Sipalocyon, Ameghino, 1887

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 : 65-71

SIPALOCYON ( HATHLIACYNIDAE )

Figures 38 , 39

Archer (1976), who described AMNH VP-9254 under one of its synonyms, Thylacodictis , was able to trace only surface details in the mesocranial area. Our scan of this specimen clears up several points that were previously in doubt.

ophthalmic neurovascular array; pglc, postglenoid canal; pglf, postglenoid foramen; rchc, rostral condylohypoglossal canal; rchf, rostral condylohypoglossal foramen; rtbs, rostral portion of transverse basisphenoid sinus; sss, sulcus for sigmoid sinus; tcf, (primary) transverse canal foramen; tcl (cbtc), (primary) transverse canal (caudal branch of transverse canal); tdss, sulcus for transverse dural sinus; tgf, fossa for trigeminal ganglion; trcf, transclival foramina; vpss, ventral petrosal sinus.

Archer (1976: 289, 307) wondered whether one or the other of two small foramina or notches (fig. 39: features 1, 2) seen on the lateral margin of each carotid foramen in AMNH VP-9254 could have accommodated true TCVs. Our CT evidence indicates that this is unlikely in both cases. Feature 2 ends abruptly and cannot be traced further rostrally along the skull (fig. 39C–E). Its nature remains uncertain, but in being directed away from the basicranial keel its routing is not that of a typical TCV as seen in marsupials. Feature 1, situated more medially (fig. 39A–F), is bordered by pterygoid material; it continues rostrally as either a canal (specimen’s left side) or sulcus (right side), to terminate in a tiny aperture located in the infratemporal fossa immediately caudal to the foramen rotundum (location indicated in fig. 38E). In this case the relations of the aperture are consistent with its being a rostral foramen for the nerve of the pterygoid canal, not a port for the TCV.

Aplin (1990) showed that in several extant marsupials the postganglionic sympathetic fibers composing the deep petrosal nerve often pass out of the carotid canal through a separate foramen, rather than adhering closely to the internal carotid artery as in many placentals ( MacPhee, 1981). The equivalent of this foramen could be represented in Sipalocyon by the small gap (feature 1) in the wall of the carotid canal seen in figure 39F. If so, the gap marks the trackway not of the transverse canal, as Archer (1976) thought, but rather that of a component of the nerve of the pterygoid canal.

It is of course possible that the TCV started to develop embryonically in Sipalocyon , but failed to persist after an early stage. In such circumstances all that can be said is that an identifiable TCF would not be expected in the adult, and that is the case here: there is nothing in the organization of the mesocranium that would suggest the presence of the TCV trunk or its branches in adult Sipalocyon .

As expected, the carotid canals pass into the endocranium lateral to the probable location of the hypophyseal fossa. Within the body of the basisphenoid the right and left carotid canals communicate with a small intramural space that positionally conforms to the caudal TBS (figs. 38B, D; 39D: feature 3), although organizationally it differs appreciably from anything seen in extant marsupials. Conceivably, this space could have permitted flow between bilateral ICVs (fig. 38D: double-headed arrow), although what purpose would be served thereby is obscure. Feature 4, a short tube that occupies the midline, could be another connector or alternatively a remnant of the notochord canal (cf. Thylacinus , fig. 30D). The TBS is not extensively developed, especially as compared with Prothylacynus (fig. 40B), and the lateral extensions only weakly connect its rostral and caudal portions.

The basijugular sulcus on the caudal basicranial surface is strongly marked in Sipalocyon and other sparassodontans (e.g., Thylacosmilus , Lycopsis ; Forasiepi et al., 2019; see also Archer, 1976: 290). It is continuous, via the caudal condylohypoglossal foramen, with the craniospinal fora- half that of the latter. From these canals, groups men on the interior lip of the foramen magnum. of interstitial canaliculi extend into the TBS (fig. This implies that the vessel that occupied the 40E–F), or invade the alisphenoid where they basijugular sulcus—presumably the EVPS, as in continue as far as the latter’s sutural contact with marsupials—anastomosed directly with the VVs, the frontal. There is nothing in their position or as well as with the highly reduced IJV and local relations to suggest that nerves were transmitted emissaria (see vascular reconstruction for Thy- through these conduits. In MACN A-5931 only lacosmilus by Forasiepi et al., 2019: fig. 26). In a single TCF is distinctly present on each side, addition to the foregoing, prominent craniospi- but additional cleaning or scanning might reveal nal foramina are also seen, as in Prothylacynus , more (see Forasiepi et al., 2019: fig. 15).

