Albanerpeton gracilis, Gardner, 2000

Gardner, JD, 2000, Albanerpetontid Amphibians from the Upper Cretaceous (Campanian and Maastrichtian) of North America, Geodiversitas 22 (3), pp. 349-388 : 375-381

Albanerpeton gracilis n. sp. ( Figs 7 ; 8 A-E; Table 4)

“undescribed middle Campanian species” Gardner 1999a: 533

“ Albanerpeton species B ” Gardner 1999d: 63 HOLOTYPE. — RTMP 95.181 .70, nearly complete left premaxilla missing palatine process and lateral end of pars dentalis, and having three broken and three intact teeth ( Fig. 7A, B ).

HOLOTYPE HORIZON AND LOCALITY. — Upper Cretaceous (middle Campanian; i.e. Judithian in age) Dinosaur Park Formation; RTMP L0410 , Dinosaur Provincial Park , Alberta.

ETYMOLOGY. — Specific name refers to the gracile build of the premaxilla.

REFERRED SPECIMENS. — Oldman Formation. Seven localities, Alberta: RTMP L0406: RTMP 95.177.82, 95.177.83, premaxillae; RTMP 95.177.80, maxilla; RTMP L0411: RTMP 96.1.57, frontals; RTMP L0413: RTMP 95.180.64, premaxilla; RTMP L1127: RTMP 96.78.187-96.78.197, premaxillae; RTMP 96.78.103, dentary; RTMP L1128: RTMP 96.78.135, frontals and incomplete right prefrontal; RTMP L1137: RTMP 96.78.185, premaxilla; RTMP L1141: RTMP 96.78.126, premaxillae.

Dinosaur Park Formation. Eight localities, Alberta: RTMP L0051 : RTMP 95.145 .67, premaxilla ; RTMP L0054 : RTMP 86.60 .110, frontals ; RTMP L0086 : RTMP 95.182 .22, premaxilla ; RTMP L0410 (holotype locality): RTMP 95.181 .72, 95.181.73, premaxillae ; RTMP 95.181 .69, 95.181.71, maxillae ; RTMP 95.181 .68, dentary ; RTMP 86.194 .8, 95.181.67, frontals ; RTMP L1108 : RTMP 95.157 .73, maxilla ; RTMP L1118 : RTMP 95.174 .59, maxilla ; RTMP L1119 : RTMP 95.163 .50, premaxilla ; RTMP L1120 : RTMP 95.171 .20, premaxilla .

Kaiparowits Formation. OMNH V6, Utah: OMNH 60321-60323, premaxillae; OMNH 60237, 60324, maxillae.

Aguja Formation. OMNH V58/TMM 43057, Texas: OMNH 60242, premaxilla; OMNH 25349, 60325, maxilla.

DISTRIBUTION ( Table 1). — Upper Cretaceous (middle Campanian; i.e. Judithian in age), North American Western Interior : Dinosaur Park and Oldman formations, Alberta ; Kaiparowits Formation, Utah; and Aguja Formation, Texas.

DIAGNOSIS. — Moderate-sized species of Albanerpeton having no recognizable autapomorphies, but differing from all congeners in a unique combination of primitive and derived character states. Differs from A. arthridion and shares with all Late Cretaceous and Tertiary congeners two premaxillary synapomorphies: suprapalatal pit low on pars dorsalis and larger, ranging from about four to 25% of lingual area of pars dorsalis depending on the species. Primitively resembles A. galaktion and A. cifellii and differs from A. nexuosus , A. inexpectatum , and unnamed Paleocene species in having premaxilla more gracile in build, paired, and with pars dorsalis relatively taller and less strongly sutured dorsally with nasal, in having maxilla (unknown for A. cifellii ) with relatively longer premaxillary lateral process, and in having frontals (unknown for A. cifellii ) with internasal process relatively broader and acute in dorsal or ventral outline; convergently resembles Tertiary congeners in having anterior end of tooth row on maxilla approximately in line with leading edge of nasal process. Most closely resembles A. galaktion and A. cifellii in one premaxillary synapomorphy, suprapalatal pit triangular to slit-like in lingual outline, but primitively differs from former species in retaining moderate-sized suprapalatal pit and smaller palatal foramen and from latter species in lacking facet and dorsally expanded lateral internal strut on lingual face of premaxillary pars dorsalis for contact with nasal.

