Abudhabia radinskyi, Jiangzuo & Flynn & Wang & Hou & Deng, 2023

FLYNN, LAWRENCE J., WINKLER, ALISA J., JACOBS, LOUIS L. & DOWNS, WILL, 2003, Chapter 23: Tedford’s Gerbils from Afghanistan, Bulletin of the American Museum of Natural History 279, pp. 603-624 : 606-615

Abudhabia radinskyi , new species

HOLOTYPE: AMNH 133507 , including an incomplete skull and lower jaws, plus a partial skeleton (consisting of vertebrae, ribs, scapula, clavicle, humerus, radius, ulna, distal tibia).

HYPODIGM: AMNH 133507–133510. AMNH 133508 includes the skull, lower jaws, and articulated cervical vertebrae. AMNH 133509 has an incomplete cranium (without lower jaws), and associated postcranial fragments (prepared elements include scapula, radius and humerus fragments, partial innominate, tibiae, proximal right femur, left pes). AMNH 133510 is blocks of matrix bearing scattered, unprepared bone.

ETYMOLOGY: After vertebrate paleontologist Leonard Radinsky, Tedford’s field companion and cofinder of the gerbil remains, in accordance with Tedford’s wish.

TYPE LOCALITY: East Base, Tor Ghar. Lataband Series, in local gray micaceous sandstone immediately above green­gray silty sandstone with limonite­stained root casts (field notes of Tedford). One specimen ( AMNH 133508) occurs as a concretion.

DIAGNOSIS: Largest known species of the genus (table 23.1). Molar dimensions at least 8% greater than in any known species of Abudhabia . Distinct from all other species except A. sp. from Kenya in lacking any trace of longitudinal crests (derived). Remnant of anterior cingulum on M2 reduced relative to A. baynunensis , A. pakistanensis , and A. kabulense (derived). Higher crowned than A. pakistanensis . Anteroconid of m1 with reduced labial arm (no protoconulid) and triangular in outline, compared to more rhomboid­shaped anteroconid of A. baynunensis and A. kabulense ; anteroconid wider on A. sp. Transverse crests of m1 not so oblique (derived toward condition of Tatera ); third loph of m1 relatively transverse compared to that of A. baynunensis and A. sp., but within

(D) lateral views. A, B, D at same scale, anterior to right.

the range of variation of A. kabulense (derived); second loph of m1 more oblique (not as derived) as that of A. sp.

DESCRIPTION OF SKULL

DORSAL VIEW (fig. 23.2A): The nasals are long and narrow, extending anteriorly beyond the incisors. Laterally, the nasals are bordered by the premaxillaries. The nasals and premaxillaries form a transverse suture with the frontals, posterior to the infraorbital foramen, at the level of the anterior extent of the orbit. The maxillaries in turn border the premaxillaries and constitute the anterior root of the zygomatic arch. The maxillaries flare laterally, then anteriorly, to form the zygomatic plate, which is more expanded than in Rattus . The lacrimals are large, typical for gerbils, but unclear in the specimens at hand. The frontals taper gently anteriorly, and are little constricted at the orbits. The supraorbital ridge runs along the dorsal margin of the orbit, forming a narrow shelf. Unlike Rattus , the ridges are less convergent, lying only along the margin and then fading anteriorly near the suture with the premaxilla. The ridges do not extend onto the dorsomedial surface. The supraorbital ridge ends posteriorly in a small knob at the convergence of the frontal, parietal, and squamosal. The frontoparietal suture is concave anteriorly. A parasagittal crest runs posteriorly from the knob at the frontal­parietal­squamosal convergence, continuing the supraorbital ridge. The crest follows the parietal­squamosal suture until about the level of the squamosal root of the zygomatic arch, posterior to which the crest continues to the supraoccipital. The parietal­squamosal suture dips laterally and follows a secondary crest, which fades out as it approaches the supraoccipital. A lambdoidal crest is formed in the region of the parietalsupraoccipital suture, more similar to that of Tatera indica than Meriones . The posterior portion of the parietal, ventral to the parasagittal crest and above the suture with the squamosal, is rectangular in shape. The large interparietal lies between the parietals and supraoccipital, tapering laterally. The anteri­ or processes of the supraoccipital, which would truncate the interparietals laterally, appear to be absent.

