Monosaulax sp.

May, Steven R. & Brown, Matthew A., 2023, Anchitheriomys buceei (Rodentia, Castoridae) from the Miocene of Texas and a review of the Miocene beavers from the Texas Coastal Plain, USA, Palaeontologia Electronica (a 7) 26 (1), pp. 1-27 : 19-22

Monosaulax sp.

( Figures 18-20 )

Description. A small castorid from two Ba1 local faunas of the Texas Coastal Plain is identified as Monosaulax sp. based primarily on dental and mandibular morphology and size. TMM 31057-300 (Moscow Local Fauna, Barringer Farm) is a partial maxilla with P4-M3 ( Figure 18 ). TMM 31244-6 was collected by the Statewide Paleontologic-Mineralogic Survey at the Wanza Farm locality in San Jacinto County (Point Blank Local Fauna). The specimen includes a partial mandible with fragmentary right and left i1, p4-m3. P4-M2 of presumably the same individual is wedged between the right and left dentaries ( Figure 18 ). This artifact of taphonomy results in a clear association of upper and lower cheek teeth. The cheek teeth are well worn in both specimens but appear to be mesodont. It is not possible to determine if the cheek teeth have two or three roots. The fourth premolar is the largest of the cheek teeth.

The incisors of TMM 31244-6 are subtriangular in cross section, rounded anteriorly, and have a slight rugosity, but lack ridges ( Figure 19 ). The width of the left incisor where it is broken at the alveolus is 3.8 mm. The left p4 of TMM 31244-6 is 4.4 mm x 4.1 mm (L x W) at the occlusal surface but the length increases to 5.0 mm near the base of the enamel. The parafossetid is curved rather than straight, one of the characters separating Monosaulax from Eucastor ( Korth, 1999) . Accessory fossettes (fossettids) are absent in these two specimens and generally rare in the genus Monosaulax as compared to the similarly sized Temperocastor ( Korth, 2008) . At the moderate level of wear in both of the specimens, the hypoflexus (id) is the only striae (id) still open. All others have closed as fossettes (ids) indicating that the striae (ids) are short, a primitive character for the genus and similar to M. tedi ( Korth, 1999) and M. typicus ( Shotwell, 1968) . Because the striae (ids) are short, the molars lack the S-pattern. At a similar degree of wear, the lingual striid is still open in M. pansus (RAM 7618) from the Barstow Formation ( Lofgren et al., 2019). Korth (1999) stated that M. pansus has deeper lingual striids than M. tedi .

Monosaulax typicus ( Shotwell, 1968) from the early Barstovian of southeastern Oregon is slightly smaller than Monosaulax sp. from the Texas Coastal Plain, but appears to be similar in terms of crown height and length of striae (ids). The occlusal patterns of M. typicus figured in Shotwell (1968) are similar to Monosaulax sp. from Texas except that the parafossettid on p4 is straighter. Shotwell (1968) stated that the cheek teeth of M. typicus were shorter crowned and possessed shorter striae (ids) than M. pansus . Monosaulax baylei from the early Hemingfordian Runningwater Formation of Nebraska ( Korth, 2004) is slightly larger,

4 mm

5 mm

5 mm but the p4 has a v-shaped parafossettid unlike the gently curved parafossettid in Monosaulax sp.

The p4 of TMM 31244-6 is significantly smaller than Monosaulax skinneri ( Korth, 2008) ( Figure 19 ). The p4-m3 tooth row length (13.1-13.4 mm) is shorter than M. skinneri ( Korth, 2008) , M. pansus (Korth, 2002) and M. baylei ( Korth, 2004) , and similar to M. tedi and M. curtus ( Korth, 1999) , but the latter has a straight parafossetid while that of TMM 31244-6 is curved. The tooth row length is just slightly longer than the measurements reported for M. typicus ( Shotwell, 1968) . The P4- M2 length of TMM 31244-6 is approximately 10.4 mm.

The preserved length of the right dentary is 35.5 mm, and the minimum depth below the diastema is 9.5 mm. The diastema is relatively shallow. A single mental foramen is located anterior to p4 near the midline of the horizontal ramus and is 1.8 mm in its long dimension ( Figure 20 ). There is no symphyseal flange. These characters are similar to the dentaries of M. typicus figured in Shotwell (1968). A virtual slice through the CT volume of TMM 31244-6 shows two roots on p4 ( Figure 20 ). Korth (1999) described this as the common character state in all species of Monosaulax except for M. skinneri .

The P4-M3 tooth row length of TMM 31057- 300 is 12.9 mm. Individual tooth measurements are P4: 4.6 mm x 4.3 mm (L x W), M1: 2.8* mm x 3.6 mm (*broken), M2: 2.8 mm x 3.2 mm, M3: 2.7 mm x 2.7 mm. The occlusal patterns of the upper cheek teeth are simple with 2-3 fossettes and a hypoflexus. Even the hypoflexus on M1 has been lost with wear so the tooth only contains two fossettes ( Figure 19 ).

Discussion. A small castorid from the early Barstovian (Ba1) Moscow and Point Blank Local Faunas is identified as Monosaulax sp. The small size, low crown height, lack of accessory fossettes(ids), short striae (ids), and lack of S-shaped molars are all interpreted to be primitive characters for the genus. The two specimens of Monosaulax from the Moscow and Point Blank Local Faunas are most similar to M. typicus , but minor differences in occlusal patterns (e.g., less curved parafossettid on M. typicus ) and the limited number of specimens from Texas causes us to be cautious about that assignment.

Monosaulax is known from a number of Barstovian localities in North America. The older and more primitive species ( M. typicus , M. tedi ) are most similar to Monosaulax sp. from the Texas Coastal Plain. The suite of primitive characters and similarity with M. typicus are consistent with a Ba1 age assignment for the Moscow and Point Blank local faunas. Korth (1999) suggested that Monosaulax was close to the basal castoroidine. Prisaulax was described as a new genus by Korth and Bailey (2006) from the late Arikareean and early Hemingfordian of Nebraska, and they proposed a basal castoroidine phylogenetic position for that taxon. Monosaulax and Prisaulax appear to be closely related, and Korth (2017a) removed the species serundi from the genus Monosaulax and placed it in the genus Prisaulax .

Figures

FIGURE 18. Monosaulax sp. A, TMM 31244-6 mandible dorsal view. B, TMM 31244-6 mandible ventral view showing occlusal surface of P4-M2 between left and right dentaries. C, TMM 31244-6 mandible ventro-lateral view showing smooth nature of anterior face of lower incisors. D, TMM 31057-300, maxillary fragment with P4-M3, occlusal view, anterior towards top.

FIGURE 19. Monosaulax sp. A - TMM 31244-6, left and right i1, cross-section drawn at natural break. B – TMM 31244-6, L p4 occlusal view. C – TMM 31057-300, maxillary fragment with P4-M3 occlusal view, anterior towards top. D – TMM 31244-6, P4-M2 associated with mandible.

FIGURE 20. Monosaulax sp. TMM 31244-6 CT data. A – vertical longitudinal slice through anterior part of left dentary showing two roots on left p4 and oblique view of incisor, anterior to the left. B – horizontal slice through anterior part of left dentary showing two roots on left p4, anterior to the top. C – vertical transverse slice of left dentary between roots of p4 and showing cross-sectional view of incisor. D – dorsal view of mandible as isosurface rendering. E– labial view of right dentary as isosurface rendering, anterior to the right.