DESCRIPTION

Descriptions below are composites, unless noted otherwise, and rely primarily on specimens from the Dinosaur Park and Oldman formations.

Premaxilla ( Fig. 7 A-I; Table 4)

With a height of just over 2.8 mm, the holotype (RTMP 95.181.70; Fig. 7A, B ) is the largest of the 27 premaxillae at hand. The most nearly complete specimen, RTMP 96.78.91 ( Fig. 7 C- E), lacks only the lateral end of the pars dentalis and the teeth in this region. The premaxilla is gracile in construction. No specimen shows evidence of having been fused medially in life to its opposite. The medial flange is medially narrow and extends up the medial edge of the bone along the dorsal one-half of the pars dentalis onto the lower one-half to two-thirds of the pars dorsalis. The pars dorsalis is relatively tall and narrow ( Table 4). In the holotype, the dorsal end of the pars dorsalis is swollen labiolingually and indented dorsolingually by a shallow concavity for receipt of the anterior end of the nasal. On smaller premaxillae, the dorsal end of the pars dorsalis is unswollen and lacks a concave depression, indicating that the pars dorsalis would have simply abutted against the nasal. The lacrimal notch is deep and markedly narrow, both in absolute and relative terms ( Table 4). The external surface of the bone is perforated by small, scattered external nutritive foramina. On large premaxillae, including the holotype, the dorsal one-third of the pars dorsalis bears a low, indistinct boss that is weakly ornamented with irregular ridges and shallow pits.

In lingual view, the suprapalatal pit lies about one-half to two-thirds of the distance across the pars dorsalis from the medial edge and is located low on the process, with the ventral edge of the pit confluent with the dorsal face of the pars palatinum. The suprapalatal pit is moderate in size ( Table 4) and occupies about four to 10% (n = 4) of the lingual surface area of the pars dorsalis. In lingual outline, the suprapalatal pit is taller than wide and varies from triangular to slit-like (cf. Fig. 7B, D , F-H). RTMP 95.171.20 (not figured) is unusual in having the opening of the suprapalatal pit subdivided into two smaller, ovoid openings. An internal strut is typically absent medial to the suprapalatal pit; where present, this strut is little more than a lingually shallow ridge that extends down the inner face of the pars dorsalis. The lateral edge of the suprapalatal pit is consistently bordered by a more prominent, but relatively narrow internal strut that is perforated laterally by one or a few tiny foramina. The strut expands lingually as it extends down the inner face of the pars dorsalis. The base of the strut rarely extends more than about one-third of the distance lingually across the dorsal face of the pars palatinum.

The pars palatinum is lingually broad and bears prominent palatine and maxillary processes ( Fig. 7E ), both of which are indented lingually by a shallow facet for contact with one or more palatal bones. The unnamed dorsal process on the lingual edge of the maxillary process is a low, labiolingually compressed ridge that is not continuous labially with the base of the more lateral internal strut. In eight of the ten premaxillae preserving the maxillary process, the ventral face of the process bears a low, drumlin-shaped knob all but identical to that on other albanerpetontid premaxillae. However, in RTMP 95.181.70 and 95.180.64 ( Fig. 7B, I , respectively), this knob is prominently developed into a ventrally directed, procurved, and bulbous process. Elaboration of this ventral process does not appear to be size-relat- ed, because RTMP 95.180.64 is about two-thirds the size of RTMP 95.181.70. The significance, if any, of this unusual process is unclear. The palatal foramen is relatively small, with a diameter not greater than one-half the diameter of the bases of the medial teeth on the bone. The canal connecting the dorsal and ventral openings of the palatal foramen extends vertically through the pars palatinum. The palatal foramen opens dorsally in the pars palatinum, in or just lingual to the opening of the suprapalatal pit, and ventrally in the pars palatinum, just lingual to the pars dentalis and in line with the second to fourth tooth positions. Up to three smaller, unnamed foramina perforate the bone ventrally at the junction between the pars palatinum and pars dentalis. In RTMP 96.78.196 ( Fig. 7H ), two such foramina are evident, one each above the second and fourth loci. I generally have not been able to determine the paths of these unnamed foramina, but in occasional premaxillae (e.g. OMNH 60242; Fig. 7G ), a canal can be trac- ed from the medialmost foramen vertically through the pars palatinum and opening dorsally in the junction between the pars palatinum and pars dorsalis, well medial to the suprapalatal pit.

