Alligator hailensis, Stout, 2020

Stout, Jeremy B., 2020, New early Pleistocene Alligator (Eusuchia: Crocodylia) from Florida bridges a gap in Alligator evolution, Zootaxa 4868 (1), pp. 41-60 : 43-53

Alligator hailensis sp. nov.

( Figures 2–10 )

Holotype. UF 224688: complete skull with lower jaws and associated postcranial remains: thoracic and caudal vertebrae, 21 osteoderms, partial right coracoid, partial right humerus, partial right ulna, partial left ilium, both femora, left tibia, left fibula, right and left metatarsal 1, left metatarsal 3, left calcaneum, left astragalus, and left radiale, three unguals of the manus, and two unguals of the pes.

Referred material. Below are all identifiable Alligator skeletal remains from Haile 7C and Haile 7G, not including probable Alligator fossils represented by nondiagnostic elements (listed in Appendix 2). Lists of preserved material are available as Supplementary Data.

Haile 7C. Individuals with associated cranial and postcranial remains are: UF 124232, UF 162502, UF 162517, UF 162532, and UF 162533. Cranial only remains are: UF 162503 and UF 162521.

Haile 7G. Individuals with associated cranial and postcranial remains are: UF 224688, UF 236900, UF 243325, UF 245621, UF 246419, UF 255281, UF 257913, UF 294858, UF 294860, UF 294862, UF 294863, UF 294864, UF 294865, UF 294866, UF 294868, UF 299884, UF 310102, UF 310103, UF 310104, UF 310105, UF 310106, UF 310111, UF 310150, UF, UF 310224, UF 310225, UF 310226, UF 310227, and UF 310228. Cranial only remains are: UF 247113, UF 257912, UF 310506, UF 310639, UF 310640, UF 310641, UF 310662, and UF 310668.

Occurrence. UF Localities Haile 7C and Haile 7G, western Alachua County, north-central Florida ( Hulbert et al. 2006), Latest Blancan, ca. 2 Ma ( Morgan and Hulbert, 1995).

Diagnosis. Alligator distinguishable from A. mefferdi in having a palatine/pterygoid suture situated anterior to the posterior edge of the suborbital fenestra and a linear frontoparietal suture. The splenial forms the medial wall of more than 40 per cent of the length of the mandibular tooth row (more than seven posterior alveoli), which distinguishes it from A. mississippiensis ( Wu et al., 1996) . It differs from A. thomsoni in possessing an anterodorsally projecting naris and a lingual foramen for the articular artery located on the surangular/articular suture ( Brochu, 1999). Autapomorphic for the species is the presence, but incomplete closure of, the anterior foramen intermandibularis oralis on the splenial.

Etymology. Named for the fossil sites Haile 7C and Haile 7G.

Remarks. Alligator hailensis was a large species comparable in size to the modern A. mississippiensis and highly derived within Alligator but basal to the latter species.

Description. In overall form, Alligator hailensis exhibits morphology consistent with other species of derived Alligator . Large at maturity, the skull of UF 224688 measures 52.5 cm from the posterior margin of the parietal to the anterior portion of the premaxillae (midline length) and 15 cm across the maxillae at the fourth maxillary alveoli. Another large though incomplete specimen (UF 162517) measures 20 cm across the widest extent of the maxillary.

Naris/Premaxilla. The external narial region is best observed on UF 224688 and UF 310226. The naris is ovate in shape, bisected completely by the thin anterior projection of the paired nasals, and projects anterorodorsally. Anteriorly it opens flush with the premaxillae. The naris exhibits a lateral step of the premaxillae (the “deep notch” of Brochu, 2011) that is similar to A. mississippiensis but less deep than that of the Moss Acres Racetrack Alligator (A. cf. A. mefferdi, Snyder, 2007 ). The dorsal posterior processes of the premaxillae are shorter in UF 224688 than that observed in A. mississippiensis or A. cf. A. mefferdi , extending only to the first maxillary alveolus ( Figure 2 ). The premaxillary/maxillary suture possesses the ventral occlusal notch for the fourth dentary tooth. Five dental alveoli are found in the premaxillae of two individuals in which a count was possible (UF 310226 and UF 257912).

