Pseudoglyptodon

McKENNA, MALCOLM C., WYSS, ANDRÉ R. & FLYNN, JOHN J., 2006, Paleogene Pseudoglyptodont Xenarthrans from Central Chile and Argentine Patagonia, American Museum Novitates 3536 (1), pp. 1-20 : 13-15

PSEUDOGLYPTODON

PHYLOGENETICS

Is Pseudoglyptodon a sloth? This of course depends on one’s definition of ‘‘sloth’’. Pseudoglyptodon clearly falls phylogenetically outside the minimally inclusive clade of which Bradypus and Choloepus are a part, i.e., it is the nearest outgroup to what has traditionally been termed sloths. Is it preferable to amend the definition of ‘‘sloth’’ such that it is applicable to Pseudoglyptodon as well, or should a different name be defined for the minimally inclusive clade of xenarthrans of which Pseudoglyptodon is a member? As mentioned earlier, the recognition of two new species of Pseudoglyptodon herein presents an excellent opportunity to rectify a longstanding nomenclatural problem, the existence of two names (Tardigrada and Phyllophaga) that have been employed nearly interchangeably in xenarthran systematics in reference to the same group. We have opted to attach the former to the crown clade, using the name Phyllophaga to refer to all xenarthrans more closely related to sloths (Tardigrada) than to anteaters or armadillos. Following this usage, we are confident that Pseudoglyptodon is a member of the Phyllophaga, but it is almost certainly not a tardigrade.

A number of features argue for the outgroup placement of Pseudoglyptodon relative to Tardigrada. In common with tardigrades, Pseudoglyptodon apomorphically possesses a short, deep skull and robust mandibles. Anteriorly the mandibles of Pseudoglyptodon bear a spoutlike structure and a large foramen like that of tardigrades. There is no posterior opening of the mandibular canal in Pseudoglyptodon , contrasting with the condition in tardigrades. There are likely only four upper and four lower teeth on each side in Pseudoglyptodon , of which the most anterior are caniniform. This represents a reduction of the dental formula from the five upper teeth/ four lower teeth considered ancestral for tardigrades ( Gaudin, 2004). The enlarged caniniform teeth of Pseudoglyptodon occlude in a manner approaching the condition seen in some tardigrades; in sloths having caniniform teeth, the upper one occludes anterior to the lower one (the reverse of the situation seen in most mammals). By contrast, in Pseudoglyptodon the caniniforms occlude nearly side by side. The cheek teeth of Pseudoglyptodon are hypselodont and trilobed, reminiscent of glyptodont teeth in these respects, but lacking the central dentine figure of glyptodontids. In our estimation, the closest approach to the dental pattern of Pseudoglyptodon among tardigrades is seen in various mylodonts, a group generally recognized as having diverged early in the history of sloths. Among these, orophodontids may be singled out. The bilobed cheek teeth of Octodontobradys puruensis from the Mio- Pliocene of western Brazil (Santos et al., 1993), may hint that this pattern was primitive for sloths. This had been suggested earlier by incomplete orophodont dentitions containing several sequential bilobed teeth. Thus, the cheek-tooth outline exhibited by Pseudoglyptodon may represent: (1) a still more primitive condition (wherein sloths may have been characterized ancestrally by trilobed cheek teeth behind simpler caniniforms); (2) an autapomorphic modification to three lobes from the bilobed condition seen in some early, Octodontobradys -like orophodonts; or (3) a completely independent derivation from an unlobed ancestral condition.

Beyond the unusual outline of the cheek teeth in Pseudoglyptodon , the departure of this taxon’s dental formula (4/4) from the pattern typical of tardigrades (5/4) should also be emphasized. Pseudoglyptodon retains fully functional caniniforms, meaning that the reduction of the upper dental count was likely achieved through the loss of the first or the last molariform. Thus, it seems inescapable that this and potentially other aspects of Pseudoglyptodon ’s dental anatomy do not typify phyllophagans ancestrally (nor any other group of xenarthrans to which this taxon is potentially related). Given the early age of Pseudoglyptodon , its high degree of aberrant dental specialization is unexpected.

In a superb recent assessment of tardigrade relationships, Gaudin (2004) identified 22 nonauditory cranial features as unambiguously diagnostic of the group. Of these, currently available specimens of Pseudoglyptodon permit scoring of only the following (using Gaudin’s character/character state numbering scheme). Gaudin’s analysis did not include glyptodonts, however, so we caution that several of these features are not unique among xenarthrans.

