Nyctitherium gunnelli, Murphey & Kelly, 2017

Murphey, Paul C. & Kelly, Thomas S., 2017, Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 2: Apatotheria, Lipotyphla, Carnivoramorpha, Condylartha, Dinocerata, Perissodactyla and Artiodactyla, Palaeontologia Electronica (Cambridge, England: 2003) 20 (2), pp. 1-51 : 14-22

Nyctitherium gunnelli new species

Figures 7 , 8.8-10 , Table 2

zoobank.org/ 22943145-6062-4D3F-92F9-A7A5CE48453B

Holotype. Associated Lp4-m2, SDSNH 110393 .

Hypodym. From UCM Locality 92189: LP 4, UCM 95769.

Type Locality. SDSNH Locality 5843, 105 m above the base of the Turtle Bluff Member, Bridger Formation , Wyoming .

Distribution and Age. Type locality and SDSNH Locality 92189 at 2 m above the base of the Turtle Bluff Member, Bridger Formation, Wyoming. Earliest Uintan (Ui1a).

Diagnosis. Nyctitherium gunnelli is larger than Nyctitherium velox , Nyctitherium serotinum , Nyctitherium krishtalkai , and Nyctitherium celatum . It further differs from N. velox and N. serotinum by having m1-2 talonid width slightly more expanded transversely relative to trigonid width (m1 tra/trp = 0.93 and m2 tra/trp = 0.97 versus mean m1 tra/trp = 0.96 and mean m2 tra/trp = 0.98 for N. velox , and mean m1 tra/trp = 0.95 and mean m2 tra/trp = 1.05 for N. serotinum ), m1-2 anterior (precingulid) and posterior (postcingulid) portions of labial cingulid more strongly developed, and m1-2 hypoconulid more reduced in size relative to size of entoconid resulting in these two cuspids being slightly more weakly twinned. It further differs from N. krishtalkai and N. serotinum by having the p4 hypoconulid slightly better developed. It further differs from N. krishtalkai by having m1-2 cristid obliqua terminating higher on the postvallid (posterior wall of trigonid) and p4 talonid width more expanded transversely relative to trigonid width (talonid wider than trigonid). It further differs from N. serotinum and N. celatum by having a complete labial cingulid. Nyctitherium gunnelli differs from Nyctitherium christopheri by having P4 with slightly more transverse expansion, a much straighter ectocingulum, a more conical parastyle, a weaker metacone and metastyle, and less posterior expansion of hypoconal shelf. The lower molars of Nyctitherium gunnelli cannot be compared directly to much younger (late Uintan-Duchesnean) N. christopheri because it is only known from upper dentitions, but they appear to be larger (see Remarks below).

Etymology. Patronym for Gregg Gunnell of the Division of Fossil Primates, Duke Lemur Center, Duke University, in honor of his many contributions on Eocene mammals.

Description. The three TBM teeth from SDSNH Locality 5843 are regarded as associated, that is represent a single individual, because they were recovered from the same small batch of matrix, their preservation exhibits the same patterns of coloration, they are all in the same wear stage (unworn), are compatible in size and occlusal morphology, and all exhibit very similar abrasion patterns on the enamel along their ventrolabial borders. Although the ventrolabial enamel is abraded off these teeth, they all apparently had a continuous labial cingulid because the underlying dentine surfaces have a distinct ridge still present that outlines the labial cingulid. Moreover, the anterior and posterior cingulids (pre and postcingulids) are very robust, and the ridged dentine outline merges anteriorly and posteriorly with their broken edges ( Figure 7.4-6 ), indicating that the cingulid was continuous, extending from the anterolabial base of the protoconid across the entire ventrolabial border and terminating at the posterolabial base of the hypoconulid.

The p4 is molariform with the talonid wider transversely than the trigonid. The cuspate paraconid is small, but distinct, positioned slightly lingual of the midline and significantly lower in height than the metaconid and protoconid. The protoconid is large, vertically orientated and larger and slightly taller than the metaconid. The talonid is well developed with a moderately deep basin. The hypoconid is robust and about equal in height to the entoconid. The hypoconulid is distinct, smaller than the entoconid, and positioned close to the entoconid (twinned condition). The cristid obliqua is tall, extending anterolingually from the hypoconid to join the postvallid (posterior wall of the trigonid) at a level about a third the way up the base of the protoconid.

