Symmetrolestes parvus, Tsubamoto & Rougier & Isaji & Manabe & Forasiepi, 2004

Tsubamoto, Takehisa, Rougier, Guillermo W., Isaji, Shinji, Manabe, Makoto & Forasiepi, Analía M., 2004, New Early Cretaceous spalacotheriid “ symmetrodont ” mammal from Japan, Acta Palaeontologica Polonica 49 (3), pp. 329-346 : 332-336

Symmetrolestes parvus sp. nov. Tsubamoto and Rougier

Figs. 2–6 .

Holotype and only specimen: NSM PV 20562 , a fragmentary right lower jaw with a incisor and five postcanine teeth found in two blocks.

Repository: National Science Museum, Tokyo, Japan.

Type locality: 36°07’41”N, 136°32’44”E; in the valley of the Sugiyama−gawa River , Kitadani−cho, Katsuyama City, Fukui Prefecture, central Japan ( Fig. 1 ) .

Type horizon: Upper part of the Kitadani Formation (= Kitadani alternation of sandstone, shale and tuff in Maeda 1958), Akaiwa Subgroup, Tetori Group; Early Cretaceous (late Hauterivian to Aptian).

Etymology: From Latin parvus , small.

Diagnosis.—As for genus. Description

The jaw preserves the first incisor and five postcanine teeth, which represent the ultimate premolariform and the whole molariform series (m1–m4). In front of these postcanine teeth, there are nine complete alveoli and somewhat ambiguous evidence for three more (Figs. 3, 4). At the very tip of the jaw is the procumbent i1, whose root is exposed in dorsal view by breakage of the alveoli. Its crown is partially covered by bone but can be seen along the ventral edge of the jaw ( Figs. 2 , 4 ).

The posterior six alveoli show distinctive interradicular septa, which typically are higher than the intermolar septa. This implies the presence of three double−rooted teeth in front of the ultimate premolariform. Given the monotonic change in size and shape of these alveoli whose sizes diminish anteriorly, it is very likely that all of them represent premolariform loci. There are three further alveoli mesial to those considered (i.e., seventh–ninth from the most posterior alveolus preserved) that are also complete. The most posterior of these (seventh) is subequal in size and similar in shape to the alveolus of the premolariform behind it. In front of the seventh alveolus there are two smaller and subequal alveoli arranged at a small angle to the alveolar process of the jaw. This change in orientation suggests that there is a change in tooth family. The likely interpretation of these alveoli would suggest that a small double−rooted canine occupied the front two alveoli and that a single−rooted first premolariform followed it. Small double−rooted canines are present among i1

i2–i4

c

p1

p2

p3

p4

p 5 m 1

m2

m3

m4

Fig. 3. S ymmetrolestes parvus ( NSM PV 20562 , holotype). A. Buccal stereo view of the dentition and alveolar margin of the jaw. B. Interpretation of tooth positions and alveoli. Abbreviations follow those in the main text. Scale bar 2 mm .

basal forms of trechnotheres and cladotheres, making this interpretation plausible. Regardless of alternative interpretations of the three mesial alveoli, we are certain of a count of four or more premolariforms. The minimum number of incisors is three and if our suggestions were accepted, there would be at least four lower incisors.

In short, the dental formula can be conservatively postulated as i3+, c1, p4+?, m4, and if the outline put forward above is followed, the likely formula is i4, c1, p5, m4.

Dentition.—The fist incisor (i1) is complete, strongly procumbent and with a long root that extends back at least up to the level of the first completely preserved alveolus, which is interpreted here as the likely anterior root of a two−rooted canine (Figs. 3, 4). The crown is rather featureless with a convex mesial surface devoid of accessory cusps and a fairly flat distal surface. No wear facets are visible and the enamel of the crown does not extend back towards the root. Given the position of the tooth on the extreme ventral edge of the jaw and the lack of wear, it is likely that this tooth was as yet unerupted and has been exposed by breakage of the anteriormost portion of the jaw.

The first preserved postcanine tooth is the ultimate premolariform and here we will assume it to be the p5 for the purpose of description ( Figs. 2–5 ). The p5 is the best−preserved tooth, although it shows some damage. The crown is buccolingually compressed and dominated by a single cusp, the sharp and trenchant protoconid, made even more prominent by the lack of a metaconid, which is lost due to breakage. The tall protoconid is in the confluence of three main crests. The mesial one is the paracristid, on the lingual aspect of which the broken base of a minute cuspule, likely the paraconid is shown. The second crest reaches the lingual surface of the protoconid and bisects this surface in two fairly equal areas. The third and last crest, the protocristid, bears a flat and distally oriented wear facet near the apex and a small cusp midway between the protoconid and the base of the now missing metaconid. A shallow groove descends on the lingual surface of the protoconid from the cuspule on the protocristid. The paraconid, as deduced from its broken base, amounted to little more than a conical cusp on the lingual end of the paracristid. The substantially more robust metaconid, as deduced from its broken base, was removed from the protocristid and much smaller than the protoconid. There are small anterior and posterior cingular cusps. The lingual surface of the crown is distinctly delimited from the roots by a thickened edge reminiscent of a cingulid. In the buccal side of the tooth the cingulid is complete and fully encircles the base of the crown from one cingular cusp to the other. The cingulid has small crenulations or cuspules along its length. The roots are somewhat transitional between the slightly oval or circular roots deduced for the premolariforms or their alveoli, and the distinctively mesiodistally compressed roots of the molariforms of spalacotheriids. The condition, however, is much closer to that of the molariforms than to the circular roots present in the alveoli for the premolariforms.

