Anchitheriomys buceei, May & Brown, 2023

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 : 8-19

Anchitheriomys buceei new species

( Figures 5-15 , Table 2)

zoobank.org/ 517DFDD0-FE18-4A1F-99D0-7342CEF189C7

Type Specimen. TMM 40197-2665 , partial skull with M3 and roots for P4-M3 .

Referred Specimens. TMM 31057-7 LM2; TMM 31057-59 Rm2; TMM 31057-141 RM 3; TMM 31057-163 Lm1; TMM 31057-185 Lm1; TMM

40622-13 LP 4; TMM 40623-12 left dentary; TMM 71-2666 distal humerus, proximal ulna, TMM 71- 2667 Rp4; TMM 71-2668 – molar; TMM 71-2669 – RM 2. F: AM 64023A - partial left maxilla with P4; F: AM 64023B - P4; F: AM 64024A edentulous partial left ramus; F: AM 64024B left lower cheek tooth. (Note: TMM 71-2770, an isolated tooth was borrowed in the 1940s and never returned. The specimen number at that time would have been TAMU 2770.)

Horizon and Locality. Lagarto Formation, Fleming Group, Burkeville, Newton Co. ( TMM 40197); Push Creek, Belts Creek, Tyler Co. ( TMM 40623, TMM 40622); Moscow (Barrington Farm), Polk Co. ( TMM 31057); Trinity River (Pit 1 AMNH specimens), Point Blank, San Jacinto Co. ( TMM 71); Texas.

Age. Early Barstovian, Ba1, (middle Miocene).

Etymology. Species named in recognition of Bucee’s roadside attraction travel centers that have repopularized beavers in Texas.

Diagnosis. Anchitheriomys buceei is larger than both A. stouti from the early Hemingfordian of Nebraska ( Korth, 2001a) and A. nanus from the early Hemingfordian of Nebraska ( Korth and Martin, 2007). The cheek teeth have much simpler enamel patterns than A. tungurensis ( Stirton, 1934) . The dentary has a well-developed symphyseal flange differing from both A. suevicus and A fluminis . A relatively large mental foramen is located on the labial side of the dentary anterior to p4 and well above the midline of the horizontal ramus with much smaller foramen just anterior to the primary foramen. The ridges on i1 are much more prominent than in A. nanus and possess secondary corrugations along their length. Anchitheriomys buceei is similar to A. fluminis in size and elongated rostrum, but differs in having grooves on the palatine, the more posterior position of the posterior palatine foramina, a single nasolacrimal foramen, the ethmoidal foramen is located more dorsal relative to the optic foramen, and the posterior divergence of the upper cheek teeth is less pronounced. The posterior terminus of the palatine in A. buceei forms a broadly curving arc as in A. fluminis and unlike the tight u-shaped terminus in A. nanus . The cheek teeth are large with a very large P4/p4 relative to the molars.

Description. TMM 40197-2665 is a partial skull of a relatively large castorid that preserves portions of the cranium and dentition, with much of the cranial anatomy preserved as an endocast ( Figures 5 , 6 ). The length of the preserved skull is 152.6 mm, but with both anterior and posterior elements missing. The greatest width of the preserved rostrum is 47.3 mm, again representing a minimum value. Although the anterior portion of the rostrum is missing, the I1-P4 diastema is at least 55 mm. The depth of the skull from palate to the frontals is approximately 51 mm. This compares well with the value of 50.8 mm for the skull of Anchitheriomys fluminis (USNM 299914, Korth and Emry, 1997). The size of the skull of A. buceei is significantly larger than the Hemingfordian taxa A. nanus and A. stouti and similar to the mid Barstovian A. fluminis (USNM 299914, Korth and Emry, 1997).