The TBM lower molars are very similar in occlusal morphology, but the m1 can be distinguished from the m2 by being slightly larger in size, having the length slightly longer relative to the talonid width (m1 trp/ap = 0.54 versus m2 trp/ap = 0.61), and the protoconid is slightly taller than the metaconid, whereas in the m2, they are about equal in height. Their trigonids are narrower and taller than the talonids, and open lingually (not compressed). The paraconid is cuspate and lower in height than the protoconid and metaconid. The protoconid is the largest trigonid cusp with its apex positioned slightly anterior of the metaconid apex. The talonid is deeply basined. The hypoconid and entoconid are robust with the latter being slightly lower in height. The hypoconulid is smaller than the entoconid and positioned lingually, close to the entoconid, and slightly lower in height than the entoconid and hypoconid. The preentoconid cristid is moderately strong, extending anteriorly from the entoconid to join the metaconid about a third the way up from its base. The cristid obliqua is tall, extending anterolingually towards the metaconid, where it terminates about halfway up the postvallid, near the posterolabial edge of the metaconid. There is a slight swelling on the cristid obliqua (incipient mesoconid?), near the anterolingual terminus of the cristid obliqua.

A partial P4 (UCM 95769) was recovered from UCM Locality 92189 that is significantly larger than those of Nyctitherium velox , but compatible in size with the p4-m2 of N. gunnelli and is tentatively assigned to the species ( Figure 8.8-10 , Table 2). Although the protoconid of UCM 95769 is broken off and a small portion of the enamel on the anterior face of the hypocone is missing, the following characters can still be discerned. The paracone is tall and robust with a moderately developed, centrally positioned rib on its labial face. The metacone is represented by a slight bulge on the short, but tall, blade-like postmetacrista, and is positioned slightly more lingually than the paracone apex. The paracone and metacone are separated by a relatively deep notch, thus a distinct centrocrista is lacking. The parastyle projects anteriorly as a robust, conical cusp with a sharp apex, and it is significantly lower in height than the paracone. The metastyle is very small and positioned at the labial termination of the postmetacrista. The protocone appears to have been relatively large based on the diameter of its broken base. The hypocone is low and moderate in size with a relatively large, posteriorly expanded hypoconal shelf. The anterior cingulum is weak and extends lingually from the anterior base of the protocone, then continues as a slight ridge around its lingual base, terminating at about the middle of the lingual base of the protocone. The posterior cingulum extends labially as a thin ridge from the hypocone and then, as it reaches the level of the base of the metacone, expands to become robust, terminating at the posterior base of the postmetacrista. The labial margin of the ectocingulum is relatively straight, wherein it extends anteriorly as a ridge from the posterolabial corner of the tooth and then descends and narrows to terminate at the anterolabial base of the paracone, resulting in a moderately developed metastylar shelf between the ectocingulum and the metacone. A paraconule and metaconule are lacking.

