Metanoiamys sp.

Kelly, Thomas S. & Murphey, Paul C., 2016, Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 1: Primates and Rodentia, Palaeontologia Electronica 7 (8), pp. 1-55 : 36-41

Metanoiamys sp.

Figure 11.1-12 , Table 7

Referred specimens. From UCM Locality 92189: M1 or 2, UCM 66311, 95693, 98815, 98816; M3, UCM 98813, 98814; p4, UCM 95751. From SDSNH Locality 5841: M3, SDSNH 110372; dp4, SDSNH 110369; p4, SDSNH 110437; m1, SDSNH 110370. From locality 5844: M1 or 2, SDSNH 110442; p4, SDSNH 110436; m3, SDSNH 110438. From SDSNH Locality 5787: dP4, SDSNH 110371; m3, SDSNH 110411. From DMNH Locality 4672: dP4, DMNH 74131, 74135; P4, DMNH 74130, 74149, 75331; M1 or 2, DMNH 74133, 74136, 74139, 74141, 75251, 75278, 75332, 75333; p4, DMNH 74143, 75276, 75277; m1 or 2, DMNH 74132, 74137, 74146, 74147, 75250, 75252, 75254, 75255; m3, DMNH 74144, 74145. From DMNH Locality 4673: M1 or 2, DMNH 75316; m1 or 2, DMNH 75318.

Description. Three teeth are identified as dP4. The anterior cingulum extends anterolabially from the anterior base of the protocone to a parastyle and then continues in a wide arc to join the anterior base of the paracone, forming an expanded (wide) cingular shelf. A small accessory lophule is present that extends from the parastyle to the protoloph (= adlophule of Chiment an Korth [1996]). The primary cusps (protocone, paracone, hypocone, and metacone) are about equal in size, with the protocone and hypocone being round and the paracone and metacone being slightly compressed anteroposteriorly. The protoloph is a complete, low crest connecting the protocone to the lingual base of the paracone. The endoloph is complete, connecting the protocone to the hypocone. On one dP4, a small mesocone is present at about the center of the endoloph, and there is also a small, posterolabially directed accessory lophule coming off the mesocone (mesoloph), whereas in the other dP4s the mesocone is vestigial (only a small swelling on the endoloph). The mesostyle is small, but distinct. The metaloph is complete connecting the hypocone to the metacone. The posterior cingulum extends labially from the hypocone to terminate at the posterior base of the metacone.

Three teeth are identified as P4. Their occlusal outlines are roughly square. The anterior cingulum extends labially from the anterior base of the protocone to the anterior base of the paracone and, in two specimens, there is a slight posterior indentation at about the middle of the cingulum. The protoloph is a complete crest, connecting the protocone to the paracone. The endoloph is a complete crest in two teeth and nearly complete in one tooth. A small mesocone is present on the endoloph in two teeth. The metaloph is complete, connecting the hypocone to the metacone. A very weak adlophule is present on two teeth. The posterior cingulum extends labially from the hypocone to terminate near the posterior base of the metacone.

Confident differentiation of M1 from M2 cannot be made for Metanoiamys . Fourteen teeth are identified as either M1 or M2. The occlusal outline is nearly square. The anterior cingulum is robust, extending lingually from near the anterior base of the metacone to terminate at a level just labial to the protocone apex, where it is separated from the protocone by a shallow valley. A small, but distinct, anterocone is usually present on the anterior cingulum (10 teeth), near its lingual terminus. An adlophule is commonly present (eight teeth) connecting the anterocone or anterior cingulum to the protoloph. A small mesostyle is usually present (11 teeth). The protoloph is complete, extending from the anterolabial edge of the protocone in a gentle arc to connect with lingual edge of the paracone. The endoloph is usually a complete crest connecting the protocone to the hypocone, but in three teeth it is incomplete extending anterolabially from the hypocone to terminate near the posterolabial base of the protocone. The mesocone varies from a slight swelling (incipient) to a small, but distinct, cuspule on the endoloph and from one to two additional, small accessory lophules (mesolophs) extend from the mesocone into the central basin. The metaloph is complete, connecting the hypocone to the metacone. The posterior cingulum is robust, extending labially from the hypocone to terminate at the posterior base of the metacone.

