DIPROTODONTIA, Owen, 1866

MacPhee, Ross D. E., Gaillard, Charlène, Forasiepi, Analía M. & Sulser, R. Benjamin, 2023, Transverse Canal Foramen And Pericarotid Venous Network In Metatheria And Other Mammals, Bulletin of the American Museum of Natural History 2023 (462), pp. 1-125 : 35-55

DIPROTODONTIA

NOTAMACROPUS AND OSPHRANTER (MACROPODIFORMES, MACROPODIDAE ) (figs. 18–20). This large family is represented in the comparative set by several specimens, including a serially sectioned perinatal specimen of Notamacropus (= Macropus ) eugenii ZIUT HL 29 mm (fig. 19). The specimen’s head was divided midsagittally prior to histological preparation, and only the right half was available for study. As a result, structures on or near the midline are not well represented. Aplin (1990) described another specimen of the same species, of known age (28- day pouch young). Judging from his micrographs, the degree of ossification of the auditory capsule (fully cartilaginous except in the region of the fenestra cochleae) is similar to that of the ZIUT specimen. Our macropodid material also includes juveniles of N. eugenii AMNH M-197003 (fig. 18A, B), the tamar wallaby, and Osphranter (= Macropus ) robustus AMNH M-80171 (fig. 18C), the common wallaroo.

In the sectioned N. eugenii , the vein identified by the red pointer in figure 19A is interpreted as the presumptive TCV because it is the only external channel that joins the ICV within the mesocranium. The TCV, ICV, and CS freely communicate around the site of the developing carotid canal in this specimen. Figure 20 illustrates the probable organization of the large vessels that would have passed through the mesocranium in O. robustus during life, although no claim for accuracy in their shapes is made. A diagnostic feature of the hybrid pattern is the absence or great reduction of the intramural RBTC pathway. Instead, only the CBTC pathway is clearly present, indicated by the very large CBF. The importance of the CBV is underlined by the fact that its foramen is equivalent in width to the canal for the TCV trunk, which makes sense because it is the latter’s only major tributary (fig. 18C).

The fact that the CBTC pathway passes immediately into the braincase is consistent with Sánchez-Villagra and Wible’s (2002: 30) probe-based observation that “in no case in our study was an intramural bilateral connection found” in any macropodiform. It is also consistent with Aplin’s (1990: 185) observation that in macropodids the connection with the CS “is made via one or more transverse canal foramina: they are usually located in the most deeply excavated region of the medial pterygoid fossa and open directly into the lateral wall of the hypophyseal fossa” (see figs. 18C, 20A– C). Aplin (1990: 184) also noted that the ICV (= pericarotid vein) was “sizeable” in his wallaby specimen and that it was “connected with the prevertebral plexus [= BVP, pharyngeal plexus] and with the posterior emissary vein [=?EVPS] via the posterior lacerate foramen.”

Aplin (1990: 191) tried to trace the formation of the TCF and its associated canal by comparing perinatal stages of several different macropodid species: “A transverse canal emissary foramen is first observed at Stage 3; …the late appearance of this emissary vein suggests that it forms as a direct consequence of the progressive excavation of the medial surface of the pterygoid fossa, perhaps through the annexation of an existing intraosseous vascular component.” Stage 3, of a total of 5 stages, is defined as representing “more advanced pouch-young” (Aplin, 1990: 150), although the only criterion for staging mentioned in Aplin’s table 3.1 is premolar/molar eruption status. How “progressive excavation” and “annexation of an existing vascular component” might lead to the formation of the TCV is insufficiently explained. Aplin’s observations might alternatively concern the developing TBS and onset of blood cell production, but this has not yet been demonstrated to occur in macropodids. Also, the number of transverse canals is variable: in O. robustus AMNH M-80171 there are accessory transverse canals (fig. 20A; see also Trichosurus , fig. 22B).

Aplin’s (1990: fig. 3.6b) venogram of an?adult Potorous tridactylus is of interest because he identified a remarkably large ICV/BVP (= pericarotid vein) in this specimen. Because of vessel overlaps, his identification is difficult to verify. Although ICV/BVP hypertrophy cannot be excluded in this species, nothing similar has been reported for other taxa. For example, in the perinatal ZIUT specimen of Notamacropus the VPS is not especially large, but neither is the ICV, and as usual the plexiform ICV/BVP rapidly diminishes caudally (fig. 19C). If relative vessel size were similar in the adult, venous return from the ICV/BVP to the IJV would be limited, also as expected. By contrast, according to its venogram the EJV is massive in Potorous , indicating a dural drainage pattern apparently dominated by the PGLVN, which is also the case in Macropus (sensu Aplin, 1990) .

