraorbitalis (= maxillary artery) in extant mammals (e.g., Bugge 1974; Wible 1984, 1987). The alisphenoid canal in UM 68074 conducts the ramus infraorbitalis to the orbit via the sphenorbital fissure ( Fig. 1 : sof; the rotundo-orbital foramen of Thewissen and Gingerich 1989), which based on the right side of UM 68074 is entirely in the alisphenoid. Within the orbit, the ramus infraorbitalis runs anteriorly with the maxillary nerve, the second division of the trigeminal ( Fig. 5 : V 2), ultimately reaching the infraorbital foramen ( Fig. 6 : iof) as the infraorbital artery. A major branch of the ramus infraorbitalis in the orbit in extant mammals is the sphenopalatine artery, but as the sphenopalatine foramen was not identifiable in UM 68074 due to damage, I left that artery out of the reconstruction.

From its origin, the ramus superior runs dorsally in a groove on the pars canalicularis and then onto a broader groove on the parietal ( Fig. 3 ). Here, the ramus superior supplies multiple rami temporales (five on the right and three on the left) that exit the endocranium via openings in the parietal ( Figs. 4 , 6 : rt; Fig. 3A : frt). Thewissen and Gingerich (1989) recognized correctly that these foramina transmit arteries and veins, but called them parietal emissary foramina; the term employed here, foramina for rami temporales, is more descriptive of their occupants. On both sides of UM 68074, one foramen for the ramus temporalis is exceedingly large, comparable to the foramen ovale, while the others are considerably smaller. There is no posttemporal canal in UM 68074, which connects the occipital artery and ramus superior via the arteria diploëtica magna in early eutherians (e.g., Prokennalestes trofimovi, Wible et al. 2001 ).

After supplying the rami temporales, the ramus superior curves forward as the ramus supraorbitalis and accompanying vein ( Fig. 6 : rso) initially in the orbitotemporal groove (sinus canal of Thewissen and Gingerich 1989) on the parietal and then on the frontal ( Fig. 3A : otg). The groove leads to a short orbitotemporal canal ( Fig. 3A : otc), initially between the frontal and orbitosphenoid and then entirely in the frontal, that, in turn, opens into another short orbitotemporal groove in the floor of the rostral cranial fossa ( Fig. 4 : rcf). This groove runs to a ventrally directed foramen in the frontal ( Fig. 1 : otc + ef); a second groove runs dorsolaterally from this foramen on the endocranial sidewall of the rostral cranial fossa. Thewissen and Gingerich (1989) correctly identified this foramen as a combined aperture for the orbitotemporal system (their sinus canal) and ethmoidal foramen, with the ethmoidal nerve, artery, and vein ( Fig. 4 : enav) occupying the groove running dorsolaterally in the rostral cranial fossa.

One last artery is shown in the reconstructions, the ophthalmic artery accompanying the optic nerve ( Fig. 5 : oa + on) in the optic canal ( Fig. 1 : oc). The presence of an ophthalmic artery in the optic canal is conjectural because mammals have two alternative origins for the artery with the optic nerve; it arises intracranially from the circle of Willis (as reconstructed here) or extracranially from the ramus infraorbitalis via the ramus orbitalis ( Bugge 1974; Wible 1987) and does not pass through the optic canal.

Regarding the cranial venous system, on the endocast of UM 68074, Thewissen and Gingerich (1989: fig. 7) described a large sinus on the posterior part of the cerebrum and anterior part of the cerebellum that they called the parietal sinus. Their parietal sinus was fed dorsally by the transverse and dorsal (superior) sagittal sinuses and had four primary exits from the cranium: (i) the parietal emissary foramina (= foramina rami temporales); (ii) the sinus canal (= orbitotemporal groove and canal); (iii) the temporal sinus (= the capsuloparietal emissary vein; see Wible 1990) out the retroarticular foramen (= postglenoid foramen); and (iv) the sigmoid sinus out the petro-occipital fissure (= jugular foramen). I agree with their identification of transverse and dorsal sagittal sinuses and the four exits from the parietal sinus, although the first two exits in the reconstruction here are primarily arterial ( Figs. 4 , 6 ). However, I question the identification of a parietal sinus by Thewissen and Gingerich 1989). To me, this is an enlargement caused by the juxtaposition of the ramus superior of the stapedial artery internal to the capsuloparietal emissary vein ( Fig. 4 : cpev) and the veins accompanying the rami temporales and ramus supraorbitalis. Thewissen and Gingerich (1989) observed that the sigmoid sinus is larger than the temporal sinus (= capsuloparietal emissary vein), and I add that the largest foramen for ramus temporalis is also larger than the jugular foramen, both of which are not solely venous in nature.