Maxilla. –Consistent with other Alligator ( Brochu, 1999) , the maxillary alveoli are round in cross-section, with conical anterior teeth and slightly laterally compressed posterior teeth with bulbous crowns. All teeth lack significant carinae. UF 257912 contains sixteen maxillary alveoli. The largest maxillary alveolus is the fourth and all maxillary teeth occlude labially with those of the dentary. The maxillary toothrow bows laterally around the fourth alveolus resulting in a rostral flare.

Though preservational compression obscures some features of the rostrum, UF 224688 and UF 310226 exhibit smooth rostral curvature and modest, but present paired dorsal sulci (canthi rostrali). The preorbital region slopes smoothly toward the rostrum enclosing its posterior margin completely (i.e. lacks antorbital fenestrae).

Posterior rostrum. The prefrontals are bordered on their medial margins posteriorly by a long anterior projection of the frontal and anteriorly by the nasals and extend farther anteriorly than the lacrimals, which is intact and visible in UF 294863 ( Figure 3 ). When viewed through the orbit, the dorsal portion of the prefrontal pillar flares into a wide anteroposteriorly projecting process making up the anterodorsal orbital surface.

The orbits are dorsally rounded, ventrally linear, and terminate in a lenticular anterior margin. As in other Alligator ( Brochu, 1999) the dorsal rim of the orbit is raised in high relief relative to the skull table forming a narrow trough composed primarily of the frontal.

Most of the ventral margins of both the orbit and infratemporal fenestra are formed by the jugal, and the quadratojugal forms the posterior portion of the infratemporal fenestra. The quadratojugal lacks an anterior projection (spine) and tapers to near termination dorsally toward the skull table. The quadrate contacts the postorbital dorsally and the quadratojugal anteriorly and forms the ventral floor of the external auditory meatus. A thin postorbital bar separates the posterior margin of the orbit from the infratemporal fenestra.

Skull table. The frontoparietal suture is linear as viewed on the dorsum of the skull table, and the parietal is hourglass-shaped, forming the medial margins of the supratemporal fenestrae, and extends to the posteriormost margin of the skull table ( Figures 2 , 3 ). Laterally the postorbital meets the squamosal via a suture that progresses ventral to the skull table anteriorly. The squamosals are heavily ossified in the mature UF 224688. The lateral bones of the skull table slope gradually into (and do not overhang) the supratemporal fenestrae.

Palate. Ventrally, the palate is formed from the complete ventral closure of the premaxillae, maxillae, and palatines from the naris to the suborbital fenestrae and is observable in two mature individuals (UF 224688 and UF 310226) and one juvenile (UF 257913), though in all individuals lateral margins are obscured by their associated mandibles. The anterior palatine process is wide, pointed medially and extends anteriorly to the tenth maxillary alveolus in UF 224688. The ventral surface of UF 224688 is crushed but shows a palatine/pterygoid suture situated before the posterior margin of the suborbital fenestra ( Figure 4 ), which differs from A. mefferdi .

Laterally and posteriorly the ectopterygoid is separated from the maxillary toothrow and tapers to a point at its anteriormost margin. Suborbital fenestrae are obscured or obliterated in all specimens, though UF 224688 shows that they are formed laterally by the maxillae and medially by the palatines. The posterior margin of the suborbital fenestra is formed by the contact between the pterygoid and ectopterygoid (seen in UF 124232 and UF 224688). The anterior extent of the suborbital fenestra is the eleventh maxillary alveolus (in UF 224688, Figure 4 ).

Though crushed, the lateral edges of the posterior pterygoids in several individuals are interpreted to flare ventrally producing a shelf, and extend farther ventrally than the ectopterygoid. The internal choana is septate (with an outward-projecting septum) and is enclosed entirely by the pterygoids.

Occipital region. The posterior of the skull and braincase of Alligator hailensis is observable in UF 224688 ( Figure 5 ) and UF 310226 (both from Haile 7G), and in the obliquely compressed UF 162533 (from Haile 7C). When viewed posteriorly the dorsal surface is nearly horizontal and made up of the squamosals laterally and by the parietal medially. Overlain by the parietal is the supraoccipital, which possesses paired posterolateral projections on its dorsal surface. The triangular-shaped supraoccipital is bordered laterally and ventrally by the exoccipitals. The exoccipitals form the dorsal and lateral margins of the foramen magnum. The basioccipital forms the ventral surface of the foramen magnum (as the occipital condyle) and continues ventrally as a laterally wide, thin plate. The basisphenoid should exist as a thin flange inferior to the basioccipital ( Brochu, 2011) but was not observed in any specimen of A. hailensis .

