MICROBIOTHERIA, Ameghino, 1889

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 : 33-35

MICROBIOTHERIA

DROMICIOPS ( MICROBIOTHERIIDAE ) (figs. 15–17). Dromiciops gliroides ZIUT HL 19 mm was probably an advamced pouch young at the time of its collection, judging from the degree of cranial ossification. The slide set, although adequate for morphological interpretation, exhibits shrinkage artifacts incurred during histological preparation. A slightly damaged near-adult skull of the monito del monte, Dromiciops gliroides MACN Ma-23607, was used for endocast reconstruction and segment illustration.

Making allowance for tissue shrinkage in the ZIUT specimen, the ICV and internal carotid artery would have filled the carotid groove (fig. 16D). Presence of the CBV within the groove could not be verified histologically, although its foramen in the scanned specimen confirms its normal presence (figs. 15C, 17C). The ICV at its point of origin from the CS is at least twice as wide as the internal carotid artery (fig. 16E), assuming consistant shrinkage. After leaving the carotid foramen the ICV’s caliber rapidly decreases (fig. 16D) before it becomes continuous with the BVP. Because of the specimen’s condition the ICV/BVP could not be satisfactorily traced to the jugular area. Exit points for the EVPS and IJV on the basicranium are quite distinct (fig. 15A).

Although not obvious in the ventral reconstruction (fig. 15E), the basicranial keel of Dromiciops is well pneumatized. Pneumatization occurs in the parasphenoid as well as the central and lateral parts of the basisphenoid and presphenoid (figs. 16A, B; 17A–D). The transverse canals (RBTCs) are fully continuous with the rostral TBS, forming a single vacuity with a minor amount of internal subdivision. Their broad continuity is consistent with the compound junction pattern (table 4). The medial aspect of the petrosal is also extensively pneumatized, although it could not be determined whether this was due to, or independent of, middle ear expansion. Sánchez-Villagra and Wible (2002: 30) record the TCF of Dromiciops as technically polymorphic (i.e., one known case of absence, in AMNH M-92147, which we confirm). Absence must be rare, however, because Beck et al. (2022: 65) report the TCF as present in every Dromiciops specimen they examined that preserved this region (N = 22 specimens).

On either side of the midline a large discontinuity interrupts the lateral margin of the presphenoid, here identified as the pterygopalatine fissure in recognition of its size (fig. 15E; see also D’Elía et al., 2016: fig. 7). This is not the TCF, which lies in its expected place on the basisphenoid or in the basisphenoid-alisphenoid interface, lateral to the carotid foramen (Beck et al., 2022). In the sectioned juvenile the pterygopalatine fissure transmits only the nerve of the pterygoid canal (fig. 16A: asterisk), despite the aperture’s size and the nearby presence of large vessels like the ophthalmic veins. Large fissures in this location occur in some other marsupials (e.g., Perameles , fig. 32A), presumably with the same content. In cases in which the fissure is small or subdivided, it might be better labelled the rostral foramen for the nerve of the pterygoid canal, to distinguish it from the foramen at the canal’s tympanic end (i.e., caudal foramen for the nerve of the pterygoid canal).

sinus; mxnc, maxillary nerve canal; mxns, maxillary nerve sulcus; onvs, sulcus for ophthalmic neurovascular array; PA, parietal; PS, presphenoid; ptc, pterygoid canal; rbtc, rostral branch of transverse canal; rtbs, rostral portion of transverse basisphenoid sinus; sc, sagittal crest; SO, supraoccipital; sof, sphenoorbital fissure; SQ, squamosal; tcf, transverse canal foramen; tgf, trigeminal ganglion fossa.