VENTRAL VIEW (fig. 23.2B): The premaxillary­maxillary suture falls in the anterior middle portion of the anterior palatine foramen. The anterior palatine foramen extends posteriorly to about the anterior margin of M1. The posterior palatine foramen is elongate, about a third the length of the anterior palatine foramen, and much longer than in Rattus . The maxillo­palatine suture intersects the posterior palatine foramen. The pterygoid fossa is short and roundly triangular, but not deep, more similar to Tatera indica than Meriones . The mesopterygoid fossa is narrow. The foramen ovale lies at the posterolateral extent of the pterygoid fossa at the anterior limit of the tympanic bulla. The presence of an accessory foramen ovale cannot be determined. In Tatera indica , a bridge off the pterygoid is present, but quite delicate, and is associated with the accessory foramen ovale. If such a bridge was present in Abudhabia , it was also quite delicate. Wahlert (1974, 1985) considered possession of an accessory foramen ovale to be a rodent synapomorphy that is lost in more advanced forms. In gerbils with the bulla more inflated than in Tatera , the accessory foramen ovale is lost. Posterolateral to the foramen ovale the medial lacerate foramen lies in the fissure anterior to the auditory bulla. A postglenoid foramen is present near the bulla, posterior to the glenoid fossa on the ventral surface of the squamosal root of the zygomatic arch. The basisphenoid and basioccipital are narrow. No complete, undistorted bullae are preserved and visible; however, the tympanic portion, while not appearing to be greatly inflated, is elongate anteroposteriorly. The mastoid portion does not appear to be inflat­ ed. Tong (1989) mentions a small foramen on the ventral surface of the maxilla anterior to M1 as present in gerbils, but it is not present in these specimens.

LATERAL VIEW (fig. 23.2D): In lateral view, the nasals are shown to extend anterior to the incisors, and the incisors are strongly recurved. Opisthodonty is emphasized by an increase of the length of arc of the incisors, extending their apices rearward (see Landry, 1957). The zygomatic plate is developed on the maxillary root of the zygomatic arch and closes off the infraorbital foramen. While the anterior margin of the zygomatic plate is not preserved in any specimen, the remaining portions are consistent with an anteriorly expanded zygomatic plate. Preserved portions clearly extend anterior to the nasolacrymal foramen and conceal it from view. The sphenopalatine foramen is large, anteroposteriorly elongate, and sits approximately over the posterior half of M1, similar to Tatera indica and Meriones , but unlike Rattus . The frontal folds ventrally to form the dorsomedial wall of the orbit. A small ethmoid foramen is present in the frontal, anterodorsal to the optic foramen, and above the level of contact between M1 and M2.

Ventral to the frontal is the orbitosphenoid. The optic foramen in the orbitosphenoid is large, its anterior end at the level of the posterior border of M2. The ventral margin is straight while the dorsal border is convex. The large sphenoid fissure sits posteroventral to the optic foramen, separated from it by a thin process of the orbitosphenoid. Its posterior limit is in the alisphenoid. Its upper limit does not reach the level of the upper border of the optic foramen.

The alisphenoid is comparable to that of Tatera indica . The foramen ovale sits opposite the lateral process of the alisphenoid, which helps define the pterygoid fossa, separated by a thin piece of bone from the medial lacerate foramen.

The squamosal forms the lateral surface of the skull dorsal to the alisphenoid and tympanic bulla and ventral to the frontal and parietal. The glenoid fossa for the articulation of the dentary is a broad troughlike groove on the ventral surface of the squamosal root of the zygomatic arch. A postglenoid foramen may be present. The zygomatic root extends a short distance anteriorly along the skull as a crest that elongates the glenoid forward. The posterior margin of the zygomatic root extends posteriorly to the mastoid region. The posterior portion of the squamosal surrounds the squamosomastoid window. The ventral squamosal process terminates in a bony triangle that contacts the dorsal squamosal process to enclose the window. The mastoid apparently is not swollen. Structures inside the bulla cannot be seen because they are internal to other bones or because they require additional preparation to observe if they are present. The paroccipital process apparently does not protrude posteriorly beyond the occipital condyle.

MANDIBLE

The dentary of Abudhabia radinskyi (fig. 23.3B, C) has a reduced coronoid process. The incisor capsule is distinct. The mental foramen is anterior to m1 along the dorsolateral margin of the diastema. The angular process reclines posteriorly. It is well developed and deflected only slightly laterally. In more derived gerbils, such as Meriones , the angular processes are deflected much more strongly to accommodate greatly inflated bullae. The inferior masseteric crest extends from the mental foramen to the ventral margin of the jaw towards the angle. The ventral margin at the angle folds medially to form a flat surface. The superior masseteric crest is distinct, but less developed than the inferior crest. It extends from the anterior margin of the coronoid process and converges with the inferior masseteric crest at the mental foramen. The mandibular foramen is high on the medial surface of the ascending ramus, posterior to the posterior border of the coronoid process, and near the posterior margin of the ascending ramus. Only the left mandible of AMNH 133508 is preserved sufficiently for measurement of the diastema: 6.08 mm to the anterior edge of m1 (5.8 mm to its alveolus). The dentary resembles that of Tatera indica in size and most details.

