Sayimys sivalensis (Hinton, 1933)

Sehgal, Ramesh Kumar, Singh, Abhishek Pratap, Singh, Ningthoujam Premjit, Gilbert, Christopher C., Patel, Biren A. & Patnaik, Rajeev, 2023, First report of rodents from the Miocene Siwalik locality of Dunera, Pathankot District, Punjab, India, Palaeontologia Electronica (a 49) 26 (3), pp. 1-27 : 16-17

Sayimys sivalensis (Hinton, 1933)

Figure 7

Holotype. GSI D284 , left partial dentary with m2– m3.

Type locality. Chinji village, Middle Miocene of Pakistan.

Referred materials. WIMF /A 4733 left M2 ( Figure 7A ), WIMF /A 4750 right M2 ( Figure 7B ), WIMF /A 4731 left M2 or M3 ( Figure 7C ); WIMF /A 4732 left m1 or m2 ( Figure 7D ).

Occurrence. Dunera locality, Punjab, India (study area), early Late Miocene Tapar locality, Kutch, India; Middle Miocene Dehari locality, Ramnagar, India; Late Miocene Haritalyangar locality, India; Middle Miocene, Sind, Pakistan; Middle to Late Miocene of Potwar Plateau, Pakistan.

Description. WIMF/A 4733 ( Figure 7A ) is identified as a likely M2 based in part on its size, which is larger than any M1 described for S. sivalensis by Munthe (1980). In occlusal view, the tooth narrows distally, and the outline of the mesial occlusal surface is rounded while the distal end is squared off. The lingual cusps (protocone and hypocone) are larger than the buccal cusps (paracone and metacone). The protocone is somewhat larger and extends more lingually than the hypocone. The paraflexus is indistinct, if present, with a slight indentation just anterior to the paracone. The mesoflexus is wider and greater in depth than hypoflexus, and these features are oriented slightly oblique to each other, separated by a small, narrow crest running mesiodistally on the lingual side of the tooth, connecting the anteroloph to the metaloph/posteroloph. A short metaflexus is present just distal to the metacone, and the posteroloph is shorter than the metaloph.

WIMF/A 4750 ( Figure 7B ) is also identified as a likely M2, again based in part on its size, which is larger than M1s described by Munthe (1980) as well as slightly larger than WIMF/A 4733. In overall shape, WIMF/A 4750 is similar to WIMF/A 4733 with the distal portion of the tooth narrower than the mesial portion in occlusal view. WIMF/A 4750 is more worn than WIMF/A 4733, perhaps accounting for some of the minor differences between the teeth. The lingual cusps are again larger than the buccal cusps, with the protocone larger than the hypocone. Both lingual cusps extend to the lingual border of the tootha and neither cusp appears to extend more lingually than the other. The paraflexus is indistinct, with the slightest hint of an indentation or facet present just anterior to the paracone, although some of this appearance could be due to slight damage and wear. The mesoflexus is wider and greater in depth than hypoflexus, with the mesoflexus oriented lingually and the hypoflexus narrower and oriented mesiobuccally. The mesoflexus and hypoflexus are separated by a tiny, narrow crest running mesiodistally and slightly distolingually on the lingual side of the tooth, connecting the anteroloph to the metaloph/posteroloph. A short metaflexus is present just distal to the metacone, and a very short posteroloph is present (shorter than the metaloph) and oriented distobuccally, but more distally oriented compared to the posteroloph in WIMF/A 4733.

WIMF/A 4731 ( Figure 7C ) is a left M2 or M3, most likely an M3. Though slightly larger, it is morphologically similar to the above described M2s. The paraflexus appears absent, with only the slightest indentation anterior to the paracone. The protocone is slightly larger than the hypocone, and the tooth narrows distally, to a greater degree than seen in the M2s. The mesoflexus and hypoflexus are more equal in size depth compared to the M2s, although the mesoflexus is still larger. They both terminate opposite each other near the midline, separated by a narrow mesiodistally oriented crest connecting the anteroloph to the metaloph/posteroloph. An extremely short metaflexus is present, represented by a shallow indentation distal to the metacone.

WIMF/A 4732 ( Figure 7D ) is most probably a left m2 based on its relatively large size but given that m1 and m2 are difficult to separate morphologically, we describe it as an m1 or an m2. The occlusal outline of the tooth is sub-rectangular, slightly narrower posteriorly, with a transversely straight anterior margin that slightly narrows posteriorly. The protoconid is ‘v’ shaped and its anterior arm connects to the metalophid terminating at the metaconid. The protoconid extends more buccally compared to the hypoconid. The mesoflexid is slightly shorter and deeper than the metaflexid. The hypolophid runs at a slightly oblique angle postero-lingually, terminating at entoconid. The hypoconid is also ‘v’ shaped and its posterior arm merges with the obliquely oriented posterolophid that runs almost parallel to the hypolophid. The connection of the anterior arm of the hypoconid with the hypolophid is narrow, meeting near the midline of the tooth. The hypoflexid is much larger and deeper compared to the mesoflexid and metaflexid.

Comparison and Remarks. Five ctenodactylid species belonging to two genera, Prosayimys and Sayimys , are currently recognized from the Neogene of the Indian subcontinent (López-Antoñanzas and Sen, 2003; López-Antoñanzas and Knoll, 2011). Prosayimys flynni is found in the Early Miocene Chitarwata Formation of Zinda Pir Dome, Pakistan, and at least four species of Sayimys are recognized at many Neogene sites across India and Pakistan (López-Antoñanzas and Sen, 2003; López-Antoñanzas and Knoll, 2011). The present specimens are very similar to those of the long ranging (Middle Miocene-Late Miocene) form S. sivalensis in having derived characters such as: semi-hypsodont teeth, a mesoflexid almost equal to the metaflexid in length on m1-m2, and a short or absent paraflexus and metaflexus on M1/M2 and M3. The present described specimens are larger in size compared to many species of Sayimys from the Siwaliks but fall within the range of Sayimys sivalensis (sensu López-Antoñanzas and Sen, 2003, including S. perplexus and S. chinjiensis ), further supporting their assignment to S. sivalensis (see Table 3).

Figures

FIGURE 7. 3D surface models of Sayimys sivalensis in occlusal views. A) WIMF/A 4733, left M2; B) WIMF/A 4750, right M2 C) WIMF/A 4731, left M2 or M3; D) WIMF/A 4732 left m1 or m2. Note: Mesial towards the left and distal towards the right in all panels.