Cladosictis , and Borhyaena . The doubling of rostral branches in this manner recalls conditions in some extant dipro- PROTHYLACYNUS (BORHYAENOIDEA) todonts, especially petauroids ( Dactylopsila ,

Dactylonax , and Gymnobelideus ) in which typi-

Figures 40 , 41

cal TCFs exist alongside large anterior ptery- The caudal cranium of YPM VPPU-15700 goid foramina according to Beck et al. (2022: shows considerable damage in the vicinity of the char. 50). Given the close proximity of all these right carotid canal and basioccipital surface. vessels in the petauroids, they probably anas- Another skull, P. patagonicus MACN A-5931, tomose within the mesocranium. However, was examined to corroborate surface details not whether it would be meaningful to call all of well preserved on the Yale specimen. them transverse canals is a different problem. Scanning reveals a complicated arrangement We recognize that Prothylacynus presents the of internal vascular channels in the rostral part same nomenclatural issue.

of the mesocranial region. These channels are The rostral branches join across the midline comparable to the rostral branches (RBTCs) seen and communicate with the carotid canals by in extant marsupials, and they are designated as means of tubes functionally similar to the caudal such here. However, there are some important branches (CBTCs) of marsupials, but there are difdifferences that need to be acknowledged. In the ferences. No potential sites of communication Yale specimen the RBTCs are connected to two between these branches and any likely location for sets of similar-sized TCFs (features 1–4, fig. 40A, the hypophyseal fossa were identified. The left D). The canals into which they feed are situated tube exhibits a particularly large interstitial conin advance of the carotid canals, and present nector that directly communicates with the TBS cross-sectional widths that are approximately (fig. 40D: asterisk). Another (fig. 40E: double FIG. 22. Trichosurus vulpecula TMM M-849 ( Phalangeridae , Diprotodontia ), adult caudal cranium, coronal segments in rostrocaudal order (data source, table 2). In A and B, rostral portion of transverse basisphenoid sinus inflates endocranial floor of basisphenoid and receives accessory transverse canal. In C–E, main transverse canals directly communicate with hypophyseal fossa via endocranial floor apertures (single asterisk with bracket) and caudal branch foramina (arrows). In F–H, note endocranial carotid groove, carotid canal, and possible sulcus for caudal branch of transverse canal. Junction pattern provisionally regarded as hybrid (see text). Key: AS, alisphenoid; astp, tympanic process of alisphenoid; atcf, accessory transverse canal foramen; atcl, accessory transverse canal (?rostral branch of transverse canal); BS, basisphenoid; cbs, sulcus for caudal branch of transverse canal; cc, carotid canal; ctbs, caudal portion of transverse basisphenoid sinus; encf, endocranial carotid foramen; encg, endocranial carotid groove; excf, exocranial carotid foramen; fo, foramen ovale; mdnc, canal for mandibular nerve; mxnc, canal for maxillary nerve; mxns, sulcus for maxillary nerve; onvs, sulcus for ophthalmic neurovascular array; ptc, pterygoid canal; rtbs, rostral portion of transverse basisphenoid sinus; tcf, (primary) transverse canal foramen; tcl, (primary) transverse canal (caudal branch of transverse canal); tgf, fossa for trigeminal ganglion; tmc, tympanic cavity.

asterisks) is pointed toward the sphenoorbital fissure, suggesting a more extensive drainage field.

In Prothylacynus (fig. 40E) the carotid canals are much smaller in caliber relative to endocast width than in Sipalocyon (fig. 38E), or indeed in most of the marsupials investigated in this paper. This difference, which is the opposite of expected, is of uncertain significance but cannot be attributed to specimen damage.