DENTAL DESCRIPTIONS

The dentitions of all specimens show moderate occlusal wear (fig. 23.4). Maximum upper tooth row lengths in millimeters (using a reticule fitted onto a Wild dissecting microscope) are: AMNH 133507, left = 6.17; AMNH 133509, right = 6.50 and left = 6.42. Maximum lower tooth row lengths are: AMNH 133507, right = 6.42; AMNH 133508, left = 6.25. Lower molar tooth row lengths are about 6% greater than diastema length. Occlusal measurements of the cheek teeth are given in table 23.1. Tooth terminology is illustrated in figure 23.5.

The upper and lower molars are lowcrowned and broad, with rounded (exaenodont) sides. Lophs are separated by deep, transverse valleys. Longitudinal connections between the lophs are absent. Lophs of the M1 are inclined slightly posteriorly. Lophs of M2 are relatively vertical. The M3 is inclined somewhat anteriorly. Lophs of the lower molars are inclined slightly anteriorly.

UPPER DENTITION: The upper incisors are preserved in AMNH 133508 and 133509 (figs. 23.2, 23.3, 23.5). Cross­sectional dimensions (mm) are: 2.62 buccolingual length X 1.07 mediolateral width, and 2.70 X 1.15, respectively. The incisor enamel wraps well onto the lateral surface, covering about 45% of the tooth in lateral view. The single longitudinal groove is located just lateral to the midline of the major axis of the eliptical cross section. The groove is a sharp step with rounded margins, but not invaginated. The groove of Tatera indica is similar, although nearer the midline and slightly invaginated.

The M1 includes three transverse lophs. The anterior loph consists of a broad triangular anterocone extended labially as an inconspicuous short cingulum. A faint indentation along the anterior wall indicates derivation from a bilobed condition. The width of the anterocone is about three­fourths or more that of the second loph. An anterolingual inflection (sensu Flynn and Jacobs, 1999) is lacking. The posterior apex of the anterior loph is in contact with the midpoint of the second loph. The second loph is formed by the joined protocone and paracone. The protocone is slightly more bulbous, relative to a more transversely elongate paracone. The posterior loph consists of the metacone and larger hypocone. This loph is slightly oblique (more anterior labially; AMNH 133507) or transverse (AMNH 133509 and AMNH 133508) to the long axis of the tooth. The cusps are confluent posteriorly, with a distinct posterior cingulum labial to the long axis of the tooth. The posterior cingulum represents an abbreviated posteroloph. There is a strong root under the anterocone; specifics of other roots cannot be determined.

The M2 includes two transverse lophs. The first has transversely elongated protocone and a smaller paracone. There is a small anterolabially directed anterior cingulum (faint on AMNH 133507) emerging from the longitudinal midline. The second loph has transversely elongated metacone and larger hypocone. The metacone is slightly anterior to the hypocone. This loph is continuous with the distinct posterior cingulum located near the midline of the tooth and confluent with the posterolabial border of the hypocone. Specifics of root morphology cannot be determined.

The M3 includes a single, wide anterior transverse cusp that represents coalesced protocone and paracone, and is fused posteriorly (via the paracone) with a prominent posterior cusp, which represents a reduced hypoconemetacone complex. The posterior cusp is located labial to the longitudinal midline of the tooth, and gives the tooth a hooklike appearance. There appears to be a single large root.

LOWER DENTITION: Lower incisors are preserved on AMNH 133507 and 133508. The lower incisor is gracile and lacks prominent ornamentation. This tooth can be measured readily on AMNH 133507. Cross­sectional dimensions are 2.13 buccolingual length, and 0.98 mediolateral width. The enamel is rounded and wraps 42% of the way onto the lateral side of the incisor.

