Alphadon praesagus ( Russell, 1952 ), 1972

Sahni, Ashok, 1972, The vertebrate Fauna of the Judith River formation, Montana, Bulletin of the American Museum of Natural History 147 (6), pp. 319-416 : 378-381

Alphadon praesagus ( Russell, 1952) , new combination

Delphodon ? praesagus RUSSELL, 1952, p. 11 3.

Delphodon ? praesagus : CLEMENS, 1966, p. 109.

HOLOTYPE: NMC 114, a right mandible with M3 and roots of M4.

TYPE LOCALITY: Four miles below the mouth of Berry Creek, southeast of Steveville, Alberta.

KNOWN DISTRIBUTION: Oldman Formation, Alberta; Judith River Formation, Montana.

REVISED DIAGNOSIS: A common Campanian didelphid with close similarities to the slightly smaller A. marshi . The type specimen, NMC 114, from the Oldman Formation, is a partial right lower mandible with M3, roots of M4, and an alveolus for the posterior root of M2. The stylar shelf of M3 is broad, and the stylar cusp is better developed than in most contemporary marsupials. It is separated from stylar cusp B by a short valley. Stylar cusp B is the largest stylar cusp, with a low ridge joining the only slightly higher paracone, another ridge extends to stylar cusp C. This latter elongate cusp is variously oriented, it may be inclined to the anteroposterior direction (Ml) or less inclined (M2) or situated on the labial margin (M3 and M4). Cusp C is smaller than stylar cusp B but larger than stylar cusp D. A high ridge links the metastyle to the metacone. The metacone is as tall or slightly taller than the paracone. The protocone is a low cusp. The lower teeth have a crista obliqua that intersects the protoconid-metaconid wall just ventral to the notch separating the posterior trigonid cusps.

DESCRIPTION: M1; specimen AMNH 77344, an M1, is assigned to Alphadonpraesagus (fig. 13C). It is less transverse than the other upper molars. The tooth is only slightly wider than it is long and the stylar shelf is wide. Stylar cusp A is well developed but smaller than the other stylar cusps. A narrow, short valley separates it from stylar cusp B, and it is connected to the paraconule by a crest. Stylar cusp B is a large cusp with at least three distinct ridges leading from its apex. One ridge extends anteriorly, another low ridge joins the paracone, and a third trends posteriorly toward stylar cusp C. Stylar cusp C is situated centrally on the incipiently bilobate stylar shelf. Its elongate orientation is not anteroposterior, but at an angle to this direction, leaving a small, flat shelf between itself and stylar cusp B. Stylar cusp D is smaller than stylar cusp C.

The paracone and metacone are well separated by a broad valley, as in A. marshi , and the metacone is noticeably taller than the paracone. No anterolingual cingulum is present. Both conules are well developed. Although the roots are broken, it is possible to make out an interradicular crest that joins the lingual side of the labial roots together.

M2; an M2, AMNH 77343 (fig. 13B), is relatively more transverse than M1 but is relatively less transverse than M3. The stylar shelf of M2 is asymmetrically bilobate. Stylar cusp A is situated at the anterolabial corner of the crown on an expanded shelf area, and is separated from stylar cusp B by a valley similar to that found in M'. Stylar cusp B is well developed, and joined by a ridge to the paracone, and the cusp is farther separated from the paracone than the corresponding cusp in M1. An indistinct ridge arises from the apex of stylar cusp B and joins the smaller stylar cusp C, situated at the stylar cleft. Stylar cusp D, although slightly worn, still appears to have been smaller than stylar cusp C. The paracone and metacone are separated by a broad valley and there does not seem to be much difference in the relative heights of these cusps. The protocone is a low cusp. The lingual portion of the crown is broken, but there appears to be a small anterolingual cingulum. An interradicular crest is present as in M1.