Maxilla ( Fig. 7 J-N)

The largest of the nine available specimens, RTMP 95.174.59 (unfigured), is about 4 mm long and would have been slightly longer when the bone was complete. The remaining specimens are from smaller individuals. The two figured specimens, RTMP 95.157.73 ( Fig. 7 J-L) and 95.177.80 ( Fig. 7M, N ), overlap in the region of the anterior one-half of the tooth row and adequately document the structure of the element. The labial surface is unornamented, with up to ten external nutritive foramina scattered across the anterior part of the pars facialis. The nasal process is low and triangular in labial outline. The pars facialis is low and becomes shallower posteriorly. At its posterior end the pars facialis labially bears a shallow facet for contact with the jugal. The ventral edge of the pars dentalis is straight to shallowly convex ventrally in labial or lingual outline. The anterior end of the tooth row is approximately in line with the point of maximum indentation along the leading edge of the nasal process.

The premaxillary lateral process is anteriorly elongate, with its length greater than the height at the base, and tapers anteriorly to terminate in a blunt end. The premaxillary dorsal process is lingually broad and ventrally bears a low horizontal ridge for contact with the posterior edge of the maxillary process on the premaxilla. The pars palatinum is lingually broad, tapers towards its posterior end, and dorsolingually has the short trough for contact with one or more unknown palatal bones. The raised bony patch on the dorsal surface of the pars palatinum for contact with the base of the lacrimal is weakly developed. The internal narial margin spans four or five loci.

Dentary ( Fig. 7 O-Q)

Two incomplete dentaries are available: RTMP 96.78.103 ( Fig. 7O ) preserves about the anterior two-thirds of the bone, whereas RTMP 95.181.68 ( Fig. 7P, Q ) lacks about the anterior one-third of the ramus and much of the area for attachment of the postdentary bones. The two specimens overlap for about ten tooth positions anterior to the opening for the Meckelian canal. Both dentaries are from moderate-sized individuals, comparable in size to those represented by upper jaws. Each dentary is relatively gracile in construction. Neither specimen is ornamented labially. A row of external nutritive foramina is present labially, as is the scar ventrally for attachment of the intermandibularis musculature. The dorsal edge of the dental parapet is straight in labial or lingual outline. RTMP 95.181.68 preserves enough of the dorsal edge immediately behind the tooth row to show that no dorsal process was present in this region. RTMP 96.78.103 preserves a moderately prominent symphyseal eminence and two intact symphyseal prongs. The remainder of the lingual structure is unremarkable.

Dentition ( Fig. 7 A-J, L, M, O-Q)

As in other albanerpetontids, marginal teeth are highly pleurodont, non-pedicellate, and tipped with labiolingually compressed, chisel-like, and faintly tricuspid crowns. Minor differences are apparent among jaws in the relative length, build, and spacing of teeth; this variation does not appear to correlate with overall jaw size. On jaws having more robust teeth, the pedicels tend to be slightly expanded mesiodistally midway up the shaft, giving these teeth the appearance of being somewhat swollen or barrel-shaped in lingual view ( Fig. 7D, J, O, P ). Teeth are weakly heterodont in size along the maxillary tooth row, with the longest teeth occupying the third to fifth positions. Judging by intact and broken teeth on the two dentaries, a similarly weakly heterodont pattern occurred on this element. Six to ten loci are present in the 12 premaxillae having a complete tooth row. One maxilla (RTMP 95.181.71; not figured) preserves a complete tooth row of 19 loci; other maxillary specimens appear to have a similarly low tooth count when complete. For example, RTMP 95.174.59 (not figured) and 95.157.73 ( Fig. 7J ) preserve the anteriormost 17 and 14 tooth positions, respectively, and each specimen is probably missing only the posteriormost one or two loci. The dentaries RTMP 95.181.86 and 96.78.103 preserve, respectively, the posterior 19 and anterior 18 tooth positions; I estimate that each held about 27 loci when the bone was complete. Most jaws preserve one or more empty tooth slots for replacement teeth. More advanced stages of tooth replacement are seen in a maxilla (RTMP 95.177.80; Fig. 7M ) with an in situ replacement crown in the tooth slot at the second locus and in a dentary (RTMP 95.181.68; Fig. 7P ) with replacement teeth at the fourth-sixth and fifteenth loci from the broken anterior end of the tooth row.