TRICHOSURUS (PHALANGERIFORMES, PHALANGERIDAE ) (figs. 21, 22). On each side of the scanned specimen of the brushtail possum, Trichosurus vulpecula TMM M-849, a primary and a smaller accessory TCF occur in close proximity (figs. 21B, 22B–F). Their suggested homologies as branches of the TCV trunk are indicated in the figures, but verification is still required (see below). Their canals open directly into the compound vacuity shared with the rostral TBS. On the endocranial floor, two large, partly overlapping apertures perforate the TBS eminence at the presumptive location of the pituitary body and CS (fig. 21A; asterisk, fig. 22B–D). In the dry skull of another specimen, T. vulpecula AMNH 48055, conditions are similar except that the apertures are much smaller and can be interpreted as a group of venous foramen; EC, ectotympanic; excf, exocranial carotid foramen; evpf + jf, external continuation of ventral petrosal sinus and jugular foramen; fo, foramen ovale; PE, petrosal; pf, piriform fenestra; pgli, postglenoid incisure; pglf, postglenoid foramen; PT, pterygoid; ptcns, sulcus for nerve of pterygoid canal; rchf, rostral condylohypoglossal foramen; smf, suprameatal foramen; tcf, transverse canal foramen.

hypophyseal canaliculi (fig. 21D). Aplin (1990: With regard to other points of interest, the 326) made no mention of large apertures on jugular foramen is larger on the left side of the the eminence of the TBS of Trichosurus , but did scanned specimen than on the right (fig. 21C, E). say that “the primary canal is connected Such asymmetries are evident in other speciintraosseously with the carotid canal [?in the mens in the comparative set (e.g., Didelphis , fig. carotid groove], and thus has no direct com- 3; Thylacinus , fig. 30) and may be a common munication with the cavernous sinus.” These variation in certain species. Both rostral and cauremarks may be contradictory, as CBFs are rep- dal condylohypoglossal foramina are present (fig. resented on the endocast of Trichosurus vulpec- 21A), but the basijugular sulcus is either not eviula TMM M-849 and therefore direct dent or noncontinuous along its usual pathway. communication with the CS does occur in this Aplin (1990: 324) found a small foramen on the taxon (figs. 21C, D; 22E, F). medial side of the foramen ovale in specimens of Clearly, additional investigation will be Trichosurus . He attributed the additional aperneeded to clear up apparent discrepancies ture to the passage of the ramus medialis of the before anything definitive can be said about mandibular nerve, but another possibility is that PCVN organization in bushtail possums (or it transmitted an emissary (in this case, v.e. phalangeriforms generally). At present the foraminis ovalis). TMM M-849 exhibits a foraavailable evidence for Trichosurus is consistent men in apparently the same location (fig. 21E: with the interpretation that the main transverse double asterisks).

canal represents the CBTC rather than the OTHER PHALANGERIFORMS. According to Aplin RBTC. This is supported by the observation that (1990: 326), the TCF is either very small or alto- CBFs can be identified in the carotid grooves, gether absent in several taxa, including most speeven though Aplin’s (1990) text as written seems cies of the phalangerids Phalanger and Spilocuscus , to exclude contact with the CS/ICV. In this sce- the petaurid Petaurus , as well as the burramyid nario the accessory transverse canal could be a Cercartetus lepidus and the related tarsipedid Tarreduced or perhaps defunct RBTC, which is the sipes rostratus. However, there is a difference interpretation favored in figure 42. A similar between “small” and “absent.” According to Beck et issue affects interpretation of Osphranter (fig. al. (2022), who checked Aplin’s results in their sur- 20A). Rather than propose yet another junction vey, the only “possum” taxon in which the TCF is pattern at this time, we provisionally conclude consistently completely absent is Tarsipes , although that the architecture seen in Trichosurus con- sporadic absence occurs in others.