The upper tooth rows of Anchitheriomys buceei diverge posteriorly as in the skull of A. fluminis (USNM 299914) described by Korth and Emry (1997), but the degree of divergence is less ( Figure 7 ). The width of the palatine on TMM 40197-2665 varies from 10.2 mm at P 4 to 13.3 mm at M3. The ratio of palatal width measurements at P4 and M3 is 0.76 for A. buceei , but 0.56 for A fluminis . The two skulls of A. nanus described by Korth and Martin (2007) illustrate variability in this character with the larger skull at 0.67 and the smaller skull at 0.56. The anterior portion of the palatine is grooved, as in A. nanus , while the posterior portion is slightly concave with no distinct grooves. There is a prominent ridge along the midline of the palatine. The palatine grooves terminate anterior to the palatine foramina, similar to A. nanus ( Korth and Martin, 2007) and unlike A. fluminis ( Korth and Emry, 1997) ( Figure 7 ). Recent preparation of A. nanus skull (TMM 1465-1.1) revealed that the palatine grooves terminate posteriorly in foramina. This appears to be the same in A. buceei , although the palatine bone is poorly preserved at that location. The palatine foramina are located entirely within the palatine bone as in other species of Anchitheriomys . Whereas A. nanus has a single palatine foramen on each side of the palatine bone, A. buceei has two, as does A. fluminis ( Korth and Emry, 1997) . The left side of the palatine includes a larger posterior foramen aligned with the middle of M3 and a smaller anterior foramen. The locations of the posterior palatine foramina in A. buceei are more posterior than in A. fluminis . The palatine/maxilla suture is not preserved, but the palatine bone extends at least to the posterior edge of M1. The posterior margin of the palatine structure in A. buceei forms a broadly curving arc as in A. fluminis ( Figure 7 ). In contrast, the posterior edge of the palatine in A. nanus , forms a narrow, u-shaped margin along with the pterygoid processes ( Figure 8C ).

Although the nasal bones are mostly missing on TMM 40197-2665, the endocast of the nasal cavities is long and narrow with a preserved length of 78.0 mm and a combined width of approximately 10.6 mm ( Figures 9 , 10 ). Fragments of both premaxilla are preserved. These fragments terminate at approximately the same position posteriorly probably reflecting the original suture with the frontal bones. A small bone fragment, presumably part of the right nasal, also terminates at approximately this position and the nasal cavity endocast exhibits a slight change in orientation at this position as well. The nasals and the premaxilla bones joined the frontals near the anterior terminus of the zygomatic arch. This geometry is similar to the skull of Anchitheriomys fluminis shown in Korth and Emry (1997).

The frontal sinus is large and bilobed with dimensions of 44.1 mm x 14.6 mm (L x maximum width) ( Figures 9 , 10 ). The infraorbital canal on the right side of the skull is relatively large with four small foramina present in two paired groups ( Figure 9 ). A post-glenoid foramen is preserved on the right side of the skull and appears to be more rounded as in Anchitheriomys fluminis compared to the elongated geometry of A. nanus ( Korth and Martin, 2007) .

Approximate dimensions of the alveolus endocast for the right I1 are 12.5 mm x 11.5 mm and similar to Anchitheriomys fluminis (10.6 mm x 9.4 mm, 10.9 mm x 9.5 mm, I1 L x W in Korth and Emry, 1997). P4 is the largest of the cheek teeth and all cheek teeth have three roots. The lingual root is large and bladelike while the two labial roots are more peg-like ( Figure 7 ). There is no evidence of cheek teeth anterior to P4.

Except for the right M3, only the roots of the cheek teeth are present in TMM 40197-2665. The left side tooth row is more complete with an alveolar length of ~ 41.6 mm. This is similar to the tooth row length of Anchitheriomys fluminis (41.4 mm, 39.2 mm in Korth and Emry, 1997). Measured at the roots, the left P4 is 13.1 mm x 15.3 mm. The right P4 at the roots is 12.0 mm x 14.1 mm.

The single isolated right M3 associated with TMM 40197-2665 is moderately worn. It had been rotated out of position and was prepared from the matrix of the original specimen. It is 7.8 mm x 8.3 mm (L x W) at the occlusal surface and 9.7 mm x 8.7 mm at the base of the enamel. The preserved crown height is 9.3 mm. The alveolar dimensions of the left M3 are 6.8 mm x 8.1 mm (L x W). The roots of the right M2 are also rotated so that they point laterally rather than ventrally.