Six teeth are identified as M3. They have a rounded occlusal outline and are smaller than the M1-2. The paracone is the largest primary cusp, slightly larger than the protocone. The hypocone is smaller than the protocone and separated from the protocone by a distinct notch along the posterolingual border of the tooth. The metacone is weak, varying from a slight bulge to a small cusp. The anterior cingulum is strong, extending labially from the anterior base of the protocone to the anterior base of the paracone. The protoloph is a complete crest, extending labially from the protocone to the paracone. The posterior arm of the protocone extends posterolabially into the central basin with from one to two small accessory lophs extending labially or posterolabially from it terminus. The posterior cingulum extends in a continuous arc from the hypocone to the posterior base of the paracone.

One tooth is identified as dp4 because it exhibits a significantly narrower trigonid and a distinct anteroconid. Its protoconid and metaconid are about equal in size and are connected by a short, low metalophulid II. The entoconid and hypoconid are robust and larger than the protoconid and metaconid. A moderate metastylid crest is present that extends posteriorly from the metaconid to terminate near the anterior base of the entoconid. The ectolophid is low, complete, and connects the protoconid to the hypoconid. The hypolophid is low and complete, extending from the lingual base of the hypoconid to terminate at the labial base of the entoconid. The posterior cingulum is very short, originating from the posterior cingulum and extending to near the posterolabial base of the entoconid.

Six teeth are identified as p4. The trigonid is narrower than the talonid, with the metaconid slightly larger than the protoconid. The hypoconid and entoconid are the largest primary cusps and are subequal in size. The anterior cingulid is distinct, extending labially from the metaconid to terminate near the anterior base of the protoconid. The mesoconid varies from a small, round cuspulid (two teeth) to a slightly transversely expanded cuspulid (three teeth), and one tooth has a short spur extending from the mesoconid into the central basin (incipient mesolophid). The ectolophid varies from being complete (three teeth), connecting the mesoconid anteriorly to the protoconid and posteriorly to the hypoconid, to incomplete, connecting the mesoconid anteriorly to the protoconid. The hypolophid is complete, extending lingually from the hypoconid to join the entoconid. The posterior cingulid is distinct, originating from about the center of the hypolophid to terminate at the posterior base of the entoconid.

Confident separation of m1 from m2 cannot be made for Metanoiamys . Ten teeth are identified as m1 or 2. The occlusal outline is subrectangular. The primary cusps (protoconid, metaconid, hypoconid, and entoconid) are somewhat lophate, being slightly anteroposteriorly compressed. The anterior cingulid is robust, extending labially from the anterior base of the metaconid to terminate near the anterior base of the protoconid, where it is separated from the protoconid by a shallow valley. An anteroconid is usually present on the anterior cingulid (seven teeth), and on three teeth an accessory cristid is present that extends posteriorly from the anteroconid towards the metalophulid II (= adlophulid of Chiment and Korth [1996]). The metalophulid II is complete, extending lingually in a gentle arc from the protoconid to join the labial edge of the metaconid. The ectolophid is usually complete (nine teeth), extending from the protoconid to the hypoconid, but in one tooth it is incomplete, only connecting the mesoconid posteriorly to the hypoconid. The mesoconid varies from a distinct, small round cuspulid (eight teeth) to a slightly transversely elongated cuspulid (two teeth). A small mesostylid is present between the metaconid and entoconid on five teeth. A short mesolophid is present that extends lingually or posterolingually from the mesoconid into the central basin. The hypolophid is complete, extending lingually from the hypoconid to join the labial edge of the entoconid. The posterior cingulid extends from the hypolophid, near its origination from the hypoconid, to the posterolabial base of the entoconid.