forms better to the hybrid pattern than the How PMD drainage is conducted in marsupicompound pattern, and include this taxon als that lack a designated TCF is of interest. In a under the former heading in table 4 (but in sectioned and stained specimen of Acrobates pygparentheses ) and in figure 42. maeus, Aplin (1990: 326) noted that a “major emissary vein,” traveling in an open sulcus on the Wegner (1964: 33) stated that the transverse mesocranium, “is connected medially to the canals are missing in vombatids, but they are pericarotid vein [i.e., ICV], and laterally, to an illustrated in his figure 22. In the common womanterior vein of the pterygoid plexus.” This emis- bat V. ursinus TMM M-2953 the rostral branches sarium was therefore conducted through the of the transverse canals produce a conspicuous carotid canal, which is different from the usual midline junction, visible endocranially (figs. CBTC pathway through an independent TCF. 24C, 25A–C) as in Didelphis (figs. 2C, 5C). These Aplin (1990) did not address this issue further, taxa differ, however, in that the opossum lacks evidently assuming that lack of a separate fora- caudal branches (simple pattern), while in the men and passage through the carotid canal did wombat they are present and large (complex not affect vessel homology. pattern; fig. 25C–G). The TCF is also said to be absent in another The rostral and caudal portions of the TBS are acrobatid that Aplin examined, Distoechurus extensive, with areas of trabecularization as well as pennatus . We found that a pseudoforamen rarefaction, as would be expected in a large-bodied occurs at the TCF’s usual site in D. pennatus taxon like the common wombat (table 2). However, AMNH M-105938. This feature is created by a TBS communication with either set of transverse partly calcified or ossified ligament (?sphenope- canal branches appears negligible, consisting only trosal ligament) that walls off a groove directly of small interstitial canaliculi. There is a patent craconnected to the carotid canal (fig. 23). This niopharyngeal canaliculus that joins the transverse suggests, but does not settle, whether the carotid canal junction (fig. 25C, D), but as in other cases its canal is the exit foramen for the putative TCV vascular role, if any, is not known. Foramina for the as well as the ICV. As in the case of Trichosurus, EVPS and IJV are well separated and the basijugudetermining the junction pattern of Distoe- lar sulcus is deeply etched. churus is problematic and placement must be provisional (see table 4).

DASYUROMORPHIA VOMBATUS (VOMBATIFORMES, VOMBATIDAE ) (figs. 24, 25). According to Sánchez-Villagra and

DASYURUS ( DASYURIDAE ) (figs. 26, 27). A Wible (2002: 30), wombats and koalas are the

scanned adult of the northern quoll Dasyurus halonly diprotodontians known to exhibit intramu-

lucatus TMM M-6921 was accessed for this study, ral union of right and left transverse canals (i.e.,

along with intact specimens of this species in the RBTCs of this paper). Aplin (1990: plate 4.12a)

AMNH collection. Archer (1976: 252), who pointed to additional PCVN-related similarities

examined a number of species in this genus, between Vombatus and Phascolarctos , and com-

explicitly described the transverse canal as bifurmented on the great size of the TCFs (frequently

cated in most specimens, one branch (CBTC) multiple) found in both (Aplin, 1990: 255).

communicating with the carotid foramen (= Extracranial continuation of ventral petrosal sinus does not cross external aspect of exoccipital; vessel passes instead directly into an intramural canal (double asterisks) linked to condylohypoglossal canals. Basijugular sulcus barely represented. Compare small size of jugular foramen to that of craniospinal foramen. Key: AS, alisphenoid; astp, tympanic process of alisphenoid; BO, basioccipital; BS, basisphenoid; cchf, caudal condylohypoglossal foramen; cspf, craniospinal venous foramen; encf, endocranial carotid foramen; EO, exoccipital; etbs, eminence of transverse basisphenoid sinus; excf, exocranial carotid foramen; fr, foramen rotundum; hpf, hypophyseal fossa; io + pf, combined incisura ovalis and piriform fenestra; jf + evpf, joint aperture formed by jugular foramen and extracranial continuation of ventral petrosal sinus; mxns, sulcus for maxillary nerve; onvs, sulcus for ophthalmic neurovascular array; PE, petrosal; PS, presphenoid; ptcs, sulcus for nerve of pterygoid canal; rchf, rostral condylohypoglossal foramen; sof, sphenoorbital fissure; SQ, squamosal; tmc, tympanic cavity; vpss, sulcus for ventral petrosal sinus.

endocranial carotid groove), the other branch (RBTC) joining the “cellular sinus in anterior midline of basisphenoid” (= TBS). The CBTC pathway is not distinct in our scanned specimen, but the large opening on the rostral wall of each carotid groove confirms that the CBV was present in life (arrows, fig. 27D). Endocranially, the rostral TBS exhibits modest relief and forms a junction with the RBTCs (fig. 27A–C). The position of the hypophyseal fossa is not marked by any special feature, but is assumed to lie as usual between the endocranial carotid foramina (fig. 27E).