37(3): Depth of mandible.22.5%, # 25% of maximum mandibular length. This number is difficult to estimate in Pseudoglyptodon due to breakage, but the maximum mandibular length is, 10 cm. The mandible of P. chilensis is 2.5 cm deep at a minimum, and probably approached 3 cm in life. Thus, mandibular depth in Pseudoglyptodon meets or exceeds the primitive sloth condition, potentially matching the most extreme deepening seen among tardigrades, e.g., Acratocnus , Megatherium , and Octomylodon ( Gaudin, 2004) . Glyptodont mandibles are also deep, which is interpreted here as homoplasy.

61(1): Fused mandibular symphysis. Fusion of the symphysis in Pseudoglyptodon argues that this feature is diagnostic of Phyllophaga. A fused symphysis occurs also in glyptodonts, presumably homoplastically.

74(1): Posterior external opening of mandibular canal. Absent in Pseudoglyptodon .

85(3): Length of snout (preorbital length measured to tip of nasal),25%, $ 15% of basonasal length. In Pseudoglyptodon the basonasal length is, 12 cm. The preorbital length is less securely known given breakage of the anterior rostral region in SGO PV 2995; we estimate it to be between 3 and 4 cm. Although there is considerable variability in the length of the snout in tardigrades, the condition in Pseudoglyptodon clearly more closely resembles the rostral form typically seen in that group than it does the form in cingulates (except glyptodonts) or myrmecophagids.

142(2): Lacrimal foramen large, diameter $ 2.5%,,3% of basonasal length (BNL). The diameter of the lacrimal foramen in Pseudoglyptodon is, 3 mm, i.e., roughly 2.5% of BNL.

145(2): Jugal with large ascending and descending processes. Although the ascending process is not preserved in Pseudoglyptodon , this taxon appears to have been marked by a strong descending process.

In addition, there are a number of derived features seen in Pseudoglyptodon that are optimized as ambiguously synapomorphic for Tardigrada by Gaudin (2004, his node 6).

2(2): Dental formula: 5 upper teeth, 4 lower teeth. Pseudoglyptodon should actually be scored with Gaudin’s character state 3 (i.e., 4 uppers, 4 lowers), a condition occurring elsewhere among xenarthrans only in Mylodon . Nevertheless, the dental formula seen in Pseudoglyptodon (4 uppers, 4 lowers) closely approaches that typical of tardigrades ancestrally.

7(1): Hypsodont cheek teeth. Pseudoglyptodon is clearly high crowned, and moreover is hypselodont. P. chilensis shows that early members of Pseudoglyptodon are substantially less hypsodont than the slightly younger P. sallaensis . Glyptodonts also have hypso- and hypselodont cheek teeth, presumably independently derived.

9(2): Modified orthodentine core of teeth, large, typically well vascularized. Although the degree of vascularization has not been assessed in Pseudoglyptodon , the teeth are nonetheless quite large for a xenarthran.

11(2): Outer layer of cementum forms thick layer around outside of teeth. There is no obvious evidence of cementum on the teeth in Pseudoglyptodon .

18(1): Upper tooth row extends anterior to lower. This condition is just barely met in Pseudoglyptodon , judging from the less distorted right side of SGO PV 2995.

20(1): Wear surface on C1/c1 oblique.

36(6): Trilobate m3. Among xenarthrans, Pseudoglyptodon is remotely comparable only to glyptodonts in this respect.

100(0): Length and width of nasals. Tardigrades are typified (ambiguously) by short wide nasals, the ratio of maximum length to width measured at midpoint,3. In Pseudoglyptodon this ratio is.6, probably approaching 12, being difficult to estimate due to anterior and posterior damage to the nasals. Slightly less elongated nasals (length:width ratio.4) uniquely (and apomorphically) typifies Scelidotheriinae among tardigrades. Pseudoglyptodon is quite unusual in this respect, probably reflecting the condition marking pilosans primitively. Glyptodonts are also characterized by short wide nasals.

153(1): Descending process of jugal present and hooking posteriorly. The bone fragment floating in isolation just lateral to the base of the coronoid process in SGO PV 2995 (see description) indicates that a descending process with this orientation was likely present in Pseudoglyptodon .