Frontals ( Fig. 8 A-E)

Two of the five pairs of fused frontals are nearly complete. RTMP 86.194.8 ( Fig. 8A, B ), the smaller of the two pairs, lacks the distal ends of the anterolateral processes and the distalmost end and left edge of the internasal process. RTMP 96.78.135 ( Fig. 8C ) consists of a more nearly complete pair of frontals, about 3.7 mm in midline length, that lacks the posterior end of the ventrolateral crest and adjacent part of the orbital margin on the right side, but has the medial end of the right prefrontal articulated in the more posterior slot on the right side (left in figure). The third figured specimen, RTMP 95.181.67 ( Fig. 8D, E ), is broken transversely between the slots for receipt of the prefrontal and lacks the anterior part of the bone. Although the largest frontal specimens at hand suggest a midline length of about 4 mm, several jaws evidently are from slightly larger individuals. Frontals are solidly fused medially. In dorsal or ventral view, the fused frontals are triangular in outline and slightly longer than wide. Based on RTMP 86.194.8 and 96.78.135, I estimate the ratio of midline length to width across the posterior edge between the lateral edges of the ventrolateral crests at between 1.1 and 1.2. The anterior processes and slots for receipt of the nasal and prefrontal are well developed. The internasal process is acute in dorsal view, with the length subequal to the width across the base, and laterally bears the groove for contact with the nasal. The dorsal and ventral edges of the slot for receipt of the prefrontal are shallowly excavated medially in most specimens. RTMP 86.194.8 is unusual in having the slot completely open dorsally ( Fig. 8A ). This condition is not an artifact of preservation, as the dorsal margin of the slot on both sides of the specimen is smooth and exhibits no evidence of breakage. Posterior to the anterolateral process, the lateral edge of the bone diverges at about 25° from the midline and the orbital margin is shallowly concave medially in dorsal or ventral view. In RTMP 86.194.8, the posterior edge of the frontal roof is shallowly concave to either side of the midline and clearly abutted in life against the paired parietals. In larger frontals, such as RTMP 96.78.135 and 95.181.67, the posterior edge is more nearly transverse in dorsal view and was sutured with the parietals.

Frontals are consistently ornamented dorsally with the usual albanerpetontid pattern of broad, polygonal pits bordered by narrow ridges. Pits vary in relative depth, being shallow on some specimens and deeper on others (cf. Fig. 8A versus D). The ventrolateral crest is relatively thick dorsoventrally. With increased frontal size, the crest becomes absolutely and relatively wider, with width of crest behind slot for receipt of prefrontal increasing from 0.40-0.65 mm (n = 4) and ratio of width of crest to width across posterior edge of bone between medial edge of crests increasing from 0.25-0.40 (n = 3). The transverse profile of the crest also changes with growth. On the two smallest frontals, RTMP 86.194.8 ( Fig. 8B ) and 96.1.57 (not figured), the crest resembles that on frontals of Albanerpeton arthridion (Gardner 1999a) in being convex ventrally in transverse view. On RTMP 96.78.135 and 95.181.67 ( Fig. 8C, D , respectively), the ventral face of the crest is slightly bevelled, with the flattened surface facing ventrolaterally. The crest does not, however, approach the triangular transverse profile seen in frontals of A. inexpectatum , A. nexuosus , and A. galaktion .