Exocranially, there is an appreciable distance between the TCF and the undivided piriform fenestra/incisura ovale (fig. 26), which appears to be the case in most dasyuromorphians. As is especially obvious in the segment series, Dasyurus (fig. 27) is very similar to Monodelphis (fig. 12) for the characters under consideration, but quite different in body size (1000–1500 g and (60–100 g, respectively). Both are considered to exhibit the complex pattern (table 4).

SARCOPHILUS ( DASYURIDAE ) (fig. 28). The only published study that includes empirical information on the cephalic vascular system of the Tasmanian devil S. harrisii is that of Shah and Nichol (1989). In their paper, principal cephalic veins are identified on a venogram and accompanying interpretative diagram (Shah and Nichol, 1989: fig. 2; partly reproduced here as fig. 28D). Neither Archer (1976) nor Sánchez-Villagra and Wible (2002) present any specific information on PCVN components in this species. From Shah and Nichol’s venogram it is possible to infer that the TCV, labelled as “PS” (pterygoid sinus), is of substantial size in this taxon. This accords with the large caliber of the TCFs on the skull (fig. 28B), but does not provide the details necessary for assigning a junction pattern.

The top of the skull of an adult no-data specimen (AMNH M-35535) was removed in order to gain access to the cranial interior (fig. 28C). A point of interest is the condition of the endocranial carotid grooves in this specimen, which are notably asymmetric. On the left side, the large carotid groove exhibits both the carotid canal and a large foramen on its rostral wall, which by its position must be the CBF. By contrast, the right groove is considerably shallower, especially rostrally, and lacks a CBF. Yet there is an external TCF on the right side, which means that there was a functional TCV in life. Probing is of little help: when inserted into the specimen’s right TCF, it always emerges through the CBF, which gives no indication whether a rostral pathway is also present, as in the majority of investigated dasyuromorphians (see Archer, 1976; Sánchez-Villagra and Wible, 2002). The endocranial evidence is similarly ambiguous: there is no convincing eminence for the RBTCs rostral to the hypophyseal fossa, but in large-bodied species a discernible ridge is not always present. The large bilateral “pterygoid sinus” veins illustrated in the venogram do not show the slight arching of the RBTC at the point of their junction as seen in Didelphis or Caenolestes ; instead, they run in fenestra (s. 29.04.04. In E and F, complex of anastomoses involving condylar emissary vein, internal jugular vein, and extracranial continuation of ventral petrosal sinus; note also persistent lateral head vein (ss. 26.05.0, 32.04.02). Key: AC, auditory capsule; AS, alisphenoid; at, auditory tube; BO, basioccipital; BS, basisphenoid; bvp, basicranial venous plexus; cca, common carotid artery; cdv, condylar vein; cs, cavernous sinus; ctn, chorda tympani nerve; EC, ectotympanic; ejv, external jugular vein; EO, exoccipital; evps, extracranial continuation of ventral petrosal sinus; GO, goniale; hgn, hypoglossal nerve; hp, hypophysis; ica, internal carotid artery; icn, internal carotid nerve; icv, internal carotid vein; icv/bvp, internal carotid vein and basicranial venous plexus; ijv, internal jugular vein; lcm, longus capitis muscle; lhv, lateral head vein; lptm, lateral pterygoid muscle; MA, malleus; MC, meckelian cartilage; mca, middle cerebral carotid artery; MD, mandible; mdn, mandibular nerve; mptm, medial pterygoid muscle; mxv, maxillary vein; pal, processus alaris; stm, stapedius muscle; tg, trigeminal ganglion;?ttcv,?presumptive trunk of transverse canal vein anastomosing with internal carotid vein vein; ttm, tensor tympani muscle; vn, vagus nerve; vps, ventral petrosal sinus.

a transverse direction, directly to the CS (fig. 28D). The venogram shows other veins in the vicinity, but none resembles a likely RBV. On this evidence all that can be said is that the CBTC is evidently dominant in Sarcophilus ; if there is a functional rostral branch, it must be highly reduced. At least in this respect, Sarcophilus is similar to Thylacinus , although the former lacks the union of transverse and carotid canals seen in the latter. In light of these uncertainties we omit Sarcophilus from the junction pattern framework (table 4).

