Synemporion keana, Ziegler & Howarth & Simmons, 2016

Ziegler, Alan C., Howarth, Francis G. & Simmons, Nancy B., 2016, A second endemic land mammal for the Hawaiian Islands: a new genus and species of fossil bat (Chiroptera: Vespertilionidae), American Museum Novitates 2016 (3854), pp. 1-52 : 7-17

Synemporion keana , gen. and sp. nov.

Lava-tube Bat

Figures 2–8 , tables 1–3

HOLOTYPE: BPBM 159269 , a nearly complete skeleton of an adult individual including skull, both dentaries, and partial body skeleton with both scapulae, humeri, ulnae, radii, and femora; right tibia and partial innominate with 2 fused sacral vertebrae; 8 metacarpals; 2 manual phalanges; 1 pedal phalanx; atlas; 2 thoracic vertebrae; and 1 rib. This individual was collected on 16 September 1982 by Francis G. Howarth and Fred D. Stone, Field No. 22 of F.G. Howarth.

PARATYPES: Specimens representing at least 110 individuals from 5 Hawaiian Islands , as listed in appendix 2, including 33 partial to nearly complete skulls, 9 dentaries, 34 associated partial post cranial skeletons, and 60 individual post cranial bones.

TYPE LOCALITY: Hawaiian Islands , Maui Island , ‘ Ulupalakua Ranch , Māhiehie Cave , 500 m, 20.63°N; 156.39°W (WGS 84 datum) .

GEOGRAPHIC AND GEOLOGIC RANGE: Hawaiian Islands, at least the five largest islands, from ca. Middle Pleistocene to Late Holocene (but apparently not into post-1778 historic period), with details as follows. Kaua‘i: infill of sinkhole of Late Pleistocene lithified calcareous dune deposits, and in Early Holocene eolian surface calcareous sand-dune deposits; O‘ahu: in Middle Pleistocene pond deposits of volcanic tuff-cone crater, as well as in later-Pleistoceneto-Holocene composite soil deposits (primarily sedimentary) within limestone sinkholes in emergent Late Pleistocene coral-algal reefs; Moloka‘i: in presumably Polynesian or early postcontact alluvial sediment on floor of a dynamically active piping cave; Maui: on exposed floor, embedded in mineralized crusts on walls and in alluvial sediments of Late Pleistocene or Holocene lava tubes; Hawai‘i: on exposed floor or in alluvial deposits of Late Pleistocene and Holocene lava tubes; approximate locations of these sites are shown in figure 1, and detailed information on all sites is provided in appendix 4.

ETYMOLOGY: The genus name Synemporion root from Greek common noun synemporos, “fellow traveler or companion,” with addition of the suffix -ion to form a neuter diminutive, in allusion to the new bat’s former co-occupation of the tectonically mobile Hawaiian Islands with the larger Lasiurus cinereus semotus . The specific name is a noun in apposition, formed from Hawaiian: the demonstrative ke, plus ana, “cave” or “lava tube,” referring to the subterranean provenience of the holotype and a majority of the paratypes.

DIAGNOSIS: A small vespertilionid with a dental formula of I1/3, C1/1, P1–2/2, M3/3 = 30–32. Distinguished from other vespertilionid genera with broadly similar dental formulae ( Scotoecus , Scotozous , Chalinolobus , Lasiurus , Nycticeius, Rhogeesa , Scotomanes , Scotophilus , Otonycteris , Bauerus , Pharotis , Nyctophilus ) by the following traits: upper I2 always absent (sometimes or always present in Scotozous and Chalinolobus ); upper P2 variably present (always absent in Nycticeius, Rhogeesa , Scotomanes , Otonycteris , Bauerus , Pharotis , Nyctophilus ); lower incisors trifid and subequal in size (all bifid in Otonycteris and Scotophilus ; i1 and i2 bifid in most Rhogeesa, all species of which have i3 reduced to a peg or spicule); M3 crown approximately 1/3 the area of M1 and M2, lacking any trace of a metacone or premetacrista (M3 crown area ½ or more than that of M1 and M2, and premetacrista present, in Nycticeius , Nyctophilus , Scotoecus , and Scotozous ); m3 with well-developed entocoinid and talonid only slightly narrower than trigoinid (entoconid poorly developed or absent and talonid markedly narrower than trigonid in Scotomanes ); skull with low rostrum and moderately well-developed forehead that rises abruptly to join with braincase, so that the rostrum profile appears concave in lateral view (rostrum profile flat or convex and forehead break absent in Lasiurus , Nycticeius , Scotomanes , Scotoecus , Scotozous , and Otonycteris ); rostrum relatively narrow (broad in Scotoecus , Scotomanes , and Lasiurus ); sagittal crest absent (present in Lasiurus , Nycticeius , Scotozous , Scotoecus, Rhogeesa , Scotophilus , Scotomanes , Chalinolobus , Otonycteris , Bauerus , Pharotis , and Nyctophilus ); metacarpal formula III> IV> V (metacarpal formula III = IV = V in Chalinolobus , Rhogeessa , Nycticeius , Scotoecus , and Scotomanes ; III> IV = V in Nyctophilus and Otonycteris ; III = IV> V in Scotophilus ).