The orbital foramina are well preserved on TMM 40197-2665 and imaged in the CT data ( Figure 11 ). The size and locations of these foramina are different in detail than either Anchitheriomys fluminis or A. nanus , but are more similar to A. nanus . According to Korth and Martin (2007), A. nanus differs from A. fluminis in the more posterior position of the optic and ethmoidal foramina which Korth and Martin (2007) interpreted as a primitive condition. The relative locations of the ethmoidal and optic foramina in A. buceei are similar to A. nanus , but the sphenopalatine foramen is elongate and dorso-ventrally oriented, and the optic foramen is sub-circular as in A. fluminis . Both the optic and interorbital foramina are larger than in A. fluminis . The ethmoidal foramen is located more dorsal relative to the optic foramen as in A. nanus . The large interorbital foramen in A. buceei is located just dorsal to a small dorsal palatine foramen. This positioning is similar to A. nanus and unlike A. fluminis . A pair of nasolacrimal foramina are present in A. buceei , as in A. fluminis , and unlike the single foramen described in A. nanus ( Korth and Emry, 1997; Korth and Martin, 2007) ( Figure 11 ).

TMM 40623-12 is a partial left dentary assigned to Anchitheriomys buceei from the Push Creek Local Fauna ( Figure 12 ). The dentary is robust. The medio-lateral width at the symphysis is 15.5 mm, at the diastema is 16.02 mm, at p4 is 21.7 mm, at m1 is 23.7 mm, and at m2 is 23.1 mm. The minimum dorso-ventral depth at the diastema is 22.9 mm while the depth below m1 is 30.6 mm (minimum measurement as the ventral portion of the dentary is missing). The diastema of A. buceei is relatively shallow compared to A. fluminis (FAM 64020 in Korth 2017b). The symphyseal flange (digastric process) is much better developed than in A. suevicus ( Stefen and Mörs, 2008) and A. fluminis ( Korth, 2017b) . Anchitheriomys stouti ( Korth, 2001a) also has a prominent symphyseal flange as does the large castorid (UF 223914) from Suwannee River, Florida ( Mörs and Hulbert, 2010). Mörs and Hulbert (2010) assigned UF 223914 to Amblycastor fluminis , but Korth (2017b) argued that the Florida specimen is of uncertain taxonomic identity. The symphysis is oval in shape and narrows ventrally. It is 38.7 mm in dorso-ventral height with a maximum width of 23.7 mm.

A relatively large mental foramen is located on the labial side of the dentary anterior to p4 and well above the midline of the horizontal ramus. A much smaller foramen is located just anterior to the primary foramen. Anchitheriomys suevicus exhibits a single mental foramen located near the middle of the horizontal ramus ( Stefen and Mörs, 2008), while A. stouti has a double mental foramen that is located below the center of the diastema at about the dorso-ventral midline of the horizontal ramus ( Korth, 2001a). TMM 40623-12 clearly differs from both of these morphologies. Anchitheriomys fluminis (FAM 64020) shown in Korth (2017b) appears to have a single small foramen located near the midline of the horizontal ramus and more posterior than in A. buceei . However, the holotype of A. fluminis figured in Matthew (1918) shows a large sub-

5 mm

circular mental foramen slightly posterior to the midline of the dentary.

The lower incisor of Anchitheriomys buceei is ovoid in cross-section with very prominent longitudinal ridges on the anterior face ( Figure 13 ). Approximately 15-16 ridges are generally sub-parallel, but occasionally anastomose. The ridges are individually corrugated with small culminations along their length as shown in Figure 14 . Because the ventral portion of the dentary is broken, the ridges can be seen to run the entire length of the incisor ( Figure 12 ). Measured at the natural break, i1 is 12.8 mm x 11.3 mm in cross section with a preserved length of 73.5 mm. The ridges on i1 are much more prominent than in A. nanus .