The endocast of UM 68074 also shows the course of the inferior petrosal sinus (= ventral petrosal sinus of Thewissen and Gingerich 1989: fig. 7), which connects from an area posterolateral to the pons to the sigmoid sinus at the jugular foramen. Despite the connection to the jugular foramen on the endocast, Thewissen and Gingerich (1989: fig. 1) reported that the inferior petrosal sinus left the cranium via a separate foramen located immediately posterior to what is described here as the rostral tympanic process of the petrosal. There is a depression on the left side here (asterisked in Fig. 1 ), which is preserved differently on the right side. Review of the CT scans reveal that this depression is not a foramen but a blind end. The CT scans also show that the sulcus for the inferior petrosal sinus ( Fig. 3B : sips) extends all the way to the jugular foramen as in the endocast. Therefore, the exit point for the inferior petrosal sinus is reconstructed here ( Fig. 4 : ips) as the jugular foramen where it joins the sigmoid sinus to form the internal jugular vein ( Fig. 4 : ijv).

During the process of making the endocast of UM 68074, Thewissen and Gingerich (1989) considered the olfactory bulbs to be too delicate and did not include them with the cast. They reconstructed the olfactory bulbs on the endocast ( Thewissen and Gingerich 1989: fig. 7) and included a bifurcated vascular structure on the dorsal midline that was labeled “canal for dorsal emissary vein,” which was not treated further in their text. These authors did describe the frontal diploic vein, extending from the dorsal midline to the orbit via the foramen caecum (= foramen for frontal diploic vein here; Fig. 1 : fdv), which presumably includes their dorsal emissary vein. The CT scans provide additional detail about this vein. The sulcus for the frontal diploic vein arises from the anterior end of the sulcus for the dorsal sagittal sinus on the dorsal midline of the anterior part of the rostral cranial fossa. After a short ventrolateral course, the sulcus enters a canal entirely in the frontal, which it occupies until it emerges in the orbit ( Fig. 6 : fdv). On the right side, there is a window into the canal where the vein would have been exposed endocranially, whereas on the left side there are two such windows, neither of which corresponds to the window on the right suggesting possible preservational differences. Posterolateral to the main entrance into the canal for the frontal diploic vein is a much smaller foramen on both sides that connects to, and was a distributary of, the main canal. Thewissen and Gingerich (1989: fig. 7) illustrated but did not label this vessel on the endocast.

The CT scans help clarify several other foramina observed by Thewissen and Gingerich (1989). First, they ( Thewissen and Gingerich 1989: figs. 1, 2) reported two small venous foramina in the medial wall of the alisphenoid canal (their alar canal) on the left side of UM 68074 that they suggested ( Thewissen and Gingerich 1989: 464) “probably carried vessels to the diploic spaces of the basisphenoid”. The CT scans show that the posterior of these two foramina ( Fig. 1 : tcf) opens into a canal passing through the basisphenoid and opening on the opposite side, whereas the anterior foramen is merely a depression. The posterior foramen represents a transverse canal foramen, which carries a vein in extant mammals ( Wible 2003, 2008). Second, the endocast of UM 68074 has the impression of a small canal posterior to the optic canal that appears to enter the diploic space of the presphenoid (illustrated but not labeled in Thewissen and Gigerich 1989: fig. 7). The CT scans show that this canal after a short course through the presphenoid joins the more dorsomedially positioned optic canal. In light of its proximity to the sphenorbital fissure and union with the optic canal, a probable occupant of this structure is the ophthalmic vein, although I am not aware of any example of such a separate conduit for this vein in extant mammals. One foramen that the CT scans did not help identify is the small mastoid emissary foramen, which Thewissen and Gingerich (1989) described as piercing the dorsal portion of the mastoid in UM 68074, although absent in UM 72623; I was not able to find this opening in the holotype.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 1. Palaeoryctid mammal Eoryctes melanus Thewissen and Gingerich, 1989, UM 68074 (UM locality SC-133, Clarks Fork Basin, Wyoming, USA; early Wasatchian Land-Mammal Age, early Eocene), isosurface of cranium rendered from CT scans in ventral view. Petrosals in gold; basi- and alisphenoids in red; pre- and orbitosphenoids in blue; frontals in green. Asterisk marks the depression identified as the foramen for the ventral petrosal sinus by Thewissen and Gingerich (1989). Arrow is directed at the Glaserian fissure. Abbreviations: as, alisphenoid; asc, alisphenoid canal; bo, basioccipital; bs, basisphenoid; cd, exposed cochlear duct; egp, entoglenoid process; eo, exoccipital; er, epitympanic recess; ewas, epitympanic wing of alisphenoid; fdv, frontal diploic vein foramen; fo, foramen ovale; fr, frontal; fri, foramen for ramus inferior; gf, glenoid fossa; gri, groove for ramus inferior; hf, hypoglossal foramen; jf, jugular foramen; mx, maxilla; npc, groove for nerve of pterygoid canal; oc, optic canal; os, orbitosphenoid; otc + ef, anterior opening of orbitotemporal canal and ethmoidal foramen; pal, palatine; pe, petrosal; pgf, postglenoid foramen; pp, paroccipital process; ppt, postpalatine torus; ps, presphenoid; ptp, posttympanic process of squamosal; sof, sphenorbital fissure; sq, squamosal; tcf, transverse canal foramen; tpas, tympanic process of alisphenoid.