DESCRIPTION: The dental formula of Synemporion is I1/3, C1/1, P1–2/2, M3/3 = 30–32, with the small anterior upper premolar (here termed P2 following Miller, 1907) variably present (figs. 2–5). Among specimens of Synemporion with complete palates, P2 is present on both sides in 4 specimens, present on one side and absent on the other side in 1 specimen, and completely absent in 3 specimens including the holotype (figs. 3, 4). Two half-palates both preserve a P2. Taken together, these observations suggest that P2 was present somewhat more than 50% of the time in Synemporion . When present, this tooth is minute and displaced lingually to lie in the angle between the lingual margins of C and P4. Selected craniodental measurements for S. keana are given in table 1.

The single upper incisor of Synemporion is roughly two-fifths to one-half height of the upper canine and has about half its crown area, with a small posterointernal cingular cusp (fig. 4). The upper canine is tall, straight, and not recurved (figs. 2, 4). The posterior upper premolar (P4), always present and relatively large, has an unworn crown height about three-fourths that of the canine. An anterolingual cingular cusp is present on P4. M1 and M2 are dilambdodont, subequal in size, and lack a discrete hypocone cusp. Both of these teeth have a postprotocrista that extends from the protocone to the base of metacone, thus closing off the trigon basin posteriorly. The preprotocrista on M1 and M2 extends from protocone to the parastyle rather than to base of paracone. M3 is quite reduced, with a crown area little more than one-third the area of M1 and M2 and it lacks any trace of a metacone or premetacrista. The postparacrista is approximately two-thirds the length of the preparacrista, and the length of the lingual portion of the tooth is greater than the length of the labial portion.

The lower incisors of Synemporion are mesiodistally broad, trifid; and the crown of i1 substantially overlaps that of i2 when seen in anterior view (figs. 5, 6). The transverse crown width of unworn i1 and i2 about twice diameter of exposed root, and that of unworn i3 (missing in the holotype although the alveolus is present) is apparently only a little smaller. The lower canine is tall, sharply pointed, and not recurved (figs. 2, 5). Two double-rooted lower premolars are present. The crown length and height of p2 is approximately two-thirds that of p4, which is equal in crown height to m1. Three lower molars are present; m1 and m2 subequal in size, while m3 is somewhat smaller than the anterior molars in all dimensions. The talonid is wider than trigonid in m1 and m2, and both of these teeth are nyctalodont (the postcristid connects hypoconid with hypoconulid, so that talonid basin is narrowly open posteromedially). The last molar (m3) has a talonid that is slightly narrower than trigonid, and lacks a hypoconulid although the hypoconid and entoconid are both well developed.

The skull of Synemporion has a low rostrum and a moderately well-developed forehead that rises abruptly to join with braincase, so that the profile of the rostrum appears concave in lateral view (fig. 2). The rostrum is of moderate length and relatively narrow (less than threefourths the width of the braincase) and has a well-developed median sulcus (figs. 2, 3). The frontal portion of braincase is inflated and is higher and slightly wider than the occipital portion. There is no obvious sagittal crest, and the lambdoidal crests are weakly developed and present only laterally. The superior temporal lines are separated about 1 to 2 mm along the sagittal suture. Postorbital processes and supraorbital ridges are entirely absent. The zygomatic arches relatively narrow throughout and lack an expanded dorsal projection. The lachrymal ridge at the anterior rim of orbit is weakly developed.