Remarks. The lower molars of nyctitheriids exhibit some occlusal similarities with those of early marsupials (especially Herpetotherium Cope, 1873c ), including the twinned condition of the entoconid and hypoconulid. In fact, the holotype of Entomacodon minutus Marsh, 1872b (= Centracodon delicatus Marsh, 1872b ) was first mistakenly identified as an insectivore ( Marsh, 1872b; Matthew, 1909), but was later shown to be a synonym of Herpetotherium knighti ( McKenna, 1960a; Robinson, 1968a). The dp3 of didelphids and the p4 of Nyctitherium are molariform and also exhibit similarity in their occlusal morphology. In contrast, the p3 of didelphids can be easily distinguished from the p4 of nyctitheriids because it is not molariform. So when first examining the three associated TBM teeth, the question arose, could these teeth represent dp3- m2 of a didelphid marsupial instead of Nyctitherium ? Although rarely recovered, dp3s have been described for Herpetotherium and Copedelphys Korth, 1994 , including Herpetotherium sp. , cf. H. knighti from San Diego area of California ( Lillegraven, 1976), Herpetotherium sp. , cf. H. knighti from the Lac Pelletier Lower Fauna of Saskatchewan ( Storer, 1996), Herpetotherium sp. , cf. H. marsupium from the Lac Pelletier Lower Fauna ( Storer, 1996), and Copedelphys . sp., cf. C. innominatum ( Simpson, 1928) from the Badwater Creek area of Wyoming ( Setoguchi, 1975; Krishtalka and Stucky, 1983; Korth, 2008). In all of these samples of Herpetotherium and Copedelphys , the dp3 is reduced relative to m1 with its length significantly shorter than that of the m1 (observed range of dp3 ap/m1 ap = 0.70-0.81 and mean dp3 ap/mean m1 ap = 0.72). A similar situation exists for the relative size of the dP3 and M 1 in Herpetotherium and other Eocene didelphids ( Setoguchi, 1975; Krishtalka and Stucky, 1983; Storer, 1996). In samples of Nyctitherium velox and N. serotinum from the Bridger Formation ( Robinson, 1968a; Krishtalka, 1976b), the p4 is much less reduced relative to the m1, where it is subequal to or slightly longer than the m1 (observed range of p4 ap/m1 ap = 0.93-1.06 for N. velox and N. serotinum , mean p4 ap/mean m1 ap = 1.03 for N. velox and mean p4 ap/ mean m1 ap = 0.98 for N. serotinum ). The TBM premolar is significantly less reduced relative to m1 (p4 ap/m1 ap = 0.94) than those of the dp3 of Herpetotherium or Copedelphys and within the observed range for those of Nyctitherium .

Additional characters also support the referral of the TBM teeth to Nyctitherium rather than Herpetotherium . In Herpetotherium and Copedelphys , the dp3 paraconid is more shelf-like, whereas in the p4 of Nyctitherium it is more cuspate, as in the TBM premolar. The p4-m2 hypoconulids of Nyctitherium and those of the TBM teeth are cuspate and positioned just slightly posterior of the entoconid, whereas in Herpetotherium and Copedelphys , the dp3-m2 hypoconulids are more shelf-like and positioned slightly further posteriorly from the entoconid. In fossil and extant marsupials, the only deciduous premolar that is replaced (diphyodont) is p3, whereas in eutherians, all of the deciduous premolars are usually diphyodont ( Flower, 1867; Cifelli, 1993; Cifelli et al., 1996; Cifelli and Muizon, 1998; Nievelt and Smith, 2005). In the extant shorttailed opossum ( Monodelphis Burnett, 1830 ), the crypt for the successor permanent p3 is present within the bone of the dentary below dp3 by 57 days of age, a well-developed, calcified p3 protoconid cap is present within the p3 crypt by 65 days of age, and p3 erupts at about 109 days of age ( Cifelli et al., 1996; Nievelt and Smith, 2005). The TBM premolar is embedded with a section of the dentary and has a large, long, well-developed posterior root and an anterior alveolus for the anterior root that is also large and long. There is no indication within the bone of the dentary below the TBM premolar of a successor crypt or premolar, further indicating that it is not deciduous. In Herpetotherium and Copedelphys , the m1 differs from the m2 by having the m1 paraconid positioned relatively further anteriorly resulting in its trigonid being more widely open lingually. In Nyctitherium and the TBM molars, the m1-2 paraconids are similarly positioned with the m1 lacking the more anteriorly positioned paraconid and widely open trigonid seen in Herpetotherium or Copedelphys . Based on the above evidence, the TBM teeth agree well with those of Nyctitherium and can be eliminated as possibly representing those of a marsupial.