OTHER EXTANT DASYURIDS. Archer (1976) described conditions in dry skulls of several dasyuromorphians (principally Thylacinus , Sarcophilus , Dasyurus , Dasycercus , Dasyuroides , Myrmecobius ). Although PCVN components were not investigated in detail, his data reveal that many dasyurid taxa possess the RBV (as inferred from the presence of the RBTC intramural pathway). The survey by Sánchez-Villagra and Wible (2002) supports this conclusion, although the authors also reported some polymorphisms, especially for Dasyurus , and were not able to investigate Archer’s (1976) remarks on variation in Planigale (see below). The status of Myrmecobius in this regard is uncertain: Archer (1976: 246) presented evidence that the numbat possesses both anterior and posterior pathways, but also stated that the intramural condition does not exist in this taxon.

As noted in Anatomical Structures, Archer (1976: figs. 2–4) provided schematic drawings of cephalic venation in injected specimens of several dasyurid species, but overlaps and apparent disparities make them difficult to interpret. In regard to the sminthopsine Planigale, Archer (1976: 265) stated:

When transverse canal foramina are present they are often not connected by canal, but rather open directly into endocranium. In other specimens where they are present, they do lead to canal which opens into endocranium via internal sulcus for entocarotid foramen. In no instance observed, does transverse canal cross basicranium to link both foramina via basisphenoid sinus.

This description is consistent with the trunk of the TCV receiving only the CBV in Planigale , with the RBV evidently being absent. This may also apply to the related Sminthopsis and the dasyurine Antechinus , although the relationship of the TCVs to the midline “transverse canal sinus” (= transverse basisphenoid sinus) implies that rostral rather than caudal branches are present. Whether dasyurids are as morphologically disparate as this is an interesting problem that could be resolved with appropriate data.

THYLACINUS ( THYLACINIDAE ) (figs. 29–31). Thylacinus cynocephalus , the only extinct Quaternary marsupial to be reviewed here, exhibits a mesocranial region that is interesting but complicated. Our treatment is based on two specimens: NMB c.2526a, an intact skull for which a scan is available, and AMNH M-144316, a coronally hemisected skull (table 1). Although several soft-tissue and perinatal specimens of Thylacinus exist in collections ( Sleightholme and Ayliffe, 2013; Newton et al., 2018), to our knowledge there are no published sources on intracranial venation in this species.

The apparent complexity of the PCVN in Thylacinus , as reflected in our endocast reconstruction, is fundamentally due to the elaborate connections between the carotid and transverse canals. All four canals briefly coalesce just distal to the endocranial carotid foramina, something that is difficult to properly show in 2D ventral views (fig. 30A, D), but that can be better appreciated by viewing the successive segments illustrated in figure 31. An additional complication for illustration is that digitally filling the sulci for the endocranial carotid groove and hypophyseal fossa, as required by our method (see Illustrations), results in a continuous isoshape for these features. This is insufficiently realistic. We correct for this in figure 30F by depicting the fossa’s shape in a different color and removing enough of the vasculature to reveal the position of the carotid grooves.

Although there are no indicia to indicate how the veins in the conjoined canals would have interacted, their basic routes can be inferred from conditions in other taxa. In our interpretation, the veins connecting the contents of the hypophyseal fossa with the trunk of the TCV would have been homologous with CBVs, not RBVs. The basis for this inference is that their osteological pathways start at the hypophyseal fossa, suggesting that they emerged at this point from the CS/ICV before descending into their respective canals. Furthermore, as there is no indication of a pathway rostral to the fossa, there is no basis for inferring the presence of RBTCs. This constitutes a major difference from the simple, complex, and compound junction patterns, but it accords in principle with the hybrid patbranch vein; cc, carotid canal; ccs, sulcus leading to carotid canal; cchc, caudal condylohypoglossal canal; con, confluence of rostral branch veins; ctbs, caudal portion of transverse basicranial sinus; encf, endocranial carotid foramen; encg, endocranial carotid groove; etbs, eminence of transverse basisphenoid sinus; etcl, eminence of transverse canal; fr, foramen rotundum; hpf, hypophyseal fossa; ica, internal carotid artery; icv, internal carotid vein; jf + evpf, joint aperture formed by jugular foramen and extracranial continuation of ventral petrosal sinus; junc, junction of transverse canals; onvs, sulcus for ophthalmic neurovascular array; pf piriform fenestra; pglf, postglenoid foramen; rbtc/le, rostral branch of transverse canal inside lateral extension of transverse basisphenoid sinus; rbv, rostral branch vein; rchc, rostral condylohypoglossal canal; rtbs, rostral portion of transverse basicranial sinus; smf, suprameatal foramen; sss, sulcus for sigmoid sinus; tcf, transverse canal foramen; tdss, sulcus for transverse dural sinus; tpss, sulcus for temporal sinus; ttcv, trunk of transverse canal vein; vpss, sulcus for ventral petrosal sinus.