M3 and M4; a maxilla fragment containing M3 and M4, AMNH 77342 (fig. 13A), has been recovered. The stylar shelf on its M3 is worn, but stylar cusp B seems to have been well developed as indicated by the labial expansion of the stylar shelf in that area. Stylar cusp A is much smaller than cusp B and is separated from it by a short valley. Stylar cusp C is smaller than cusp B, and is situated on the labial margin of the stylar shelf at the stylar cleft. Stylar cusp D is present and is smaller than cusp C. A high, worn ridge links the metastyle to the metacone, and the paracone is broken and missing. Both conules are present, and the premetaconular wing terminates at the rounded anterolingual side of the metacone. The protocone is relatively low. A short but distinct anterior cingulum is present, and extends from about the base of the paraconule to the anteroventral side of the protocone.

M4; few M4s have been found in association with other teeth of Alphadon and consequently their assignment to the genus is tentative. Two M4s of A. rhaister were described by Clemens (1966), and a complete maxilla with M1-M4 of A. marshi from the upper Edmonton Formation of Alberta was described by Lillegraven.

M4 is smaller than M3 and is characterized by an anterolabial expansion of its stylar shelf. Stylar cusp A is present and a well-developed cingulum connects this cusp to the paraconule. Stylar cusp B is the best-developed stylar cusp. It is connected by a high ridge to the conical paracone, which is much larger and taller than the metacone. Stylar cusp C is present on the labial side of the crown and is smaller than cusp B. Posterior to cusp C, the stylar shelf rapidly narrows and forms the posterolabial part of the metacone. Both the paraconule and the metaconule are present, and the protocone is a low cusp. An anterior cingulum, similar to that found on M3, occurs on M4.

Russell (1952) described an incomplete lower jaw with M3 collected below the mouth of Berry Creek, southeast of Steveville, in the Oldman Formation, as Delphodon ? praesagus . Clemens (1966, p. 109) suggested that this molar per- tained to the genus Alphadon but, in the absence of upper molars, the assignment could not be certain. A large number of isolated teeth and two jaw fragments with teeth, from Clambank Hollow, confirm Clemens's suggestion. This is the most common marsupial from the Judith River Formation. Dimensions of the complete lower molars are given in table 1 . Fragmentary molars are useful only in establishing the relative frequency of occurrence of Alphadon praesagus . Comparisons of lengths of the upper and lower molars of A. praesagus agree with the general principle proposed by Clemens (1966, p. 83) that in didelphoid marsupials the lengths of the "lower molars tend to be equal to or greater than the lengths of their upper counterparts."

P3; some premolars have been found that most likely belong to A. praesagus . It is not possible to clearly distinguish between isolated P2 and P3 because they are morphologically and dimensionally very similar. Only a few differences can be made out, principally in the degree and development of the lingual and labial cingula adjacent to the posterior accessory cusp.

Specimen AMNH 77355 (fig. 13D, E) is tentatively identified as P3 on the basis of a large anterior "spirelike" cusp (protoconid), the development of cingula that are lingual and labial to the posterior accessory cusp, and on the basis of size. The mean length of P3 in Alphadon marshi is 1.75 mm. ( Clemens, 1966), whereas the length of AMNH 77355 is much greater (2.4 mm.). This difference is in keeping with size of molars of A. praesagus , which are larger than those of A. marshi .