The dimensions of p4 on TMM 40623-12 measured at the roots (alveolar) are 14.0 mm x 13.6 mm (L x W). This is much larger than A. stouti ( Korth 2001a), A. nanus ( Korth 2004) and near the largest reported dimensions for A. fluminis and A. suevicus ( Stefen and Mörs, 2008) , but smaller than the large castorid described by Mörs and Hulbert (2010). The alveolar tooth row length is difficult to measure, but is at least 39 mm.

Isolated Cheek Teeth: Six isolated upper cheek teeth are illustrated in Figure 15 including two P4s, two M2s and two M3s at various stages of wear. These teeth are from the Burkeville, Moscow, Belts Creek, and Point Blank Local Faunas. Dimensions are summarized in Table 2. TMM 40197-2665 ( Figure 15 ) is a moderately worn R M3 that was associated with the skull of the same specimen number. This tooth was embedded in the matrix surrounding the skull, but was rotated out of the alveolus. It has a well-developed mesoflexus that curves posteriorly and an antero-lingually oriented hypoflexus. There are four anterior fossettes and three posterior fossettes. The fossette pattern is somewhat simpler than A. fluminis ( Korth and Emry, 1997) , but more complex than A. nanus ( Korth and Martin, 2007) . The size is slightly smaller than A. fluminis , but significantly larger than A. nanus .

TMM 40622-13 ( Figure 15 ) is a moderately worn left P4 with a well-developed mesoflexus that curves posteriorly and an antero-lingually oriented hypoflexus. The parafossette is broad and the metafossette is relatively narrow with accessory fossettes on either end. The lack of accessory anterior fossettes seems to differ from A. fluminis (Emry and Korth, 1997) . This specimen is 9.6 mm x 10.7 mm at the occlusal surface (L x W) and broadens to 10.2 mm x 11.6 mm at the base of the enamel.

A 5 mm B C

5 mm

D

5 mm

F

E

5 mm

Four isolated lower cheek teeth are illustrated in Figure 16 including one p4, two m1’s, and one m2. Three of the four specimens are from the Moscow Local Fauna (TMM 31057), and one is from the Point Blank Local fauna (TMM 71). TMM 71- 2667 is the only p 4 in the collection ( Figure 16 ). It is very worn and broken, but illustrates some of the occlusal pattern. It is a large tooth measuring 12.3 mm x 10.8 mm near the occlusal surface, but broadens to 13.3 mm x 12.5 mm at the base of the enamel. This is consistent with the alveolar measurements of p4 on TMM 40623-12 of 14.0 mm x 13.6 mm (L x W). The dimensions of p4 for A. buceei are similar to those reported for A. suevicus and for A. fluminis and much larger than A. nanus and A. stouti ( Stefen and Mörs, 2008) . The occlusal pattern is similar to A. suevicus with crescentic and branching fossettids.

TMM 31057-185 and TMM 31057-163 are both very worn left m1s. The posteriorly directed hypoflexid is distinct. The anterior parafossetid complex includes four separate fossettids on TMM 31057-185 and only two on TMM 31057-163, while the mesofossettid is a single loop at this stage of wear. The metafossettid is a single loop on TMM 31057-185 but includes a pair of fossettids on TMM 31057- 163 in addition to a small accessory fossettid. This variability is at least partly due to slight variation in the amount of wear on these two teeth.

TMM 31057-59 is a moderately well-worn right m2 with a distinct posteriorly directed hypoflexid, and a short mesoflexid. The parafossettid is crescent shaped as seen in specimens of A. suevicus ( Stefen and Mörs, 2008) . The metafossettid appears to be two separate fossettids.

Post Cranial: Two post-cranial elements were recovered from the Point Blank Local Fauna (TMM 71) ( Figure 17 ). A distal right humerus (TMM 71- 2666) and a proximal right ulna (TMM 71-2666) were associated with the box of isolated teeth labeled “ topotype material” discussed above. Both specimens are broken and appear to have been tumbled. The epicondylar width of the humerus is 32.0 mm. The trochlear surface is shallow as is the olecranon fossa. There is no trochlear foramen. The lateral supracondylar ridge is not strongly developed as it is in modern Castor canadensis . The olecranon process of the partial right ulna is relatively short. The trochlear notch is relatively shallow and the coronoid process is not strongly developed. Based on multiple measurements of the ulna from A. buceei and multiple specimens of C. canadensis in the TMM comparative collection, this element of A. buceei was approximately 25- 30% larger than adult specimens of the modern American beaver.

