Dyskritodon indicus, Prasad & Manhas, 2002

Prasad, Guntupalli V. R. & Manhas, Brijesh K., 2002, Triconodont mammals from the Jurassic Kota Formation of India, Geodiversitas 24 (2), pp. 445-464 : 448-452

Dyskritodon indicus n. sp. ( Figs 2 A-E; 3A-E)

HOLOTYPE. — VPL /JU/ KM/13 , a lower left molar.

ETYMOLOGY. — Species is named after India, the country of its origin.

HORIZON AND LOCALITY. — Mudstones associated with the limestone bands of Kota Formation, west of Paikasigudem village, Rebbana Mandalam, Adilabad District, Andhra Pradesh (State), India.

DIAGNOSIS. — Dyskritodon indicus n. sp. differs from the type species D. amazighi Sigogneau-Russell, 1995 in its smaller size (length of lower molar = 1.24 mm, maximum width = 0.46 mm), in having cusp b hardly lingual to a -c line; almost equally developed cusps e and f enclosing a relatively broader anterior notch.

DESCRIPTION

The tooth is well-preserved except for the breakage of the tips of cusps a and c. The crown is transversely narrow with three principal cusps in line anteroposteriorly. Cusp formula for this tooth is a> c> d> b. Cusp a is the largest cusp, anterior in position, and is connected to a small cusp b at its anterior base. Although the tips of a and c are broken, it seems that the posterior bor- der of these cusps is slightly longer than the anterior one. Cusp d is much smaller than c and is placed perpendicular to the line of a and c, with a flat anterior face and a convex posterior face, both faces being separated by a crest ascending from the lingual cingulum. Cusps a, c, and d are separated from each other by V-shaped notches. Cusps a and b are closely appressed and separated from each other probably very close to the tip of b ( Figs 2A ; 3A ). At the anterolingual base of cusp b is a small, vertical accessory cuspule e ( Fig. 2B ). The slightly crenulated cingulum is equally prominent all along the lingual margin of the tooth and terminates posteriorly as a vertical crest on cusp d ( Fig. 3A ); a narrow shelf is even present between the main cusps and the lingual cingulum. The lingual faces of cusps a and c are angulated. Their labial faces are strongly convex, a more so than c. These cusps are also swollen labially at their bases, but there is no labial cingulum. There is a small vertical, columnar cuspule f at the labial base of cusp b ( Figs 2A ; 3B ). Cuspules e and f are nearly equal in size. In labial view, cuspule f is separated from cusp a by a shallow sulcus. Cuspules e and f enclose a welldeveloped sulcus anteriorly. Labially, a, c and d are separated from each other by broad, vertical sulci. Cusp d has a short labial face separated from the posterior face by a ridge. This ridge bears a very small cuspule at its mid-height (this could be an artefact of wear) ( Figs 2D ; 3C ). The crown overhangs the roots on all sides. The anterior root is sub-spherical in cross section. The posterior root, which extends to the posterior base of a, is relatively longer anteroposteriorly. It is also labiolingually flattened and has a rounded posterior face. The roots taper ventrally. The tooth was least affected by wear except for narrow abraded facets on the posterolabial border of a and c ( Figs 2B, D ; 3B, C ). In addition, a subspherically worn face is present on the posterolabial base of cusp a ( Figs 2B ; 3B ).