The primary cusp on P3 is a large, transversely compressed cusp with steep anterior and more gentle posterior margins. A short anterolingual cingulum is present at the base of the cusp. The posterior accessory cusp is much smaller but well separated from the larger anterior cusp by the presence of a short valley. Two ridges, one originating from the apex of the large primary cusp, and the other from the apex of the posterior accessory cusp, terminate in this valley. Two other ridges arising from the posterior accessory cusp extend lingually and labially. The lingual ridge is higher and longer than the labial one. A basal cingulum is present on the posterolabial corner of the crown. The tooth is two rooted. DP3; a left DP3, AMNH 77398. is similar to the deciduous premolar figured by Clemens (1966, p. 108). This tooth is tentatively assigned to Alphadon praesagus on the basis of size (length 2.5 mm.). Its trigonid is anteroposteriorly elongated, and the protoconid is situated anterior to the metaconid. The talonid cusps are distinct and well separated (fig. 13K, L). MI; two teeth have been tentatively identified as M1s. They are smaller but otherwise similar to M2s. M2; there are at least four isolated lower molars similar in size and morphology to M2 in the mandible of Alphadon praesagus (AMNH 77358, illustrated in fig. 13M, N). A comparison with UCMP 50299 and 50300, a right and left mandible, respectively, of A. marshi ( Clemens, 1966, p. 89) reveals that they are significantly smaller but otherwise morphologically very similar to the Campanian mandible, AMNH 77358. The posterior extension of the symphysis in both Lance species, Alphadon lulli and A. marshi , is below P2, and a similar condition is present in the Campanian mandible. A prominent mental foramen occurs ventral to M1 in the Lance mandibles, and also in the Campanian specimen. These features confirm identification of the preserved teeth as P3 and M2. The molar has a well-developed, anteriorly projecting paraconid. The paraconid is the smallest cusp of the trigonid and is well separated from the metaconid. All trigonid cusps are conical with broad bases. An anterobasal cingulum extends ventrolabially from the paraconid. The talonid is slightly wider than the trigonid, and the crista obliqua connects the prominent hypoconid to the posterior wall of the trigonid, ventral to the notch separating the protoconid from the metaconid. The hypoconulid and the entoconid are twinned, and the hypoconulid is posterior to the lingually situated entoconid. A broad cingulum extends posterolabially to the hypoconid. M3; an isolated M3, AMNH 77356, is assigned to Alphadon praesagus (fig. 13H-J). Its trigonid is anteroposteriorly compressed, and the protoconid is the tallest cusp and is situated slightly anterior to the metaconid. The metaconid is only slightly higher than the paraconid. An anterior cingulum is feebly developed. The width of the trigonid (2.0 mm.) of AMNH 77356 is greater than that of the talonid (1.8 mm.). The hypoconid is well developed and separated from the other two talonid cusps. A prominent posterolabial cingulum is present at its base. Entoconid and the hypoconulid are elevated in position and situated close to each other. The entoconid is completely lingual in position.

Specimen AMNH 77356 and an additional M3, AMNH 77366, resemble the type specimen of Alphadon praesagus . Length and width of AMNH 77356 is 3.2 mm. and 2.0 mm., respectively, whereas in the type specimen, NMC 114, the corresponding dimensions are 3.3 mm. and 2.2 mm., respectively.

M4; an isolated molar, AMNH 77361, is probably an M4, as based on its trigonid/talonid width ratio and on its large anteroposteriorly elongated posterior root (fig. 13F, G). The protoconid is the highest trigonid cusp, and the metaconid is equal to, or slightly taller than, the paraconid. An anterobasal cingulum is present. The hypoconid is a prominent cusp with a posterolabial cingulum, and the hypoconulid is twinned with the lingual entoconid. The crista obliqua intersects the protoconid-metaconid ridge at a point posterior to the metaconid.

Tables

TABLE 1 DIMENSIONS (IN MILLIMETERS) OF LOWER MOLARS OF Alphadon praesagus

T Not Nth aoumer aaoumer C		Length g	Trigonid Width	Talonid Width
M1	AMNH 77359	2.5	broken	1.8
M1	AMNH 77365	2.6	1.6	1.5
M2	AMNH 77358	2.8	1.6	1.7
M2	AMNH 77357	2.8	1.8	1.6
M2	AMNH 77363	2.8	1.6	1.6
M2	AMNH 77364	2.8	2.0	1.8
M3	AMNH 77356	3.2	2.0	1.8
M3	AMNH 77366	3.3	1.8	1.6