Paikasigudodon yadagirii ( Prasad & Manhas, 1997 ) 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 : 453-457

Paikasigudodon yadagirii ( Prasad & Manhas, 1997) n. comb. ( Figs 4 ; 5 )

Kotatherium yadagirii Prasad & Manhas, 1997: 565 , 566.

HOLOTYPE. — VPL /JU/ KM/10 , left upper molar.

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

DIAGNOSIS. — Same as for genus.

DESCRIPTION

The tooth is a double rooted, bean-shaped, excellently preserved left upper molar (maximum length = 1.881 mm, anterior width = 1.063 mm, posterior width = 0.787 mm). The principal and accessory cusps form an obtuse angle in occlusal view. A is the largest and highest cusp with a vertically oriented long axis, a nearly flat or slightly convex labial face, and a more convex lingual face. Similarly, cusps B and C have convex lingual bases. Cusp B is the next largest cusp and is separated from cusp A at a slightly higher level than cusp C by a narrow, V-shaped notch ( Fig. 5E ). It is connected to A by a sharp crest and has a convex anterolingual face. Labially, it has a broad, flat face that is slightly obliquely oriented relative to the labial face of A. A sharp transverse crest joins cusp B with a relatively large anterolabial cusp F (less than half the size of cusp B). The tip of cusp F projects labially. It has a flat posterolingual face, which lies at a right angle with respect to the labial face of cusp B, and a rounded anteri- or face. A very small cuspule (possibly cusp E) also occurs at the anterolingual base of cusp B. Cusp C is slightly smaller than cusp B but larger than the anterolabial cusp F. In occlusal view, it lies noticeably labial to the A -B line. It has a convex posterior face and moderately convex anterolabial and anterolingual faces that are separated by a crest. A small cingular cusp occurs at the anterolabial base of cusp C, and another slightly larger one at its anterolingual base ( Fig. 5C, D ). A weakly developed, narrow labial cingulum, beaded with minute cuspules, connects cusp C with cusp F ( Figs 4B, C ; 5C, E ). These cingular cusps are minute and more or less of the same size. A narrow shelf is present anteriorly. In occlusal view, the labial cingulum defines a slight ectoflexus because of the labial projection of cusps C and F. The lingual base of the crown is swollen, but no distinct cingulum is present except for a thickening of the lingual margin between A and B ( Figs 4D ; 5C, D ), and the small cingular cusps posterolingually and anterolingually. In anterior view, the crown has a transverse aspect, only the posterior crest of B and anterior crest of A are in line. There are two subcylindrical roots.

The tooth is quite unworn. A weakly developed abrasion facet extends lingually from the posteri- or tip of cusp A to its base. This facet continues across the notch between A and C to just below the tip of C ( Fig. 5C, D ). Another very faint or indistinct abrasion facet can be seen on the anterolingual face of A extending from the notch between B and A. This facet is broad at the base of A and gradually narrows toward its tip to disappear before reaching it ( Fig. 5C, D ). The posterolingual face of B appears to be unworn. VPL/JU/KM/10 is considered to be an upper molar because of the presence of a rudimentary lingual cingulum between A and B with small anterolingual and posterolingual cingular cuspules in addition to the continuous labial cingulum. The non-linear arrangement of cusps as in the upper molars of certain triconodonts ( Gobiconodon and Megazostrodon ) and the absence of anterior interlocking mechanism also preclude its placement in lower molars.

COMPARISONS

Prasad & Manhas (1997) assigned VPL/JU/KM/10 to a new species of Kotatherium , K. yadagirii (family Tinodontidae ), based on literature then available to them. Comparison of these specimens with originals and casts of Mesozoic mammals housed in BMNH and MNHN led the authors to the conclusion that VPL/JU/KM/10 is not a symmetrodont but a triconodont. VPL/JU/KM/10 differs from Kotatherium haldanei , a symmetrodont described from the Kota Formation ( Datta 1981), in its bean-shaped crown (in occlusal view), vertically oriented symmetrical cusp A without any labial tilt, absence of an anterolingual shelf, presence of prominent cusp B slightly larger than C, posterolingual cuspule and a large anterolabial accessory cusp as large as the “metacone” of Datta (1981). In other early symmetrodonts ( Kuehneotherium Kermack, Kermack & Mussett, 1968 , Eurylambda Simpson, 1929 , and Woutersia Sigogneau-Russell, 1983 ), the lateral cusps are separated from the main cusp close to their tips by shallow notches, whereas in VPL/JU/KM/10, B and C are broadly separated from A by deep notches. In symmetrodonts, the lateral cusps are symmetrically arranged on either side of the main cusp. In contrast to this, cusps are asymmetrically arranged and the crown is also asymmetrical in occlusal view on VPL/JU/KM/10. Although symmetrodonts have anterolingual cusp E ( Kuehneotherium , Woutersia ), they lack posterolingual cuspule and the large anterolabial cusp F of VPL/JU/KM/10. Furthermore, VPL/JU/KM/10 differs from primitive symmetrodonts ( Kuehneotherium , Woutersia , and Mictodon Fox, 1984 ) in the absence of parastylemetastyle interlock. These morphological differences are in favour of removing VPL/JU/KM10 from the order Symmetrodonta .