Figures

FIG 40. Prothylacynus patagonicus YPM VPPU-15700 (Borhyaenoidea, Sparassodonta), adult specimen, endocast reconstruction showing osteological features associated with pericarotid venous network and related vasculature (data source, table 2; color key, fig. 4). Views: A, ventral; B, same, with transverse basisphenoid sinus superimposed; C, oblique right lateral; D, closeup of transverse canals, ventral aspect; E, closeup of transverse canals, rostral aspect; F, horizontal section through transverse canals and mesocranium; G, ventral and H, left ventrolateral surfaces of intact caudal cranium. In A–F, features 1–4

FIG. 38. Sipalocyon gracilis AMNH VP-9254 (Hathliacynidae, Sparassodonta), adult specimen, endocast reconstruction showing osteological features associated with pericarotid venous network and related vasculature (data source, table 2; color key, fig. 4). Views: A, ventral; B, same, with transverse basisphenoid sinus superimposed; C, oblique right lateral; D, closeup of endocranial surface of mesocranial region; and E, caudoventral surface, intact caudal cranium showing extracranial continuation of ventral petrosal sinus. In A–C, rostral portion of transverse basicranial sinus lacks identifiable intramural connections with carotid canals or cavernous sinus. Feature 1, identified by Archer (1976) as a possible transverse canal in Sipalocyon, is more likely a channel for nerve of pterygoid canal. Feature 2 is a sulcus of unknown function, unrelated to either nerve of pterygoid canal or typical transverse canal. Feature 3, small channel within basisphenoid sinus tissue, presumably venous, passes between bilateral carotid canals (double-headed arrow in D) caudal to hypophyseal fossa. Arrangement is different from that of CBTC of investigated marsupials. Feature 4, another small canaliculus, either interstitial or remnant of notochord canal, connects feature 3 with rest of sinus. Key: cc, carotid canal; cchc, caudal condylohypoglossal canal; ctbs, caudal portion of

FIG. 39. Sipalocyon gracilis AMNH VP-9254 (Hathliacynidae, Sparassodonta), adult caudal cranium, selected coronal segments in rostrocaudal order (data source, table 2). As in extant marsupials, large carotid canals probably carried internal carotid vein as well as artery. However, channels for transverse canal veins resembling those of most marsupials or Prothylacynus (fig. 40A) are not in evidence. Small channels on lip of extracranial carotid foramen, thought by Archer (1976) to be possible transverse foramina, are for components of nerve of pterygoid canal and another structure that cannot be securely identified (features 1, 2; cf. fig. 38). Right and left internal carotid veins could have interacted with each other and with caudal portion of transverse basisphenoid sinus via features 3 and 4 (cf. fig. 38D). Key: AS, alisphenoid; BS, basisphenoid; cc, carotid canal; ctbs, caudal portion of transverse basisphenoid sinus; encf, endocranial carotid foramen; encg, endocranial carotid groove; excf, exocranial carotid foramen; fo, foramen ovale; hpf, hypophyseal fossa; mdnc, canal for mandibular nerve (to foramen ovale); mxns, sulcus for maxillary nerve; onvs, sulcus for ophthalmic neurovascular bundle; rtbs, rostral portion of transverse basisphenoid sinus; SQ, squamosal; 1, pterygoid canal; 2, sulcus of unknown function; 3, small canaliculus joining right and left carotid canals (presumably venous, for anastomosis between internal carotid veins); 4, small connector between 3 and rest of caudal transverse basisphenoid sinus; possibly retained portion of notochord canal.

FIG. 41. Prothylacynus patagonicus YPM VPPU-15700 (Borhyaenoidea, Sparassodonta), adult caudal cranium, selected coronal segments in rostrocaudal order (data source, table 2). Rostral branches of transverse canals (features 1–4, cf. fig. 40A, D) are situated relatively more rostrally than in extant marsupials. However, they connect with transverse basisphenoid sinus and carotid canals via caudal branches as well as interstitial canaliculi, similar to conditions in certain marsupials. In B, asterisk indicates dorsally directed interstitial canaliculus originating from transverse canal. Key: AS, alisphenoid; br, breakage; BS, basisphenoid; cbtc, caudal branch of transverse canal; cc, carotid canal; ctbs, caudal portion of transverse basisphenoid sinus; encf, endocranial carotid foramen;?hpf, hypophyseal fossa; junc1, junc2, junctions of rostral branches of transverse canals; mxnc, canal for maxillary nerve; mxns, sulcus for maxillary nerve; obf, fossa for olfactory lobes; onvs, sulcus for ophthalmic neurovascular array; PT, pterygoid; ptc, pterygoid canal; rtbs, rostral portion of transverse basisphenoid sinus; SQ, squamosal.