Mandible. The mandible is best preserved and viewed in UF 224688 (Haile 7G) and UF 162533 (Haile 7C). The left mandible of UF 162533 contains 21 alveoli. The fourth dentary alveolus is larger than, and separated from (not confluent with), the third alveolus and the anterior teeth exhibit an anterodorsal projection. The dentary symphysis extends to the fourth alveolus. The curvature of the dentary between the fourth and tenth alveoli appears to be variable in A. hailensis , with most specimens displaying slight curvature whereas the holotype is nearly linear across this range. The largest dentary tooth and alveolus is the fourth (as in other Alligator, Brochu, 1999 ) while the next largest is the 14 th, observable in UF 162533.

Lingually and anteriorly, the splenial is excluded from the mandibular symphysis, but as seen in UF 162533, it occupies greater dorsoventral space than that seen in A. mississippiensis , even at its anterior-most terminus. In addition, UF 162533 exhibited a new character state altogether, which is assumed to be diagnostic for the species: the anterior foramen intermandibularis oralis (FIO) is present but open anteriorly, which differs from both the A. mefferdi state (in which the FIO is present and fully enclosed by the splenial) and the A. mississippiensis state (the anterior FIO is absent) and subsequently required a revision to the matrix description of Brochu, 2011 ( Figure 6 shows all three character states). This character was only observed in one individual presumably because taphonomic bias tends to obliterate the narrow anterior portions of splenials in derived Alligator (where the anterior FIO is located). However, the presence of a dorsoventrally expanded anterior portion of the splenial (which contains the anterior FIO) can be inferred as a scar on the dentary ventral to the 5 th alveolus (as counted from anterior to posterior), and is observable in UF 162532, UF 162506, UF 162517, UF 224688, UF 294867, and UF 310226.

Posteriorly, the splenial forms the alveolar wall for greater than or equal to 40 per cent of the mandibular toothrow (more than seven posterior alveoli, Figure 7 , the basal A. mefferdi condition from Wu et al., 1996). The splenial contains a singular posterior perforation for cranial nerve V (posterior foramen intermandibularis oralis) observable in UF 224688 and UF 162533. The coronoid was not preserved intact in any specimens observed.

Labially and posteriorly, the angular and surangular suture meet posterior and dorsal to the external mandibular fenestra. The anterior extent of both the surangular and angular are approximately equal in mature individuals, though UF 294867 exhibits a longer anterior process of the angular. The external mandibular fenestra is large and variable relative to overall size (see Table 1) and is enclosed anteriorly by the dentary, ventrally by the angular, and dorsally by the surangular. Dorsally the surangular abuts the posterior dentary toothrow. The articular/surangular suture is simple and the articular joins the angular and surangular at its ventral and anteriormost extent.

The lingual foramen for the articular artery lies on the articular/surangular suture. Both right and left articulars of a large individual are best observed on UF 236900 but neither preserve the articular foramen aerum, which suggests its presence at the lingual margin. The retroarticular process is large, projects posterodorsally, and its posteriormost margin is formed by the angular and articular only.

Axial and epaxial skeleton. Postcranial measurements of associated individuals are found with cranial data in Table 1.

Axial and epaxial ( Figures 8 , 9 ) elements of A. hailensis appear to be largely similar in morphology to other Alligator , though the amount of complete and nearly complete postcranial material across ontogenetic stages is greater than in any other known fossil Alligator . The presacral centra are strongly procoelous and the vertebrae possess tall neural spines ( Figure 8 ).

Alligator hailensis lacks ventral armor (as in other Alligator, Brochu, 1999 ). The dorsal osteoderms are keeled ( Figure 9 ), with midline osteoderms being square or nearly square in shape and with smooth margins. Articulation and pattern of osteoderms is best preserved in the articulated remains of UF 310226, a mature individual that shows eight rows of non-overlapping dorsal osteoderms, though preservation obscures precise determination of number and orientation.

Appendicular skeleton. Preserved appendicular elements of the holotype ( UF 224688) are shown in Figure 10 . The scapula, coracoid and humerus are observable in articulation in UF 310226. The deltoid crest of the scapula is wide and with a broad margin. The deltopectoral crest emerges sharply from the proximal end of the humerus and is concave. The ulna (preserved in several individuals) is gently curved and robust relative to the radius, with a proximal end approximately twice as wide as its distal end, and possesses a wide and rounded olecranon process. Forelimb elements are nearly equal in length to homologous hindlimb elements ( Table 1) .