REMARKS

I recognize Albanerpeton gracilis as a new albanerpetontid species on the strength of distinctive jaws and frontals that are associated by structure, size, and provenance. Although I have not been able to identify any autapomorphies for A. gracilis , the species differs from its congeners, including the sympatric species A. nexuosus and A. galaktion , in the unique combination of primitive and derived character states given in the diagnosis above. The holotype premaxilla exhibits two unusual features: 1) distal end of pars dorsalis swollen and concave dorsolingually for contact with nasal; and 2) unnamed ventral process on maxillary process is a bulbous, procurved projection. As both features vary among specimens, neither should be used to diagnose the species until this variation is better understood. Maxillae of A. gracilis differ from those of A. nexuosus and A. galaktion in having fewer teeth (20 versus 25) and a less prominent bony patch, for contact with the lacrimal, on the dorsal surface of the pars palatinum. It is unclear to me whether these maxillary features are taxonomically significant or simply associated with the smaller body size of A. gracilis .

Assignment of the new species to Albanerpeton is appropriate because referred frontals possess the diagnostic suite of character states for the genus, whereas the holotype and referred premaxillae exhibit synapomorphies of the suprapalatal pit that are diagnostic for less inclusive clades within the genus. The moderate-sized suprapalatal pit locat- ed low on the pars dorsalis places A. gracilis in the unnamed subgeneric clade containing all congeners, except A. arthridion , whereas the slit- to triangular-shaped suprapalatal pit further nests A. gracilis in the less inclusive gracile-snouted clade. In lacking the respective autapomorphies of A. cifellii and A. galaktion , A. gracilis is the most generalized member of the gracile-snouted clade.

Figures

FIG. 7. — Jaws of Albanerpeton gracilis n. sp., middle Campanian (Judithian in age), North American Western Interior; A, B, RTMP 95.181.70 (holotype), incomplete left premaxilla; A, labial view; B, lingual view; C-E, RTMP 96.78.191, nearly complete right premaxilla; C, labial view; D, lingual view; E, occlusal view; F, OMNH 60321, incomplete right premaxilla, lingual view; G, OMNH 60242, incomplete left premaxilla, lingual view; H, RTMP 96.78.196, incomplete right premaxilla, lingual and slightly occlusal view; I, RTMP 95.180.64, incomplete left premaxilla, lateral view; J-L, RTMP 95.157.73, nearly complete left maxilla; J, lingual view; K, dorsal view; L, occlusal view; M, N, RTMP 95.177.80, incomplete left maxilla; M, lingual view; N, dorsal view; O, RTMP 96.78.103, anterior part of left dentary, lingual view; P, Q, RTMP 95.181.68, medial part of left dentary; P, labial view; Q, lingual view. Provenances: A, B, J-L, P, Q, RTMP specimens, Dinosaur Park; C-E, H, I, M-O, RTMP specimens, Oldman formations, all from Alberta; F, OMNH specimens, Kaiparowits Formation, Utah; G, Aguja Formation, Texas. Scale bar: 1 mm.

FIG. 8. — Frontals of Albanerpeton gracilis n. sp. and atlas of indeterminate albanerpetontid genus and species, Campanian (Judithian and Aquilan in age), Alberta; A-E, A. gracilis n. sp.; A, B, RTMP 86.194.8, fused and nearly complete frontals, A, dorsal view, B, ventral view; C, RTMP 96.78.135, fused and nearly complete frontals with medial end of right prefrontal (arrow) preserved in articulation, ventral view; D, E, RTMP 95.181.67, fused frontals missing anterior part, D, dorsal view, E, ventral view; F-H, genus and species indeterminate; F, G, UALVP 16234, nearly complete atlas missing part of left wall of neural arch, F, anterior view, G, posteri- or view; H, right lateral view, with arrows indicating foramen on right side for exit of first spinal nerve. Provenances: A, B, D, E, A. gracilis n. sp., Dinosaur Park, C, Oldman formations, middle Campanian (i.e. Judithian in age); F-H, genus and species indeterminate, Milk River Formation, lower Campanian (i.e. Aquilan in age). Scale bar: 1 mm.