Among the various early mammalian groups, triconodonts exhibit a crown morphology nearest to that of VPL/JU/KM/10. The subfamily Amphilestinae is known only by lower molars and hence no comparison is possible with VPL/JU/KM/10. Among the different members of Triconodonta , gobiconodontines have molars with central cusp substantially taller and larger than the adjacent accessory cusps, which are nearly equal in size, as is the case in VPL/JU/KM/10. The upper molars of Gobiconodontinae ( Gobiconodon borissiaki Trofimov, 1978 , G. hoburensis [ Trofimov, 1978], G. ostromi Jenkins & Schaff, 1988 ) show an incipient triangular arrangement of cusps and the degree of triangular arrangement increases posteriorly, reaching an angle of 145° to 158° on M3 and M4 (Kielan-Jaworowska & Dashzeveg 1998). In VPL/JU/KM/10, this angle is 145°. The upper molars of Gobiconodon change from oval to subrectangular anteriorly to rectangular crowns posteriorly. They show a labial indentation as in VPL/JU/KM/10. The labial cingulum of Gobiconodon is beaded as in VPL/JU/KM/10. The lingual cingulum is also well-developed in Gobiconodon , in which it is wider at the anterior and posterior ends than in the middle. In VPL/JU/KM/10, the lingual cingulum is incipient anteriorly and the tooth bears small cuspules anteriorly as well as posteriorly. Cusps E and F are known to occur in Gobiconodon (Kielan-Jaworowska & Dashzeveg 1998) as in VPL/JU/KM/10, but in the latter, cusp F is relatively very large in size. VPL/JU/KM/10 further differs from Gobiconodon in its bean-shaped crown, and in the presence of relatively high, anteroposteriorly long principal cusp A and asymmetrically placed B and C. In Gobiconodon , cusp A, though largest of the crown, is moderately higher than B and C. In VPL/JU/KM/10, wear facets are not present on the anterior and posteri- or parts of lingual cingulum as in Gobiconodon , rather they are restricted to the anterolingual and posterolingual faces of cusp A and the valley between A and C.

Another triconodont taxon in which the central cusp is high and the lateral cusps are smaller (nearly equal to each other) is Jeholodens jenkinsi described from the Late Jurassic/Early Cretaceous of Liaoning, China ( Ji et al. 1999). But in Jeholodens , the cusps are aligned anteroposteriorly. There is no cingulum on the labial and lingual sides of M1, but a faint cingulum is present on the labial side of M2 and M3 of Jeholodens . No cingular cuspules are seen on the labial and lingual sides of cusp B on the exposed M1 of Jeholodens .

The triconodont which most closely approximates to VPL/JU/KM/ 10 in crown morphology seems to be Megazostrodon rudnerae Crompton & Jenkins, 1968 , particularly M1 of this genus. As in VPL/JU/KM/10, the symmetrical main cusp dominates the bean-shaped crown, a labial medi- an indentation is present, the anterior end of the crown is broader than the posterior one (this is also the case in Gobiconodon ), a large cusp F is present labial to B, and the three main cusps form an obtuse-angled triangle. The lingual border of Megazostrodon upper molars is markedly convex as in VPL/JU/KM/10. In M. rudnerae , cusp A becomes less dominant on M2 and more posteri- or molars. By analogy, VPL/JU/KM/10 could be considered as M1. But in M. rudnerae , the main cusp is longer anteroposteriorly and extends to the labial margin in the middle. In addition, the height of the main cusp is greater than its anteroposterior length in VPL/JU/KM/10, whereas it is nearly equal in M. rudnerae . Cusp B is smaller than cusp C in M. rudnerae , while it is slightly larger than C in VPL/JU/KM/10. In this respect, the latter is more similar to Austrotriconodon , but in the latter genus cusp B is the largest of all cusps and the crown morphology is quite distinct from that of VPL/JU/KM/10. Cusp D is very small in M. rudnerae , whereas it is not present on VPL/JU/KM/10. Finally, unlike M. rudnerae , the labial cingulum of VPL/JU/KM/10 is uninterrupted in the middle and the tooth does not bear a distinct lingual cingulum although an incipient cingulum is present anterolingually between cusps A and B. In view of the above discussed morphological divergences from Megazostrodon, VPL /JU/KM/10 is placed under a new genus of the family Morganucodontidae closely allied to Megazostrodon .

Figures

FIG. 4. — SEM photomicrographs of Paikasigudodon yadagirii (Prasad & Manhas, 1997) n. comb., left upper molar, holotype (VPL/JU/KM/10); A, posterior view; B, labial view; C, occlusal view; D, lingual view; E, anterior view. Scale bar: 0.5 mm.

FIG. 5. — Paikasigudodon yadagirii (Prasad & Manhas, 1997) n. comb., left upper molar, holotype (VPL/JU/KM/10); A, anterior view; B, posterior view; C, occlusal view; D, lingual view; E, labial view. Abbreviations: A-F, cusps A-F; lacc, labial cingular cuspules;?lic,?lingual cingulum; plicc, posterior lingual cingular cuspule. Scale bar: 0.5 mm.