Alphadon rhaister

Figure 14 L-P

The presence of a large didelphid, only slightly smaller than Boreodon matutinus, is suggested by two teeth in the collection. AMNH 77372 is an upper molar which resembles the teeth described by Clemens (1966, p. 15) as Alphadon cf. A. rhaister obtained from the Lance Formation of Wyoming. Another tooth, AMNH 77371, is also a large lower molar morphologically similar to teeth of Alphadon cf. A. rhaister. These two teeth are provisionally included within Alphadon cf. A. rhaister. Both teeth are morphologically different from corresponding teeth of Boreodon matutinus. The status of Alphadon cf. A. rhaister is subject to question. It is unlikely that the two specimens figured by Clemens (1966, fig. 12) are in fact an '' extreme in intraspecific variation " of A. rhaister. More probably they represent a new metatherian genus. Additional material from Clambank Hollow indicates that two taxa are present. All stylar cusps are present in AMNH 77372. Stylar cusps B, C, D are well developed, and A and E are distinct (fig. 140 - P). Stylar cusp A is small and conical, and has a cingulum leading from it to the paraconule. Stylar cusp B is a large anteroposteriorly elongated cusp, only slightly smaller than the paracone, and connected to the paracone by a high ridge which intersects cusp B anterolingually. Cusp C is smaller than cusp B and is situated slightly lingual to it. Stylar cusp D is larger than cusp C. A ridge extends from the posterior metastyle to the metacone. The stylar shelf is broad and the labial margin relatively straight. The paracone and metacone are separated by a broad valley, and the metacone is the larger of the two cusps. The disposition of the crests arising from the paracone is characteristic. The anterior crest, originating from the metacone, is anterolabially directed toward stylar cusp C and meets the posterior crest originating from the paracone at a high angle. This condition is uncommon in species of Alphadon and Boreodon. Both paraconule and the metaconule are present. The protocone is a low cusp with relatively short ridges leading to the conules. Although its size and greater length / width ratios would indicate that it is a DP 3 of Boreodon, this is unlikely. The tooth, AMNH 77372, is highly molariform, its stylar shelf is straight and very different from the condition seen for Boreodon upper molars. Its cusps are well developed and anteroposteriorly elongate, its metacone is higher than the paracone, and both conules are present. Also, the tooth bears little resemblance to AMNH 77386, considered here to be a DP 3 of Boreodon, and also to DP 3 of Didelphodon vorax (Clemens, 1966, p. 70). The following features set this group of teeth apart: large size, straight stylar shelf, prominent development of all stylar cusps, particularly cusps B and D, the presence of a ridge linking cusp B with the paracone, a metacone slightly higher than the paracone, a stylar shelf narrower than in Boreodon, metacone and paracone ridges directed posteriorly, W-shaped, bulbous teeth usually less transverse than in other marsupials, and finally the presence of a low protocone. Specimen AMNH 77371 from Ankylosaur Point is probably an M 4 of this species (fig. 1 4 L- N). Its trigonid is anteroposteriorly compressed, the paraconid is as high as the metaconid, and the protoconid is broken. A short anterobasal cingulum is present. The talonid consists of a prominent hypoconid and a closely twinned, posterolingually placed, hypoconulid and entoconid. The orientation of the crista obliqua is more lingual than in molars of contemporary marsupials, and connects the hypoconid with the posterior margin of the metaconid. This results in narrowing of the talonid (talonid width 1.9 mm., trigonid width 2.4 mm.). The valley separating the protoconid and hypoconid is wide and shallow. A posterolabial cingulum encircles the hypoconid. The posterior root is larger than the anterior. Characteristics of AMNH 77371, a presumed M 4, and Boreodon molars are compared in table 2. M 4, AMNH 77371, is the expected size for M 4 of Eodelphis browni Matthew (1916), the type and only species of the genus. Eodelphis browni is based on a single jaw, and resembles Alphadon as much as it does Boreodon or Didelphodon, genera to which it is usually assigned.

DIscUSSION: Cretaceous marsupials are differentiated primarily on the degree and development of the stylar cusps of the upper molars, and on the orientation of the crista obliqua on the lower molars. Other features used are the relative heights of the paracone and metacone, the width of the stylar shelf, and the relative development of the conules. The stylar shelf in Alphadon consists of five cusps of which cusp B is the best developed. Cusp B is separated from the parastyle (cusp A) by a valley. It is also connected to the slightly higher paracone by a high ridge. Cusp C is larger than cusp D. Specific differences between the Lance species of Alphadon were made chiefly on the basis of size (Clemens, 1966). The orientation of the crista obliqua of the lower molars has been used for generic differentiation (Clemens, 1966). The three Campanian species of Alphadon, A. praesagus, A. halleyi, and Alphadon cf. A. rhaister, are ancestral to later Maestrichtian forms. A. halleyi is poorly known and similar in size to A. lulli. Alphadon cf. A. rhaister may be synonymous with Eodelphis browni but this cannot be shown conclusively as yet. Among the commonest marsupials in the two faunas are A. praesagus in the Campanian and A. marshi in the Maestrichtian. Apart from a size difference between the two forms, other morphological dissimilarities seem to be present, but these are at a specific, rather than at a higher level. The most obvious of these differences are in the structure of the lower premolars, structure of the upper molar stylar shelf, and the morphology of M 4. Clemens's (1966) and Lillegraven's (1969) detailed discussions of the ancestors and descendants of Alphadon greatly increase knowledge about marsupial origins, and also about their Tertiary history. At the time Clemens published on the Lance marsupials little was known about the earlier Campanian fauna. Clemens rightly pointed out that Boreodon, although primitive in certain respects, is more advanced than Alphadon. Boreodon, however, shares with Alphadon common features, such as the great width of the stylar shelf, and the prominence of stylar cusp B and prominence of the ridge connecting this cusp to the paracone. Holoclemensia texana (Slaughter, 1968 b, 1968 c) from the Albian Trinity Formation of Texas seems appropriate as an ancestor for all the Campanian marsupials.

Alphadon cf. A. rhaister CLEMENS, 1966, p. 34.