Nyctitheriids have also sometimes been mistaken for bats because of similarities in their molar occlusal morphologies (e.g., Matthew, 1918; Gingerich, 1987) and some investigators have suggested that chiropterans may have arisen from an early nycitheriid ( Matthew, 1918; Robinson, 1968a; Rose, 1981; but for conflicting opinions, see also Smith, 1995; Smith et al., 2012; and Manz and Block, 2015). Where known for primitive Eocene bats, the dp4 is molariform and significantly smaller than m1 ( Sigé, 1991; Sigé et al., 1998), whereas the p4 is usually not molariform ( Jepsen, 1966; Smith et al., 2007, 2012). As noted above, the TBM premolar is not deciduous and is only slightly smaller than the associated m1, eliminating it as a molariform chiropteran dp4. In the early Eocene bat Icaronycteris Jepsen, 1966 , from the Green River Formation of Wyoming, the p4 is pre-molariform with a three-cusped trigonid and a wide rounded, basined talonid ( Jepsen, 1966; Smith et al., 2012). The TBM premolar differs from the p4 of Icaronycteris by having a much better developed metaconid (relatively larger and taller) and a fully developed, V-shaped talonid with three distinct cuspids (hypoconid, hypoconulid, and entoconid), like that of the p4 of Nyctitherium . Therefore, the associated TBM teeth can also be eliminated as representing a bat instead of Nyctitherium .

The phylogenetic relationships and content of the Nyctitheriidae have long been debated (e.g., Marsh, 1872b; Matthew, 1909, 1918; Simpson, 1928; Robinson, 1968a; West, 1974; Krishtalka, 1976b; Bown and Schankler, 1982; Gingerich, 1987; Butler, 1988; Smith, 1995; Sigé, 1997; Lopatin, 2006; Secord, 2008; Beard and Dawson, 2009, Rose et al., 2012; Christiansen and Stucky, 2013; Manz and Bloch, 2015). Recently, Christiansen and Stucky (2013) and Manz and Bloch (2015) provided cladistic analyses of the Nyctitheriidae , which significantly increased our understanding of the systematics of the family. The p4-m2 from the TBM are assigned to Nyctitherium because they exhibit the following characters ( Robinson, 1968a; Krishtalka, 1976b; Gunnell et al., 2008b; Manz and Block, 2015): 1) small size; 2) p4 molariform with a well developed talonid and a low paraconid; and 3) m1-2 with sharp, vertically orientated primary cusps, a relatively uncompressed, lingually open trigonid, a cuspate paraconid, a cristid obliqua tending towards the metaconid (vespiform condition), and a hypoconulid positioned close to the entoconid (twinned condition).

The taxonomy of the Bridgerian nyctitheriids has a complicated history with many species previously assigned to Nyctitherium now referred to different genera. Marsh (1872b) named a number of 'insectivores' from the Bridger Formation, including Nyctitherium velox , Nyctitherium priscum , Nyctilestes serotinus , Entomacodon angustidens , Entomacodon minutus , Centracodon delicatus , and Talpavus nitidens . Matthew (1909) transferred Nyctilestes serotinus to Nyctitherium as N. serotinum , transferred Talpavus nitidens to Nyctitherium as N. nitidus , placed Centracodon delicatus as a junior synonym of Entomacodon minutus , and described two new taxa from the Bridger Formation, Myolestes dasypelix and Nyctitherium curtidens . Robinson (1968a) regarded M. dasypelix as an aberrant representative of Nyctitherium and E. angustidens as a junior synonym of N. serotinum , which was followed by later investigators (e.g., Gazin, 1976). McKenna (1960a) first noted that the material referred to E. minutus may actually represent a marsupial, which was confirmed by later investigators ( Robinson, 1968a; Gazin, 1976; Kristhtalka and Stucky, 1983). McKenna (1960b), Krishtalka (1976b), and Bown and Schankler (1982) regarded Myolestes as generically and familially distinct from Nyctitherium and referred it to the Geolabididae . McKenna and Haase (1992) replaced Myolestes with Marsholestes because the name Myolestes was preoccupied. McGrew (1959) named Diacodon edenensis from the Bridger Formation at Tabernacle Butte. Robinson (1966a) transferred N. priscum and D. edenensis to the geolabidid genus Scenopagus McKenna and Simpson, 1959 , as S. priscus and S. edenensis , respectively. Based on a partial maxilla with M1-3 and isolated upper teeth, West (1974) described Pontifactor bestiola from the Twin Buttes Member of the Bridger Formation. Most recently, Christiansen and Stucky (2013) described a third species of Nyctitherium , N. krishtalkai , from the late Wasatchian Lost Cabin Member of the Wind River Formation, Wyoming. Thus, prior to the discovery of Nyctitherium gunnelli from the TBM, only three nyctitheriid species were recognized from the Bridger Formation; Nyctitherium velox , Nyctitherium serotinum and Pontifactor bestiola .