tern as seen, in a differently derived format, in macropodids (see above and fig. 18C).

Archer (1976: 238), however, came to a different set of conclusions. He noted that, depending on the direction in which he probed a transverse canal, a hair might pass from one TCF to the other through the basisphenoid (indicating that, in our terminology, the RBTC pathway was present), or it might simply go into the endocranium (suggesting that the CBTC pathway was also present). He found that most of the thylacine specimens that he examined (N = 8) showed evidence of “two partly divergent canals or paths through basisphenoid” (p. 238), which would imply the existence of a junction pattern resembling the complex or compound arrangement rather than the hybrid. However, it is pertinent to note that he also mentioned some other features that he thought might represent individual variation: for example, a sagittally sectioned skull appeared to have only one pathway, while another skull showed “just mesial to entrance of transverse foramen, [a] fork in [?transverse] canal with bony median wall” (p. 238). These differences were not analyzed further, except to observe in conclusion that Thylacinus might be significantly polymorphic for mesocranial traits. Sánchez-Villagra and Wible (2002: 30) were unable to replicate Archer’s results, as they found no intramural pathway in their material.

The CT scan of NMB c.2526a sheds some light on possible reasons for these conflicting interpretations:

(1) Archer was correct in pointing to a caudally directed pathway (“fork”) that branches off from the transverse canal immediately medial to the location of the TCF (fig. 30D: feature a). But this pathway, which originates in the TBS, does not qualify as a CBTC, at least as defined in this paper, because it communicates with the proximal part of the canal for the accessory VPS, not the CS. The pathway in question is, in effect, a greatly enlarged interstitial canaliculus. Consistent with the variability associated with such features, on the specimen’s right side the canaliculus is large; but on the left side, the accessory VPS is highly reduced, and so is the fork that joins it (fig. 30D: feature d).

(2) Having a candidate for the CBTC in the shape of the fork, Archer (1976) evidently concluded that the other branch that he had detected was an RBTC. Our definition of the rostral branch pathway requires that it should run intramurally from the trunk of the TCV to meet its partner rostral to the position of the hypophyseal fossa, communicating with the TBS along the way but displaying little or no contact with the CS/ICV. No branch of this type was found in NMB c.2526a.

(3) As Archer (1976) noted, it took careful maneuvering to pass a probe from one TCF to the other. But the reason that it was possible to do at all was not because Thylacinus skulls exhibit a rostral pathway like that of Didelphis or Caenolestes , but because the merger of all four of the major mesocranial canals creates a continuous space linked on either side to the TCFs via the transverse canals (fig. 31C–G). Merger, however, occurs caudal to the position of the hypophyseal fossa and involves a different TCV pathway (CBTC) than the one found in Didelphis (i.e., RBTC). In short, the result is what might be called a false intramural passage, which blind probing cannot differentiate from the real thing. Archer (1976: 238) also noted that, with different maneuvering, his probe would “appear inside cranium in sulcus for internal carotid artery,” but he did not state that it emerged through a separate foramen.