Discussion. Stenzel et al. (1944) discussed the stratigraphy and paleontology of Miocene sedimentary rocks exposed near Burkeville, TX. The area includes rocks of both brackish marine and non-marine facies including both marine invertebrate and non-marine vertebrate fossils. The significance of this stratigraphic relationship had been recognized for years and discussed in publications by Veatch (1902, 1906), Dall (1913), Ellisor (1936), Stenzel and Turner (1944), and others. In his 1942 paper, Hesse lists Amblycastor n. sp. (skull) from

A 5 mm B 5 mm

the A. & M. Museum Locality No. 42 (TAMU 42) (p. 175). This is undoubtedly a reference to TMM 40197-2665. Stenzel et al. (1944) list Amblycastor n. sp. Hesse, without a date, but cite a paper by C.J. Hesse listed as “Fossil Vertebrates of the Burkeville Area, Newton County, Texas ”, American Journal of Science, in press (1944). Stenzel and Turner (1944) improperly list the name Amblycastor stirtoni citing the same Hesse paper as “in preparation”. Hesse never described this material nor officially proposed the name in a publication, as he died in 1945. An untitled, unpublished manuscript housed in the archives at the Texas Vertebrate Paleontology Archives (Mark Francis Papers, Series I: Correspondence/Manuscripts), includes a note from Hesse: “For a description of this material see Amblycastor stirtoni , in the discussion of the Burkeville fauna.” There is no record of that part of the manuscript, and discussion of the Burkeville fauna only includes the name Amblycastor n. sp. Although it seems clear that Hesse intended to name a new species of castorid, in fact, the specimens were never described in the literature and no holotype was ever specified. Furthermore, much of the cranial anatomy has only become observable since 2019 through preparation of TMM 40197- 2665. The use of Amblycastor stirtoni , therefore, does not conform to the International Code of Zoological Nomenclature Article 13.1, and the name used in Stenzel and Turner (1944) is a nomen nudum.

Four specimens of a relatively large castorid were recovered from the Trinity River Pit 1 locality by Frick collectors in 1964 and are now curated at the AMNH (F: AM 64023 A, B, F: AM 64024 A, B). 64023A is a partial left maxilla with P4 and 64023B is an isolated P4. F: AM64024 A is an edentulous partial left ramus, while 64024B is an isolated left lower cheek tooth. Due to inaccessibility during the Covid pandemic, these specimens have only been

PALAEO- ELECTRONICA.ORG studied in photographs, but are consistent in size and morphology with A. buceei .

Matthew (1918) described the type material (AMNH 17213) for Amblycastor fluminis from the Snake Creek fauna of Nebraska. The holotype is a waterworn partial mandible with p4 and a partial i1. Additional specimens of A. fluminis from the Ba1 Olcott Formation have been described by Stirton (1935) and Korth (2017b). These Ba1 occurrences of A. fluminis include dentaries, fragments of dentaries, and isolated teeth. The skull of A. fluminis is only known from the Ba2 Valentine Formation ( Korth and Emry, 1997). No skulls of A. suevicus are known from Europe. Anchitheriomys tungurensis is known from dentaries, partial dentaries, and isolated teeth. Stefen and Mörs (2008) reviewed the age constraints for A. tungurensis and concluded that it is from latest Barstovian to earliest Clarendonian equivalent faunas and therefore younger than both A. fluminis (Ba) and A. suevicus (MN5-MN6). The only other species of Anchitheriomys known from skull material is A. nanus from the early Hemingfordian Runningwater Formation in Nebraska ( Korth and Martin, 2007).

The skull, dentary, and isolated teeth of A. buceei from the Ba1 Lagarto Formation of the Texas Coastal Plain are similar in size to A. fluminis , but exhibit a number of characters that differ from that taxon. Some characters are similar to A. nanus such as the grooved palatine. A. buceei represents a permissible member of the A. nanus – A. fluminis lineage.