The left ilium, both femora, and three metatarsals of the holotype are preserved and shown in Figure 10 . UF 310226 preserves semiarticulated pelvic and hindlimb elements and they are indistinguishable to this author from other late Cenozoic Alligator .

Ichnofossils. In addition to the skeletal remains, several coprolites attributed to Alligator have been recovered from both Haile 7C and Haile 7G and are assumed here to have been deposited by A. hailensis . Two of these from Haile 7C (UF 162529 and UF 162530) contain inclusions identifiable as fish (Osteicthyes). Some of the better preserved excreta are shown in Figure 11 .

Figures

FIGURE 2. Holotype of Alligator hailensis (UF 224688) skull shown in dorsal view with interpretive line drawing below. Abbreviations are angular (a), dentary (d), frontal (f), jugal (j), lacrimal (l), maxilla (m), nasal (n), parietal (p), postorbital (po), prefrontal (pf), premaxilla (pm), quadrate (q), quadratojugal (qj), squamosal (sq), and surangular (sa). Scale = 4 cm.

FIGURE 3. Posterior skull fragment of Alligator hailensis in dorsal view (UF 294863). Abbreviations are frontal (f), jugal (j), lacrimal (l), parietal (p), postorbital (po), prefrontal (pf), squamosal (sq), and supratemporal fenestra (STF). Scale = 2 cm.

FIGURE 4. Ventral view of the Alligator hailensis holotype (UF 224688) skull during laboratory preparation. Inset enlargement and drawing shows suture between left palatine and pterygoid. Abbreviations are: palatine (pal), pterygoid (pt), and suborbital fenestra (SOF). Scale = 2 cm.

FIGURE 5. Occipital region of Alligator hailensis holotype (UF 224688) in posterior view (top) and line drawing (bottom). Abbreviations are: basioccipital (bo), exoccipital (eo), foramen magnum (FM), parietal (p), posttemporal fenestra (PTF), squamosal (sq), and supraoccipital (so). Scale = 2 cm.

FIGURE 6. Comparison of anterior portion of splenial in lingual view. (A.) Alligator mississippiensis (ETMNH-Z 265) showing absence of anterior foramen intermandibularis oralis (FIO, splenial outlined in red), (B.) A. sinensis (ETMNH-Z 6953) with anterior FIO present (splenial outlined in red), and (C.) A. hailensis (UF 162533) showing autapomorphic incompletely closed anterior FIO (preserved portion of splenial outlined in white). Scale in each = 2 cm.

FIGURE 7. Occlusal view of left Alligator lower jaws showing the role of the splenial in lingual alveolar support. Alligator hailensis (UF 162517), top, shows extensive splenial support of the posterior toothrow, more than seven posterior alveoli (splenial outlined in white). The dentary alone provides most of the lingual toothrow in the derived condition seen in A. mississippiensis (ETMNH-Z 265), five posterior alveoli or less, bottom. Abbreviations are: dentary (d) and splenial (spl). Numbers refer to alveoli (counted from posterior to anterior). Scale = 2 cm.

FIGURE 8. Representative thoracic vertebra of the Alligator hailensis holotype (UF 224688) in (clockwise from top left) anterior, posterior, right lateral, and left lateral views. Scale = 2 cm.

FIGURE 9. Dorsal osteoderms of the Alligator hailensis holotype (UF 224688). Scale = 2 cm.

FIGURE 10. Appendicular elements of the Alligator hailensis holotype (UF 224688). (A.) Proximal portion of right humerus in posterior view. (B.) Anterior portion of left ilium in lateral view. (C.) Right and left femora in posterodorsal view. (D.) Left tibia in posterior view. (E.) Left metatarsal I and III, and right metatarsal I, all in dorsal view. (F.) Left calcaneum and astragalus, lateral view. (G.) Unguals of the manus, lateral view. Scale in each = 2 cm.

FIGURE 11. Coprolites from Haile 7C (right, UF 162527, and top left, UF 162528) and Haile 7G (left middle, UF 310185, and left bottom, UF 310180) attributed to Alligator hailensis. Scale = 2 cm.