Because the conformation of the hypophyseal fossa plays a role in how mesocranial vasculature is to be interpreted in Thylacinus cynocephalus , a few more details concerning its osteological appearance are worth exploring. Archer (1976) remarked that he could not locate the hypophyseal fossa in his material with certainty, but the segmental data establish that the fossa in NMB c.2526a is located as usual on the same coronal plane as the endocranial carotid foramina. However, in this case the fossa does not conform to the simple broad depression seen in most other members of the comparative set, but is instead a additional sites where the TBS is connected to deep, flask-shaped pocket (fig. 31A, B). An the transverse canals by interstitial canaliculi, are extension of the fossa that projects rostrodorsally presented in figure 30D (asterisks). toward the likely position of the optic chiasma Ultimately, we cannot specify from osteological on the endocast (fig. 30D) is apparently an ossi- evidence alone how the mesocranial veins (i.e., fied sheath for the hypophyseal stalk or infun- TCV trunk, CBTC, and ICV) would have interdibulum. Obviously, how closely the endocast acted while running through the merged carotid conforms to the shape of the pituitary body or and transverse canals. Our assumption, however, the CS cannot be ascertained. is that the ICV would not have been lost to anas-

Archer (1976) noted the presence of a midline tomosis, but would have instead remained indeaperture on the basioccipital, which he called the pendent up to the point of its departure from the median basioccipital foramen and thought that it exocranial carotid foramen (cf. similar assumpmight have carried a nutrient vessel. This feature tion for Osphranter , fig. 20). In this scenario the is seen in NMB c. 2526a (fig. 31I: feature 3), but TCV trunk would have likewise remained intact, there is nothing at the equivalent place in AMNH leaving the skull through its proper port. A good M-144316 (fig. 29A). In NMB c.2526a the feature analogy would be plexiform vertebral veins, which communicates with the left accessory VPS (fig. share branches and anastomoses at different ver- 30D). We consider it to be either a remnant of tebral levels but retain their longitudinal identity the notochord canal or an unpaired transclival ( MacPhee, 1994: fig. 23). venous foramen.

Proceeding rostrocaudally through the seg-

PERAMELEMORPHIA ments presented in figure 31, it may be seen that the trackway for the VPS after it leaves the vicin- PERAMELES ( PERAMELIDAE ) (figs. 32, 33). The ity of the hypophyseal fossa becomes, succes- auditory capsule of Perameles sp. ZIUT HL 17.5 sively, a deep sulcus and then a canal (= internal mm (table 1) is partly ossified, particularly in the jugular canal of Archer, 1976) (fig. 31J). The VPS region of the fenestra cochleae. A particularly canal continues onward to penetrate the exoc- valuable feature of this specimen is that it procipital, where it coalesces with a series of con- vides direct evidence of the confluence of the tinuous channels related to the passage of the RBVs, represented by the thin-walled vessel seen VV, SS, transverse dural sinus, and condylar in the midline in figure 33A. Confluence occurs veins (fig. 31K, L). On the external surface of the within the large chamber formed by the fusion of basicranium, the short but wide basijugular sul- the rostral TBS and transverse canal junction cus connects the large foramen for the EVPS to immediately rostral to the root of the ossifying the caudal condylohypoglossal foramen (fig. processus alaris. The chamber’s internal walls 29A–C), as in other marsupials and many pla- exhibit rows of osteoclasts and osteoblasts on centals. The true jugular foramen is smaller than opposing trabecular surfaces, indicating that the foramen for the EVPS, also as in many extant intensive bone remodeling was in progress at the marsupials. The postglenoid foramina are com- time of death. Although Azan is not a convenparatively large, signifying the importance of the tional hematological stain, abundant erythro- PGLVN in this species. Other features connected cytes can be identified by their shape within the with mesocranial circulation, including several chamber. Other erythroid cells in various stages carotid foramen; EO, exoccipital; fm, foramen magnum; hpf, hypophyseal fossa; io + pf, incisura ovalis and piriform fenestra; jf, jugular foramen; MA, malleus; PE, petrosal; rbtc, rostral branch of transverse canal; rtbs/junc, junction of transverse canals with rostral portion of transverse basisphenoid sinus; SQ, squamosal; tcf, transverse canal foramen; tmc, tympanic cavity. of differentiation can be seen, as well as darkerstaining cells that may be megakaryocytes (see Fawcett, 1986; Old et al., 2004; Young et al., 2014; see Discussion).

In the stained sections, the RBV and CBV communicate with the TCV trunk medial to the TCF (fig. 33B–D, H). The CBV, which is extremely short, passes from the ICV through the CBF to directly join the trunk. The CBF can be made out on the damaged mesocranium of AMNH M-154403 by obliquely tilting te skull (fig. 32C, D). In this specimen there is also a small hypophyseal canaliculus on the specimen’s left side that opens into the carotid groove. The right and left RBVs meet in the compound vacuity formed by the rostral TBS and transverse canals. As usual, because RBVs travel intramurally they are not seen on the endocranial floor, although there is a slight swelling marking their presence (eminence of the transverse canals).