COMPARISONS

VPL/JU/KM/13 presents the typical triconodont lower molar morphology with cusps b, a, c, and d arranged linearly in an anteroposterior direction, a well developed lingual cingulum and an anteri- or sulcus for receiving cusp d of the preceding tooth. At present, the order Triconodonta includes four named families: Morganucodontidae , Amphilestidae , Austrotriconodontidae , and Triconodontidae , and four unnamed families represented by Jeholodens jenkinsi Ji, Luo & Ji, 1999 , Tendagurodon janenschi Heinrich, 1998 , and Dyskritodon amazighi and Ichthyoconodon jaworowskorum Sigogneau-Russell, 1995 , respectively. VPL/JU/KM/13 differs from morganucodontids, in which cusp a occupies a more substantial part of the crown; and a prominent central lingual cingular cusp g or kuehneocone is present. As in morganucodontids, the main cusp a is also much larger than the adjacent nearly equal cusps b and c in amphilestines, gobiconodontines, Tendagurodon janenschi (a> c> b> d> e), and Austrotriconodon Bonaparte, 1992 (a> b> c> d). Moreover, the long axes of accessory cusps b and c diverge away from the vertically oriented long axis of main cusp a in some amphilestines, in gobiconodontines, and Jeholodens jenkinsi . In Gobiconodon , the base of main cusp is constricted on both anterior and posterior faces ( Jenkins & Schaff 1988: fig. 10A). Finally, triconodontids differ from VPL/JU/KM/ 13 in possessing subequal cusps b, a, and c. Although amphilestids, triconodontids, and Jeholodens also lack the cingular cusp g as does VPL/JU/KM/13, the lingual cingulum is medially arched in some amphilestids and discontinuous in Jeholodens . VPL/JU/KM/13 also differs from Klamelia zhaopengi Chow & Rich, 1984 , an amphilestid known from the Middle or Late Jurassic of north-western China, in which the cingulum is not only continuous lingually, but also welldeveloped labially at the anterior and posterior ends of the crown. The crenulations mentioned here on the lingual cingulum are also known to occur in some amphilestids and triconodontids. There is no trace of a lingual cingulum in Tendagurodon Heinrich, 1998 . Ichthyoconodon jaworowskorum Sigogneau-Russell, 1995 has a high, narrow, trenchant blade-like crown with sub-equal posteriorly inclined and deeply separat- ed b, a, and c (c is being slightly more dominant) and only very faint cingulum.

In the molar interlocking mechanism also, VPL/JU/KM/13 differs from morganucodontids, Jeholodens , and Triconolestes Engelmann & Callison, 1998 . In Morganucodon Kühne, 1949 and Megazostrodon Crompton & Jenkins, 1968 , the broad posterior margin of the lower molar crown fits into a shallow embayment between b and e. In Jeholodens , the cingular cuspules e and f are absent. The crescent-shaped cusp d of the preceding tooth fits into the concave anteromedial margin of cusp b of the succeeding molar, thus lacking the molar interlocking mechanism of Morganucodon and Megazostrodon or for that matter that of VPL/JU/KM/13. In the amphilestine genus Triconolestes , known by fragmentary left lower molar from the Upper Jurassic of USA ( Engelmann & Callison 1998), the interlocking mechanism is absent. But Dinnetherium , a triconodont of uncertain familial status known from the Lower Jurassic Kayenta Formation, Arizona, USA ( Jenkins et al. 1983), shows interlocking by means of anterior cuspules e and f and overhanging of crown over roots as in VPL/JU/KM/13. Dinnetherium was placed in Amphilestidae by Stucky & McKenna (1993), but it has been considered closer to Morganucodon in occlusal pattern by others ( Crompton & Luo 1993; Luo 1994; Kielan- Jaworowska & Dashzeveg 1998). A similar interlocking system is also seen in gobiconodontines. Cretaceous triconodontids developed tight interlocking between the adjacent teeth through a tongue and groove mechanism that extends well down the molar roots between e and f. In Tendagurodon , there is no f cuspule and no anterior indentation for the reception of cusp d of the preceding tooth. Similarly, Ichthyoconodon Sigogneau-Russell, 1995 also lacks the anterior interlocking mechanism of lower molars, so does licc

Klamelia Chow & Rich, 1984 . In the latter, the most anterior cusp b of one molar is medial to the most posterior cusp d of the preceding molar ( Chow & Rich 1984). Moreover, VPL/JU/KM/13 can be further distinguished from Klamelia in the cusp formula (a> c> b> d) of the latter. Finally, in marked contrast to VPL/JU/KM/13, in Klamelia cusp a is considerably longer and taller than b and c.