Fig. 2. Palaeoryctid mammal Eoryctes melanus Thewissen and Gingerich, 1989, UM 68074 (UM locality SC-133, Clarks Fork Basin, Wyoming, USA; early Wasatchian Land-Mammal Age, early Eocene), isosurface of left petrosal rendered from CT scans in ventral view. A. Solid petrosal, medial section of caudal tympanic process (1), lateral section of caudal tympanic process, medial and lateral to stapedius fossa (1, 3, respectively). B. Transparent petrosal with internal carotid artery system (in red). C. Transparent petrosal with facial nerve (in yellow) and stapedius muscle and its tendon (in red and white, respectively). Neurovascular structures redrawn from Avizo segmentation. Abbreviations: cf, cochlear fossula; ci, crista interfenestralis; cn VII, cranial nerve VII; cp, crista parotica; er, epitympanic recess; ew, epitympanic wing; fi, fossa incudis; fn, facial nerve; fsa, foramen for stapedial artery; gg, geniculate ganglion; gpn, greater petrosal nerve; ica, internal carotid artery; jn, jugular notch; pc, promontory canal; pcf, posterior carotid foramen; pp, paroccipital process; pr, promontorium; ri, ramus inferior; rs, ramus superior; rtp, rostral tympanic process; sc, stapedial canal; sf, stapedius fossa; sm, stapedius muscle; smn, stylomastoid notch; smt, stapedius muscle tendon; th, tympanohyal; tt, tegmen tympani; ttf, tensor tympani fossa.

Fig. 3. Palaeoryctid mammal Eoryctes melanus Thewissen and Gingerich, 1989, UM 68074 (UM locality SC-133, Clarks Fork Basin, Wyoming, USA; early Wasatchian Land-Mammal Age, early Eocene), isosurfaces rendered from CT scans in medial view. A. Left endocranium (petrosal in gold; parietal in yellow; basi- and alisphenoid in red; pre- and orbitosphenoid in blue; frontal in green). B. Left petrosal. C. Left petrosal with neurovascular structures reconstructed (redrawn from Avizo segmentation; arteries in red; nerves in yellow). Abbreviations: as, alisphenoid; bo, basioccipital; bs, basisphenoid; cc, cochlear canaliculus; cn VII, cranial nerve VII; crp, crista petrosa; eo, exoccipital; ew, epitympanic wing; fr, frontal; frt, foramen for ramus temporalis; gpn, greater petrosal nerve; gri, groove for ramus inferior; grs, groove for ramus superior; hF, hiatus Fallopii; hyf, hypophyseal fossa; iam, internal acoustic meatus; ica, internal carotid artery; M3, upper third molar; mcf, middle cranial fossa; os, orbitosphenoid; otc, orbitotemporal canal; otg, orbitotemporal groove; pa, parietal; pal, palatine; pc, promontory canal; pca, pars canalicularis; pco, pars cochlearis; pe, petrosal; ps, presphenoid; ri, ramus inferior; rs, ramus superior; rtp, rostral tympanic process; saf, subarcuate fossa; sips, sulcus for inferior petrosal sinus; so, supraoccipital; sof(br), sphenorbital fissure (broken); sq, squamosal; sss, sulcus for sigmoid sinus; tt, tegmen tympani; va, vestibular aqueduct.