Figures

FIGURE 5. TMM 40197-2665. Anchitheriomys buceei (holotype), A – right side (anterior to right), B – dorsal view, C – left side (anterior to left), D – ventral view. Photographs in normal light.

FIGURE 6. TMM 40197-2665. Anchitheriomys buceei (holotype), A – right side (anterior to right), B – dorsal view (anterior to right), C – left side (anterior to left), D – ventral view (anterior to top). Images rendered from CT data volume.

FIGURE 7. TMM 40197-2665. Anchitheriomys buceei (holotype), ventral view of skull, isosurface rendered from CT volume, anterior to left, illustrating roots of the upper cheek teeth, palatine grooves and foramina, posterior geometry of palatine.

FIGURE 8. A - TMM 1465-1.1. Anchitheriomys nanus, isosurface rendering of CT data showing the ventral view of the skull. Bones of the palate and associated facial features are rendered in gold, anterior to the left. B – dorsal view of the skull. C – details of the palatine morphology, anterior to the left. The right pterygoid process has been restored to near its original position in the image. Grooves in the palatine are clear as is the foramen at the posterior terminus of the groove. The two teeth are right and left P4.

FIGURE 9. TMM 40197-2665. Anchitheriomys buceei (holotype), dorsal view of skull, isosurface from CT volume, anterior to the right, nc – nasal cavity, fs – frontal sinus.

FIGURE 10. TMM 40197-2665. Anchitheriomys buceei (holotype), right oblique view of skull illustrating endocast of brain, frontal sinus, nasal cavity, infraorbital canal, and endocast of I1 alveolus, isosurface rendered from CT volume, anterior to right.

FIGURE 11 (left). TMM 40197-2665. Anchitheriomys buceei (holotype), lateral view of left orbital illustrating orbital foramina from CT volume. n – nasolacrimal, s – sphenopalatine, e – ethmoidal, o – optic, dp – dorsal palatine, ito – interorbital, anterior to the left. Isosurface rendered from CT volume. B – closer view of the same from a more anterior angle.

FIGURE 12. TMM 40623-12. Anchitheriomys buceei, partial left dentary, A – labial view anterior to the left, B – lingual view anterior to the right, C – oblique anterior view showing well developed ridges on i1 and symphyseal flange, D – ventral view showing posterior continuation of i1.

FIGURE 13. TMM 40623-12, Anchitheriomys buceei, line drawing showing cross section of i1 at natural break near alveolus with general location of well-defined ridges along the anterior face.

FIGURE 14. TMM 40623-12, Anchitheriomys buceei, oblique view of i1 showing corrugations on ridges of incisor.

FIGURE 15. Anchitheriomys buceei, isolated upper cheek teeth (all measurements are L x W, anterior up). A - TMM 40197-2665 R M3, molar associated with holotype skull 7.9 mm x 8.2 mm; B - TMM 31057-141 R M3, 7.1 mm x 7.5 mm; C - TMM 31057-7 L M2, 7.33 mm x 7.38 mm; D - TMM 40622-13 L P4, 9.4 mm x 10.3 mm; E – 2666 L P4 very worn, 14.5 mm x 12.8 mm, occlusal view above, posterior view below with large blade-like root on the lingual side of the tooth; F – TMM 71-2669 R M2, 8.9 mm x 9.6 mm.

FIGURE 16. Anchitheriomys buceei, isolated lower cheek teeth (all measurements are L x W, anterior up). A – TMM 31057-185 L m1, 8.8 mm x 9.2 mm; B - TMM 31057-163 L m1, 7.4 mm x 9.6 mm, very worn; C - TMM 31057-59 R m2, 8.0 mm x 7.5 mm, D – TMM 71-2667 R p4, 10.6 mm x 10.8 (broken along dashed line).

FIGURE 17. Anchitheriomys buceei, A, B - TMM 71-2666, proximal right ulna. C, D – TMM 71-2666 distal right humerus.