Four teeth are identified as m3 and are very similar in occlusal morphology to the m1-2, but differ by having the talonid narrower than the trigonid, a more robust hypolophid and a much weaker posterior cingulid. In addition, the m3 ectolophid exhibits variation. It is complete on two teeth, connecting the mesoconid anteriorly to the protoconid and posteriorly to the hypoconid and incomplete on two teeth, wherein on one tooth it is a low cristid that only connects the mesoconid anteriorly to the protoconid, and on the other it is a low cristid that only connects the mesoconid posteriorly to the hypoconid.

Remarks. In the sample of rodent teeth from SDSNH Locality 6242, no teeth representing Sciuravidae are present. All 26 small rodent teeth from the locality agree well with those of Metanoiamys and are compatible in size and occlusal morphology, supporting their referral to a single species. Additional teeth from four other localities within the TBM are morphologically indistinguishable from the sample from SDSNH Locality 6242 and are included in the species. Metanoiamys sp. is the most abundant rodent from the TBM.

Previous investigators have noted a similarity in the occlusal morphology of the basal eomyid Metanoiamys to that of certain Sciuravidae (e.g., Pauromys ) and have considered Metanoiamys and thus Eomyidae to have been derived from a sciuravid ancestor ( Lindsay, 1968; Walsh, 1997; Walton and Porter, 2008). In addition, Walsh (1997) discovered that the early Uintan Metanoiamys agorus Chiment and Korth, 1996 , from southern California retains a very small P3 based on an intact upper dentition preserving a small, single rooted P4 alveolus along with the common occurrence of small anterior appression facets on isolated dP4s and P4s assigned to the species. Where known, all Sciuravidae also retain a greatly reduced P3, thus this character is a pleisomorphy for M. agorus and Sciuravidae . Although similar in occlusal morphology, the lower molars of Metanoiamys sp. can be distinguished from those of Sciuravidae (see also remarks of Sciuravidae above). In Metanoiamys , the metalophulid II (= metalophid) is complete, forming a smooth arc between the protoconid and metaconid, and its junction with the metaconid is positioned more anteriorly, whereas in sciuravids the metalophulid II usually terminates slightly more posteriorly, near the posterolabial base of the metaconid. The ectolophid of Metanoiamys is relatively taller, and has a much greater tendency to be complete, usually connecting the mesoconid anteriorly to the protoconid and posteriorly to the hypoconid. The hypolophid of Metanoiamys is better developed, usually a continuous lophid joining the hypoconid to the entoconid and the entoconid is not separated from the lingual terminus of the posterior cingulid by a distinct valley. Metanoiamys sp. further differs from Pauromys by having the p4 much less reduced relative to the m1, with the mean p4 ap equal to 87% of the mean m1 ap.

As noted above in the remarks on the Sciuravidae , the upper molars of Metanoiamys are more difficult to distinguish from those of Sciuravidae than are the lower molars. However, the upper molars of Metanoiamys differ from those of Sciuravidae by having the protolophs and metalophs slightly taller and with a greater tendency to be complete, a greater tendency for the endolophs to be complete (connecting the mesocone to the protocone anteriorly and to the hypocone posteriorly), and small accessory lophules (mesolophs) that extend from the mesocone or endoloph into the central basin.

