Dipodomys, Gray, 1841

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 : 59-61

DIPODOMYS ( HETEROMYIDAE , DIPODO-

MYINAE) (fig. 36A, B). In his detailed monograph on the banner-tailed kangaroo rat ( Dipodomys spectabilis ), Howell (1932: 480) noted that on each side of the ventral midline of the skull there is a large mesocranial foramen. This “foramen pterygoideus” released an equally large vein which, “emerging from between the external and internal pterygoid muscles and extending laterally to the base of the ear… joins with the facial and temporal veins to form the external jugular.” According to Howell (1932), similarly positioned but smaller foramina occur in myomorph dipodoids ( Allactaga , Stylodipus , etc.), indicating that, although infrequent, they are not restricted to a single major clade of rodents.

Wahlert (1985) added some important new details concerning this feature, based on his dissection of a related species, Ord’s kangaroo rat ( D. ordii ) as well as comparisons with other extant and extinct geomyoids. Borrowing terminology from Hill (1935), he used the names sphenopterygoid vein and sphenopterygoid canal for the mesocranial vessel and its channel. Wahlert (1985) verified Howell’s observation that a large vein passes through the sphenopterygoid canal, but also noted that the medial pterygoid muscle invades the canal and partly arises from its walls.

Neither Howell nor Wahlert conducted an intracranial investigation of Dipodomys and therefore did not observe whether the sphenopterygoid veins actually drain the CS, although given their location this seems practically certain. (Char. 111 of Meng et al. [2003], which summarizes various literature references to the sphenopterygoid foramen in rodents and their relatives, states that the sphenopterygoid foramen gives passage to the internal maxillary artery; this suggests that in some cases it has been conflated with the alisphenoid canal.) Howell (1932) speculated that these veins might actually be the main channels for encephalic drainage in Dipodomys , given that the IJV was apparently either small or absent in his specimens. Brylski’s (1990) investigation was concerned with cephalic arteries rather than veins, but there is no indication in his vascular reconstruction that the sphenopterygoid foramen also transmits an artery in Dipodomys .

Howell (1932) made no mention of the fact that the sphenopterygoid veins are at least positionally similar to PMDs of marsupials, and it may not have occurred to him that such a comparison could be usefully made. Indeed, he avoided discussing the possible homologies of the sphenopterygoid vein, allowing only that “in primitive Mammalia it may possibly occur as a small vessel and this, if of aid in draining the blood sinuses at the base of the brain (as inferior petrosal or cavernous sinuses) could easily, if the need arose, develop to assume the entire duty of a true internal jugular” ( Howell, 1932: 480). Whether he meant by this statement that the sphenopterygoid veins might functionally replace the IJVS and its major tributaries, such as the VPS, is unclear. The suprameatal foramen and postglenoid incisure are both relatively large in the kangaroo rat, suggesting that large transverse dural sinuses must contribute significantly to encephalic return.

Wahlert (1985) concluded that a large sphenopterygoid canal counts as a shared derived feature of Heteromyidae and Geomyidae , and it would be of interest to know whether versions of these canals (and their contents) were more widespread in the larger clade, Castorimorpha, to which these families belong. Wahlert (1985) additionally identified, as the “transverse foramen,” a small hole variably situated in the caudolateral wall of the sphenopterygoid canal or in the medial wall of the alisphenoid canal. This feature was said to be lacking in some geomyid and most heteromyid specimens. The thin bone of the basisphenoid portion of the keel lacks a sphenoid sinus of any sort, and there is nothing corresponding to the lateral extensions of the TBS. The much smaller size, inconstant location, and low incidence of such apertures outside of geomyoids places their homology in question, although these veins would still qualify as emissaria.

An equivalent to the ICV is not mentioned in Howell’s study, and in any case the route of the internal carotid artery in Dipodomys (see Wahlert, 1985; Brylski, 1990) is so different from the one characteristic of most other mammals that their homology is questionable. The basicapsular fenestra is widely open in Dipodomys and there are no obvious indicia marking the external route of the VPS or the precise location of the internal jugular vein’s exit.