The m1 has three lophs. The first is a triangular anteroconid, which (on AMNH 133507, fig. 23.5C; labial only on AMNH 133508) has gentle labial and lingual indentations suggesting derivation from a tripartite condition. The anteroconid is continuous labially as a cingulum, which is bulbous on AMNH 133507. The second loph includes a transverse protoconid and metaconid. The

133509. B, C, Stylized upper and lower first molars.

metaconid is slightly larger than the protoconid. This loph is somewhat oblique, with the metaconid anterior. A low labial cingulum abutts the protoconid and hypoconid. Lingual cingula are absent. The third loph is transverse, and connects the entoconid with the larger hypoconid. This loph is continuous with the prominent conical posterior cusp, or cingulum, that fuses rapidly with the hypoconid. Root development is difficult to determine, but includes a very large anterior root plus one large or a double root under the posterior end of the tooth.

The m2 has two transverse lophs. The first has a broadly confluent protoconid and metaconid. The protoconid is bulbous relative to the more transversely elongate metaconid. There is a small anterolabial cingulum on AMNH 133508, which is more weakly developed on AMNH 133507. The second loph has a broadly confluent entoconid and slightly anteriorly located hypoconid. These cusps are continuous with a weak, somewhat lingually located, posterior cingulum on the right m2 of AMNH 133507. The posterior cingulum is greatly reduced to absent on the other specimens. This tooth has anterior and posterior roots, but specifics are indeterminate.

The m3 consists of a single transversely elongated cusp, more rounded than that of Tatera indica . The tooth is positioned lingually in the tooth row with respect to the other molars. The roots are not exposed except for the posterior portion, which angles sharply posterolabially relative to the crown, suggesting that a separate anterior root may be present.

POSTCRANIA

Representation of the postcranial skeleton is indictated above in the hypodigm, but further preparation is required. Specimen AMNH 133509 includes both tibiae and the proximal two­thirds of one femur. The femur compares with Meriones persicus in size. Its third trochanter (fig. 23.6C) is an extended, long flange that reaches farther distally than in Meriones and would supply greater surface area for muscle attachment and a longer lever arm for the muscle fibers. Type specimen AMNH 133507 includes a left scapula, humerus, radius, and ulna in anatomical association (fig. 23.3A). The elbow joint is hyperflexed with olecranon dislocated from its fossa. The ulna is lacking most of its olecranon process, about 3 mm by comparison with modern gerbils. These fossils allow computation of limb element length ratios (table 23.2).

Tong (1989: 46) selected a number of gerbils to compute limb element ratios and judge relative degree of saltatorial adaptation. She measured the tibia/femur + tibia ratio (which differs from the crural index, tibia/femur), and the hind/forelimb ratio (femur + tibia/humerus + ulna; this differs from the intermembral index, which is humerus + radius/femur + tibia).

The posterior/anterior limb ratio of Tong (1989) did not sharply distinguish gerbils from other muroids, although African taterillines exceeded 1.5, a value greater than typical muroids. Our own measurements of a specimen of Tatera indica from Pakistan yield a ratio of 1.52. The unadjusted data for Abudhabia radinskyi yield a greatly underestimated ratio of 1.4, which is observed in some gerbils and is approached by Rattus . Estimating length of the incomplete femur (table 23.2) gives a ratio of>1.6, beyond any values computed by Tong. This indicates a long hind limb for Abudhabia relative to studied gerbils.

The second ratio discussed by Tong (1989) is the proportion of the tibia to the total hind limb length, which clearly distinguishes gerbils from other muroids. Gerbil tibiae are found to be quite long. Our measurements for one specimen of Tatera indica give a ratio of 54%. For Abudhabia radinskyi , the uncorrected values are 39.4/39.4 + 22 = 64% (beyond any observed values). Femur length must be corrected. Estimates based on Meriones and Mesocricetus (table 23.2) yield 56% and 54%, respectively, comparable to values seen in the most derived gerbils. The estimated ratios, albeit imprecise, are indicative of long tibia proportions for Abudhabia , and are conservative since they are ratios.

We conclude that Abudhabia radinskyi had a long hind limb, longer than typical gerbils. Its lengthened femoral crest and bladelike third trochanter would allow powerful extension. These observations are consistent with efficient hopping, at least as efficient as that seen in modern gerbils. The hind limb is not quite as long as in the truly saltatorial kangaroo rat, Dipodomys merriami . Measurements on one specimen of D. merriami yield posterior limb ratio of (34.2 + 24.5)/(12 + 19.7) = 1.85. Tong (1986) hypothesized the Miocene fossil Myocricetodon irhoudi from North Africa as sister taxon to extant gerbils, but showed its limbs to be typically murine in proportions except for a slightly elongated tibia. Abudhabia radinskyi was clearly derived with respect to M. irhoudi .