Despite severe damage to m1–m3 the main features can be described, and these elements will be treated together here because a composite of all of them has to be made in order to describe the molar morphology ( Figs. 2–5 ). The m4 will be described separately because it is morphologically distinct form more mesial molariforms. The protoconid is the dominant cusp and is mesiodistally compressed with flat paracristid and protocristid implying prevallum−postvallum shearing surfaces. The teeth reduce the size of the protoconid in the mesial to distal direction. At the same time, the protoconid becomes more compressed and the paraconid and metaconid approach each other more closely. Both para− and metaconids are heavily damaged in all teeth but fragments of them are preserved on the smaller of the two blocks. Both cusps are connated and subequal in size. The paraconid is slightly procumbent and the metaconid inclined slightly posteriorly. The lingual bases of these two cusps are connected with distinct anterior and posterior cingular cusps, which can be seen on the fragment of the m3 on the smaller of two blocks. As in the case of the last premolar, the lingual margin of the crown is indicated by a thickened ridge that connects both cingular cusps. The main feature on the buccal surface is the prominent cingulid that runs on the base of the crown. It is strong mesiodistally but it becomes weaker in the buccal edge of the protoconid. It is interrupted in the m2 but likely continuous as a feeble ridge on m1 and m3. Therefore, the cingulid is continuous or almost continuous in Symmetrolestes . The m3 has the para− and metaconids much lower than the same cusps of the preceding molariform; this difference is not present between m1 and m2. The molariform roots are strongly compressed mesiodistally as other spalacotheriids. The interlocking between succeeding teeth is typical of spalacotheriids involving a single cusp mesially and distally.

The m4 is the smallest molariform ( Figs. 2–5 ). The m4 is also damaged, missing the apex of the protoconid. The paraconid is tall and trenchant, united lingually to a robust anterior cingular cusp. The metaconid was not present on the m4, and has a skewed outline typical of a last molariform. The buccal cingulid is fairly complete and formed by a blunt crest. It is likely that a single root supported the tooth. There is a prevallid surface (for shearing against the distal surface of M3) but no postvallid surface, suggesting the lack of functional M4. Therefore, Symmetrolestes probably has only three upper molariforms, or a very reduced M4.

Dentary.—Although the dentary is divided between two blocks, much of the anatomy of the jaw can be described ( Figs. 2 , 6 ). The corpus (alveolar process) of the jaw is gracile, slender, never reaching more than 1.5 times the height of the teeth. The dentary becomes progressively more robust posteriorly up to the level of m3; from there back, the jaw margin is slightly upwardly turned to become continuous with the condylar process. In dorsal view, the corpus of the jaw is relatively straight anterior and posterior to the level between p4 and p5, where it slightly curves medially. On the lingual surface of the jaw, the more prominent feature is the long and narrow mandibular symphysis that is only moderately rugose and extending back to the level of the tooth we interpret as p1 ( Fig. 2 ). More posteriorly and about midheight of the corpus there is a sulcus from the level of m1 that extends back through the preserved portion of the jaw. The groove represents a poorly developed Meckel's sulcus (Meckel's groove). Further back, the mandibular foramen and canal can be seen through the broken bone of the dentary. It is a proportionally large foramen. The coronoid process was extensive and erect; the anterior coronoid crest is blunt and broad, demarcated by a prominent ventral masseteric ridge and a deep masseteric fossa. At the anteroventral corner of this fossa is the masseteric (buccal mandibular) foramen. The dorsalmost portion of the coronoid process is lost. The ventral masseteric ridge is sharp and continuous with the ventral edge of the jaw anteriorly and with the condylar process posteriorly. The condylar process is broken and slightly downwardly reflected. There is no distinct angular process, and the condyle itself is small, mostly dorsally facing, and of triangular cross−section. The condyle is dorsal to the level of the alveolar edge. In ventral view, a strong pterygoid crest can be seen projecting medially; however, the morphological details of its lingual view are not accessible.

Figures

Fig. 2. Symmetrolestes parvus (NSM PV 20562, holotype). A. Lingual stereo view of the dentary. Note the ledge of matrix left along the ventral edge of the jaw, which continues toward the back bearing impressions of the condyloid and coronoid processes. B. Drawing of the jaw in approximately the same position as in A. The back of the coronoid process has been reconstructed from impressions left in the main block and the fragments preserved in the smaller block (Fig.6). Scale bar 2 mm.The arrow indicates the position of Meckel's sulcus.The dark circle towards the back of the jaw is the mandibular foramen.

Fig. 4. Symmetrolestes parvus (NSM PV 20562, holotype). A. Occlusal view of the dentary as preserved in the main block. B. Outline drawing of the dentition in occlusal view. Scale bar 2 mm.

Fig. 5. Symmetrolestes parvus (NSM PV 20562, holotype). A. Detail stereo view of p5–m4 in lingual view. B. Drawing of the dentition in lingual view. Scale bar 1 mm.

Fig. 6. Symmetrolestes parvus (NSM PV 20562, holotype). A. Stereo view of the posterior portion of the jaw in buccal view, as preserved in the smaller of the two blocks. B. Drawing of the preserved portion of the jaw. Scale bar 2 mm.

Fig. 1. Topographic map showing the fossil locality (asterisk), in the valley of the Sugiyama−gawa River, Kitadani−cho, Katsuyama City, Fukui Prefecture, central Japan (topographic map “Echizenkatsuyama”, Geographical Survey Institute of Japan).