Based on a partial dentary with m1-3, Cope (1875) described Diacodon celatus from the late Wasatchian San Jose Formation of New Mexico. Matthew (1918) transferred the holotype of D. celatus along with a referred, associated partial upper and lower dentition (originally cited as AMNH 15103, but subsequently reassigned to AMNH FM 113783) from the Wasatchian Willwood Formation of Wyoming to Nyctitherium , as N. celatum . Subsequently, AMNH FM 113783 was reassigned to Pontifactor sp. by Krishtalka (1976b), leaving N. celatum known only from the holotype. However, Gingerich (1987) described a new genus, Wyonycteris , which he first regarded as a chiropteran, but is now regarded by most recent investigators to belong to a Wyonycteris-Pontifactor clade nested well within Nyctitheriidae (e.g., Beard and Dawson, 2009; Christiansen and Stucky, 2013; Manz and Bloch, 2015). It now seems most likely that AMNH FM 113783 actually represents a species of Wyonycteris rather than Pontifactor ( Gingerich, 1987) , which makes Pontifactor a monotypic genus that is known only from upper teeth.

One species of Nyctitherium , N. christopheri from late Uintan and early Duchesnean localities of the Badwater Creek area of Wyoming was described by Krishtalka and Setoguchi (1977), and is based only on upper teeth (holotype partial maxilla with P4-M3, three isolated upper molars and a P4). The upper cheek teeth of Nyctitherium christopheri differ from those of Bridgerian N. velox and N. serotinum ( Krishtalka and Setoguchi, 1977) by the following: 1) slightly larger size; 2) relatively more transversely expanded molars, especially M2; 3) a more expanded hypoconal shelf on P4- M2; 4) a more reduced precingulum (anterior cingulum) on P4-M2; and 5) a deeper M2 ectoflexus. In N. velox and N. serotinum , where larger samples of both upper and lower molars are available ( Robinson, 1968a; Krishtalka, 1976b), the m1 ap is 5- 9% longer than the M1 ap, and the m2 ap is 4-5% longer than the M2 ap. Assuming the upper and lower molar proportions of N. christopheri are similar to those of N. velox and N. serotinum , then the extrapolated m1 ap for N. christopheri would be about 1.8-1.9 mm and that for its m2 ap would be about 1.7 mm. Although speculative, these extrapolated dimensions suggest that the m1-2 of Nyctitherium gunnelli are probably significantly larger than those of N. christopheri because its m1 ap = 2.23 mm and m2 ap = 2.08 mm. In N. velox , the M1 ap is smaller than the P4 ap (mean M1 ap = 81% of mean P4 ap), whereas in N. christopheri the M1 ap is slightly less reduced relative to that of the P4 (M1 ap = 87% of P4 ap). The M1 tr is equal to the P4 tr in N. christopheri , but slightly smaller in N. velox (M1 tr = 97% of P4 tr). Thus, the P4 of N. christopheri differs from that of N. velox by being slightly larger anteroposteriorly and transversly relative to the M1 ( Figure 8.8-10 ). The tentatively referred P4 (UCM 95769) of N. gunnelli is similar in size to that of N. christopheri and, based on the large sizes of the m1-2 of N. gunnelli , the relative sizes of its M1-2 to its P4 are speculated to be similar to those of N. velox . Moreover, if the referral of UCM 95769 to N. gunnelli is correct, then the following additional differences distinguish the P4 of N. gunnelli from that of N. christopheri ( Krishtalka and Setoguchi, 1977) : 1) slightly more transversely expanded (P4 ap/P4 tr = 0.82, whereas that of N. christopheri = 0.87); 2) the ectocingulum is nearly straight, whereas that of N. christopheri exhibits a distinct concavity along the labial edge of the metastylar shelf; 3) a more conical parastyle, whereas that of N. christopheri is expanded slightly anteroposteriorly; 4) a more weakly-developed metacone and metastyle; 5) and a less posteriorly expanded hypoconal shelf. In addition, N. christopheri is significantly younger (late Uintan to early Duchesnean, <44 Ma) than N. gunnelli (earliest Uintan, ~47 Ma).