The premaxilla in Synemporion lacks a palatal branch and is fused to the maxilla (figs. 2, 3). The narial emargination relatively broad in dorsal view, about 1.5 times as wide as deep, and the anterior palatal emargination even broader, essentially twice as wide as deep. The hard palate extends posteriorly well beyond the tooth row and orbit area. A pair of well-developed but shallow basisphenoid pits are present and are separated by a medial ridge. The cochleae are large, and distance between the cochleae is equivalent to approximately three-fourths the diameter of the cochlea. The dentary is essentially the same as found in most other vespertilionids, with a relatively low coronoid process and a well-developed mental foramen between the roots of C and P2 (fig. 2).

Selected postcranial measurements of Synemporion are presented in tables 2 and 3. The clavicle is rodlike and lacks any kind of midshaft enlargement or projection. The scapula has a large dorsal articular fossa that is nearly as large as the glenoid fossa, and has an assymetrically bifid coracoid process with the longer branch directed medially. The humerus (figs. 7, 8) has a well-developed greater tuberosity that projects proximally well beyond humeral head; the lesser tuberosity, also well developed, is smaller and does not project beyond head. The bicipital groove along lateral surface of deltoid tuberosity is deep and well defined, and the deltoid tuberosity is high with ridge extending distally for almost one-quarter the total length of humerus. The distal articular surface lies in line with shaft, the trochlea noticeably greater in proximodistal diameter than capitulum, and the medial epicondylar spine projects distally 0.5 mm beyond trochlea. The ulna is fused proximally with radius, and the distalmost ulnar remnant is evident as small, proximally directed, flat spine fused to distal end of lateral border of radius. Metacarpal formula III> IV> V, with metacarpal IV length approximately 94% of that of metacarpal III, and metacarpal V length approximately 87% that of metacarpal III.

The vertebral column and pelvis of Synemporion are unremarkable and resemble those of other vespertilionids. There is no evidence of vertebral fusion other than in the sacrum. The femur is of usual vespertilionid morphology with the lesser trochanter projecting further proximally than greater trochanter, each distal condyle markedly compressed mediolaterally, and an intercondylar notch that is narrow and deep. The tibia is relatively long and slender, and the fibula is ossified for approximately half of the length of the tibia, the remainder being either threadlike or cartilaginous (not preserved among the material found).

Figures

FIGURE 2. Lateral views of the skull and left dentary of A, the holotype of Synemporion keana (BPBM 159269) compared with B, Lasiurus cinereus semotus (BPBM 184506). Note the difference in the rostral profile between the two species, including the more robust lower jaw dentition in Lasiurus compared with Synemporion.

FIGURE 3. A, Dorsal view and B, ventral views of the skull of the holotype of Synemporion keana (BPBM 159269) compared with C, dorsal and D, ventral views of the skull of Lasiurus cinereus semotus (BPBM 184506). Note that the rostrum of Synemporion is narrower than that of Lasiurus, and that Synemporion also lacks a sagittal crest.

FIGURE 4. Lateral and occlusal views of the upper dentition of A, the holotype of Synemporion keana (BPBM 159269) compared with that of B, Lasiurus cinereus semotus (BPBM 184506). Note that a small anterior upper premolar (P2), visible in occlusal view in Lasiurus, is absent in the holotype of Synemporion keana. This tooth is variably present in Synemporion, occurring somewhat more than 50% of the time and sometimes present on one side but absent on the other side in the same individual (see text for discussion).

FIGURE 5. Lateral and occlusal views of the lower dentition of A, the holotype of Synemporion keana (BPBM 159269) compared with that of B, Lasiurus cinereus semotus (BPBM 184506).

FIGURE 6. Anterior view of the lower incisors and canine of A, the holotype of Synemporion keana (BPBM 159269) compared with those of B, Lasiurus cinereus semotus (BPBM 184506). The third lower incisor (i3) is absent in the holotype of Synemporion but paratypes preserve this tooth, which is always trilobed.

FIGURE 7. A, Lateral, B, anterior, C, medial, and D, posterior views of the humeri of the holotype of Synemporion keana (on the left in each pair; BPBM 159269) and Lasiurus cinereus semotus (on the right; BPBM 184506).

FIGURE 8. Proximal view of the humerus of A, the holotype of Synemporion keana (BPBM 159269) compared with that of B, Lasiurus cinereus semotus (BPBM 184506). Anterior is toward the top of the page in each figure.