Based on isolated teeth from the early Bridgerian Powder Wash locality of Utah, Dawson (1968) described a sciuravid species that she left in open nomenclature as sciuravid sp. Dawson (1968) noted that the m1-2 of the Powder Wash sciuravid sp. exhibit some occlusal similarity to those of Sciuravus bridgeri , but differs from this species and other species of Sciuravus by having the p4 metaconid very reduced resulting in the p4 trigonid being significantly narrower than the talonid. In Metanoiamys , the p4 talonid is also significantly narrower than the talonid. The lower molars of the Powder Wash sciuravid sp. are characterized by the following ( Dawson, 1968): 1) an incomplete metalophulid II, extending lingually from the protoconid to terminate either a short distance into the central basin or near the posterolabial base of the metaconid; 2) a hypolophid that varies from incomplete (only a short lophid extending from the entoconid into the central basin) to nearly complete, extending from the entoconid to near the lingual base of the hypoconid; 3) a commonly complete ectolophid, connecting the protoconid to the hypoconid; and 4) a small mesoconid on the ectolophid. The M1-2 of the Powder Wash sciuravid sp. are characterized by the following ( Dawson, 1968): 1) a protoloph that extends labially to terminate at a small protoconule near the anterolingual base of the paracone; 2) a relatively complete metaloph connecting the hypocone to the metacone; and 3) a complete endoloph connecting the protocone to the hypocone. Although the lower molars of the Powder Wash sciuravid sp. exhibit the typical sciuravid configuration of an incomplete metalophulid II and hypolophid, the complete ectolophid is very similar to that of Metanoiamys . Similarly, although the M1-2 of the Powder Wash sciuravid sp. have the typical sciuravid configuration of an incomplete protoloph, the endoloph is complete, a character that is lacking in all other Bridgerian and Uintan sciuravid genera in which the upper molars are known ( Sciuravus and Taxymys ), but present in Metanoiamys . It is unlikely that Metanoiamys was derived from Pauromys because the p4 of Metanoiamys is not as greatly reduced. However, the fact that the Powder Wash sciuravid sp. shares certain dental characters with Metanoiamys that are generally lacking in other sciuravid taxa, suggests that a morphotype similar to the Powder Wash sciuravid sp. could have given rise to Metanoiamys .

As detailed above in the remarks on the Sciuravidae , most of the isolated teeth, including the holotypes, from the Uintan of Texas assigned by Walton (1993) to two species of Pauromys ( P. texensis and P. simplex ) are now regarded as belonging to Metanoiamys ( Walsh, 1997; Walton and Porter, 2008). However, a reanalysis and formal revision of the Texas species and their hypodyms is still needed. Metanoiamys sp. is similar in size to Metanoiamys agorus and M. marinus Chiment and Korth, 1996 , larger the M. lacus ( Storer, 1987) and smaller than M. fugitivus ( Storer, 1984) , M. korthi Kelly and Whistler, 1998 , M. fantasma ( Lindsay, 1968) , M. texensis ( Walton, 1993) , and M. simplex ( Walton, 1993) (see also Walton and Porter, 2008; Kelly et al., 2012). Metanoiamys sp. is the oldest recorded species of the genus and appears to represent a new species that is not far removed from its sciuravid ancestor. However, until the systematic relationships of the Texas species of Metanoiamys are clarified so that adequate comparisons can be made, the TBM species is left in open nomenclature as Metanoiamys sp.

Figures

FIGURE 11. Metanoiamys sp. from TBM: 1, LdP4, DMNH 74135; 2, LP4, DMNH 75331; 3, LM1 or 2, DMNH 74136; 4, LM1 or 2, DMNH 75251; 5, RM1 or 2, SDSNH 110442; 6, LM3, UCM 98813; 7, Rdp4, SDSNH 110369; 8, Lp4, DMNH 74143; 9, Lm1 or 2, DMNH 75252; 10, Lm1 or 2, DMNH 75250; 11, Lm1 or 2, DMNH 75254; 12, Lm3, DMNH 74144. All occlusal views. Scale bar equals 1 mm.

FIGURE 12. Elymys? emryi new species from TBM, macrophotographs with corresponding line drawings of specimens: 1 and 9, LM1, SDSNH 110463; 2 and 10, RM1, SDSNH 110465; 3 and 11, RM1 (holotype), SDSNH 110466; 4 and 12, RM2, SDSNH 110444; 5 and 13, LM3, SDSNH 110448; 6 and 14, Rm1, SDSNH 110456; 7 and 15, Lm2, SDSNH 110457; 8 and 16, Lm3, SDSNH 110461. All occlusal views. Scale bar equals 1 mm.