As noted above, P. bestiola is known only from upper dentitions, so a direct comparison of the lower cheek teeth of N. gunnelli cannot be made. However, based on unambiguous synapomorphies of the upper dentition of P. bestiola and species of Wyonycteris, Manz and Bloch's (2015) cladistic analysis placed Wyonycteris as the closest sister taxon to P. bestiola in a Wyonycteris-Pontifactor clade. Their data strongly suggests that the lower molars of P. bestiola should be similar to those of Wyonycteris . The lower molars of N. gunnelli differ from those of Wyonycteris by the following: 1) p4 paraconid lower in height; 2) p4 anterior cingulid (precingulid) much better developed; 3) m1-2 paraconid cuspate and unreduced; 4) m1-2 trigonid much less compressed and more open lingually; 5) m1-2 hypoconulid positioned more lingually and twinned with entoconid; 6) m1-2 cristid obiqua tends more towards the metaconid resulting in a more vespiform constriction between the trigonid and talonid; and 7) m1-2 labial cingulid more complete, extending across the hypoconid. Therefore, it is very doubtful that the lower molars of N. gunnelli could instead represent the lower molars of P. bestiola . Furthermore, the tentatively referred P4 of N. gunnelli is quite different from that of P. bestiola ( West, 1974, figure 1), including the following: 1) a quadrate occlusal outline due to an expanded hypoconal shelf; 2) a more anteroposteriorly compressed paracone; 3) a straighter ectocingulum; 4) a more lingually positioned protocone; and 5) a small parastyle present (lacking in P. bestiola ).

In summary, Nyctitherium gunnelli can be easily distinguished from Wasatchian and Bridgerian species of Nyctitherium by the differences listed above in the diagnosis. Despite acknowledging that we cannot directly compare the p4-m2 of N. gunnelli to N. christopheri or P. bestiola until their lower cheek teeth are known, we believe the circumstantial evidence strongly supports recognizing N. gunnelli as distinct from these nyctitheriids. Furthermore, assuming that the tentative referral of the P4 to N. gunnelli is correct, then additional differences can be recognized that distinguish N. gunnelli from N. christopheri and P. bestiola .

Figures

FIGURE 7. Nyctitherium gunnelli new species from TBM, associated Lp4-m2 (SDSNH 110393, holotype): Lp4 in occlusal (1), labial (4), and lingual (7) views; Lm1 in occlusal (2), labial (5), and lingual (8) views; Lm2 in occlusal (3), labial (6), and lingual (9) views. Scale bar equals 1 mm.

FIGURE 8. Nyctitheriidae from TBM. 1-6, Nyctitherium velox: RM2 (DMNH 75292) in occlusal view (1); RM2 (UCM 95771) in occlusal view (2); RM2 (UCM 68581) in occlusal view (3); RM3 (DMNH 75291) in occlusal view (4); partial Lm2 (DMNH 75317) in occlusal view (5); partial Lm2 (SDSNH 110409) in occlusal view (6). (7), Pontifactor bestiola, RM1 (UCM 71129) in occlusal view. 8-10, Nyctitherium gunnelli new species, LP4 (UCM 95769) in occlusal (8), labial (9), and lingual (10) views. Scale bar equals 1 mm.

FIGURE 9. Chart comparing m1-2 anteroposterior length (ap) versus m1-2 talonid width (trp) in mm of specimens of Nyctitherium velox (including two referred specimens from TBM, SDSNH 110409 and UCM 68967) and holotype m1- 2 (SDSNH 110393) of Nyctitherium gunnelli new species. Black solid filled squares/circles are m1s and white filled squares/circles are m2s. Except for SDSNH 110409 and UCM 68967, measurements of other specimens of N. velox from Robinson (1968a) and Krishtalka (1976b).

FIGURE 10. Oligoryctidae and unnamed lipotyphlan from TBM. 1-4, unnamed apternodontid sp.: partial LM1 or 2 (SDSNH 110348) in occlusal (1) and anterior (2) views; partial Rm1 or 2 (SDSNH 110349) in occlusal (3) and labial (4) views. 5-6, unnamed lipotyphlan sp., partial dentary with Rp4 (DMNH 75258) in occlusal (5) and labial (6) views. Scale bars equals 1 mm.