Sigogneau-Russell (1995) described an unusual mammalian tooth and made the holotype of Dyskritodon amazighi , from the Early Cretaceous of Morocco. She discussed at length the molar morphology in different families of Triconodonta ( Morganucodontidae , Amphilestidae , Austrotriconodontidae , Triconodontidae ) and arrived at the conclusion that Dyskritodon amazighi possibly represents a new family of Triconodonta . The new tooth from the Kota Formation (VPL/JU/KM/13) recalls the crown morphology of Dyskritodon amazighi in a large number of characters. In both the Moroccan and Indian specimens, the crown is narrow labiolingually with cusps a, c, and d decreasing progressively in height posteriorly; cusps a and c dominate the crown and have slightly longer posterior crests than the anterior ones; cusp d is placed perpendicular to the a-c line and has a flat anterior face, a convex posterior face, and a short labial face separated from the posterior face by a ridge; cusps a and c have angulated lingual and labial faces and swollen labial bases separated by broad sulci; e and f cuspules are present anterior to cusp b (although Sigogneau-Russell [1995] reported the absence of cusp e in D. amazighi , reexamination of the holotype by GVRP and DSR revealed the presence of a very faint bump at the anterior base of cusp b, possibly a rudimentary e cuspule); in both, the anterior root is sub-spherical in cross section and the posterior root is anteroposteriorly longer, labiolingually flattened, and expands below cusp a; and the crown overhangs the roots on all sides. All these morphological similarities between Dyskritodon amazighi and VPL/JU/KM/13 favour inclusion of the latter in the genus Dyskritodon .

However, the two specimens exhibit the following morphological differences. VPL/JU/KM/13 (length = 1.24 mm) is smaller in size than D. amazighi (length = 1.85 mm). In VPL/JU/KM/13, cusp b is slightly lingual to a -c line ( Fig. 3E ), whereas it is a lingual cingular cusp in D. amazighi ; it is relatively smaller than cusp d, but not as small as in D. amazighi . In VPL/JU/KM/13, two equally developed cuspules e and f occur at the anterolingual and anterolabial bases of cusp b enclosing a well-developed vertical sulcus between them, while cuspule e is hardly indicated in D. amazighi and is in the form of a very small bump at the anterior base of cusp b. Cuspule f of D. amazighi is situated at the anterolabial base of a, whereas it is at the anterolabial base of cusp b in VPL/JU/KM/13. It extends obliquely to the anterior part of a in D. amazighi , while it is more vertical on VPL/JU/KM/13. The lingual cingulum is also more developed in VPL/JU/JM/13 than in D. amazighi and thus a narrow and better-developed shelf occurs between the main cusps and the cingulum. The posterior root of VPL/JU/KM/13 is not as long anteroposteriorly as in D. amazighi . These morphological differences with D. amazighi justify its placement in a new species.

Figures

FIG. 2. — A-E, left lower molar, holotype (VPL/JU/KM/13); A, lingual view; B, labial view; C, occlusal view; D, posterior view; E, ante- rior view; F, G, Triconodonta indet., fragmentary left lower molar (VPL/JU/KM/15); F, labial view; G, lingual view. Scale bar: 0.5 mm.

FIG. 3. — A-E, Dyskritodon indicus n. sp., left lower molar, holotype (VPL/JU/KM/13); A, lingual view; B, labial view; C, posterior view; D, anterior view; E, occlusal view; F, G, Triconodonta indet., fragmentary left lower molar (VPL/JU/KM/15); F, lingual view; G, labial view. Abbreviations: a-f, cusps a-f; licc, lingual cingular cuspule;?w. f.,?wear facet. Hatched areas represent wear facets and crossed hatching demarcates broken areas in all figures. Scale bar: 0.5 mm.