Fig. 4. Palaeoryctid mammal Eoryctes melanus Thewissen and Gingerich, 1989, UM 68074 (UM locality SC-133, Clarks Fork Basin, Wyoming, USA; early Wasatchian Land-Mammal Age, early Eocene), isosurface of right endocranium rendered from CT scans in ventral view, with neurovascular structures reconstructed based in part on Avizo segmentation (petrosal in gold; arteries in red; veins in blue; nerves in yellow). Segment of ramus superior is transparent to show hidden capsuloparietal emissary vein; holes in squamosal are damage.Abbreviations: ar, annular ridge; as, alisphenoid; ccf, caudal cranial fossa; cpev, capsuloparietal emissary vein; enav, ethmoidal nerve, artery, and vein; eo, exoccipital; fr, frontal; ica, internal carotid artery; ijv, internal jugular vein; ips, inferior petrosal sinus; mcf, middle cranial fossa; os, orbitosphenoid; pa, parietal; pca, pars canalicularis; pco, pars cochlearis; rcf, rostral cranial fossa; ri, ramus inferior; rs, ramus superior; rso, ramus supraorbitalis with accompanying vein; rt, ramus temporalis with accompanying vein; so, supraoccipital; sq, squamosal; ss, sigmoid sinus; tc, transverse crest; ts, transverse sinus; tt, tegmen tympani; va, vestibular aqueduct.

Fig. 5. Palaeoryctid mammal Eoryctes melanus Thewissen and Gingerich, 1989, UM 68074 (UM locality SC-133, Clarks Fork Basin, Wyoming, USA; early Wasatchian Land-Mammal Age, early Eocene), isosurface of cranium rendered from CT scans in ventral view, with neurovascular structures reconstructed based in part on Avizo segmentation (left petrosal in gold; arteries in red; veins in blue; nerves in yellow). The alisphenoid canal is broken on the left side in UM 68074 (see Fig. 1), but is reconstructed here to show the course of the ramus infraorbitalis.Abbreviations: gpn, greater petrosal nerve; ica, internal carotid artery; icn, internal carotid nerve; ijv, internal jugular vein; npc, nerve of pterygoid canal; on + oa, optic nerve and ophthalmic artery; pgv, postglenoid vein; ri, ramus inferior; rio, ramus infraorbitalis; rm, ramus mandibularis; rso, ramus supraorbitalis; V1, ophthalmic nerve, first division of trigeminal nerve; V2, maxillary nerve, second division of trigeminal nerve; V3, mandibular nerve, third division of trigeminal nerve.

Fig. 6. Palaeoryctid mammal Eoryctes melanus Thewissen and Gingerich, 1989, UM 68074 (UM locality SC-133, Clarks Fork Basin, Wyoming, USA; early Wasatchian Land-Mammal Age, early Eocene), isosurface of cranium rendered from CT scans in right lateral view, with neurovascular structures reconstructed based in part on Avizo segmentation (right petrosal in gold; arteries in red; veins in blue; nerves in yellow). As noted in text, the sphenopalatine foramen is not preserved due to damage and, therefore, the expected sphenopalatine artery was omitted from the reconstruction. Abbreviations: as, alisphenoid; ejv, external jugular vein; en + rso, ethmoidal nerve and ramus supraorbitalis with accompanying vein; fdv, frontal diploic vein; fr, frontal; iof, infraorbital foramen; ionav, infraorbital nerve, artery, and vein; lac, lacrimal; lp, lambdoid plate; mx, maxilla; pa, parietal; pal, palatine; pe, petrosal; pgv, postglenoid vein; rm, ramus mandibularis; rt, ramus temporalis with accompanying vein; sq, squamosal; V1 + rso, ophthalmic nerve and ramus supraorbitalis with accompanying vein; V2 + rio, maxillary nerve and ramus infraorbitalis; V3, mandibular nerve.

Fig.7. Lipotyphlan mammal Solenodon paradoxus Brandt, 1833, left basicranium in ventral view. A. CM 18069, photograph of adult (A1); close-up of ear region (A2). B. Drawing of juvenile AMNH 28272 with arterial reconstruction based on MPIH 6863 (modified from Wible 2008: fig. 26C). Caudal tympanic process of the petrosal is broken in CM 18069, exposing the cochlear fossula. Asterisk in A2 is on the membrane of the piriform fenestra in the location of the tensor tympani muscle. Tympanohyal does not contact caudal tympanic process in juvenile AMNH 28272 but does in adults (Wible 2008). Abbreviations: as, alisphenoid; bo, basioccipital; bs, basisphenoid; cf, cochlear fossula; cin, carotid incisure; ctp, caudal tympanic process of petrosal; egp, entoglenoid process; eo, exoccipital; er, epitympanic recess; ew, epitympanic wing of petrosal; fm, foramen magnum; gf, glenoid fossa; ica, internal carotid artery; jf, jugular foramen; me, mastoid exposure; pf, piriform fenestra; pgf, postglenoid foramen; pr, promontorium of petrosal; ri, ramus inferior; rp, ramus posterior; rs, ramus superior; rtp, rostral tympanic process of petrosal; sa, stapedial artery; sm, stapedius muscle; sq, squamosal; th, tympanohyal.