In the sectioned specimen the intracranial ICV is significantly larger than the internal carotid artery (fig. 33A–D). As usual the BVP is plexiform and small in caliber relative to the VPS (fig. 33E). By contrast, the anastomosis formed by the SS and VV is very substantial (fig. 33J). If also true of the adult, this would imply that encephalic return is mostly discharged into the CSVS rather than the IJV in this marsupial.

In the adult skull of Perameles nasuta AMNH M-160199, the osteological foramen ovale is subdivided into daughter apertures by a bony bridge (fig. 32A: feature b). The lateral aperture is possibly for the large emissary vein (v.e. foraminis ovalis) seen issuing from this location in company with the mandibular nerve in the perinatal specimen (fig. 33H: asterisk). The specimen exhibits a large TCF on its left side, but the foramen on the right side is much smaller (fig. 32A, B), despite the size of the fossa in which it is located. In P. gunnii AMNH M-106102 (not illustrated), both TCFs are larger than the exocranial foramina of the carotid canals. Some taxa are known to display a significant degree of variation in TCF dimensions (Aplin, 1990; Sánchez-Villagra and Wible, 2002), but whether this is particularly the case in Perameles has not been reported.

One of the very few published mentions of PCVN components in Perameles is Cords’s (1915: 40) observation that the carotid canal contains a vein (i.e., ICV) as well as an artery. Cords (1915: 22) also argued that the IJV of Perameles is possibly not homologous with that of “higher” mammals. The basis for her observation was that she noticed a vein that she took to be the IJV passing through a foramen situated well rostral to the track of the glossopharyngeal and vagus nerves, as in nonmammals like the chicken Gallus . A more likely interpretation is that Cords mistook the EVPS for the true (and notably smaller) IJV, and incorrectly assumed that the former’s exit aperture was the jugular foramen (see fig. 32).

Figures

FIG. 20. Osphranter robustus AMNH M-80171 (Macropodidae, Diprotodontia), juvenile caudal cranium, selected coronal segments in rostrocaudal order (data source, table 2). A–F, Conjectured arrangement of mesocranial blood vessels (red, arterial only; blue, venous only), sizes exaggerated for clarity.Macropodids often have multiple transverse canal foramina (e.g., Notamacropus, fig. 18B). In this specimen, accessory foramina (arrow in A) are tiny and open into cancellous tissue, but do not form recognizable junction or communicate directly with endocranium or much larger main foramen (cf. Trichosurus, fig. 22A, B). Nevertheless, they may qualify as RBTCs. In C–F, external apertures for main transverse canal and carotid canal can be seen passing through floor of endocranial carotid groove, where their pathways merge. As a result there is no separate caudal branch foramen because entire trunk consists of caudal branch vein.Whether it retains its separate identify or anastomoses with internal carotid vein in carotid groove as suggested in C (?icv + cbv) is unknown. In any case, in this reconstruction internal carotid vein is shown as departing as a separate vessel through main transverse canal foramen, as in other taxa. Key: AS, alisphenoid; bbs, basisphenoid-basioccipital synchondrosis; BS, basisphenoid; cbf, caudal branch foramen; cbv, caudal branch vein; cc, carotid canal; cs/ov, opthalmic vein entering cavernous sinus; cs/vps, cavernous sinus releasing ventral petrosal sinus; ctbs, caudal portion of transverse basisphenoid sinus; encf, endocranial carotid foramen;encg, endocranial carotid groove; fo, foramen ovale; hpf, hypophyseal fossa; ica, internal carotid artery; icv, internal carotid vein;?icv + cbv, possible anastomosis of caudal branch vein of transverse canal and internal carotid vein; le, lateral extension of transverse basisphenoid sinus; mca, middle cerebral artery; mxns, suclus for maxillary nerve; oa, ophthalmic artery; ov, ophthalmic vein; PT, pterygoid; ptc, pterygoid canal; rtbs, rostral portion of transverse basisphenoid sinus; SQ, squamosal; tcf, transverse canal foramen; tgf, trigeminal ganglion fossa; ttcv, trunk transverse canal vein; vps, ventral petrosal sinus.