Siamosorex debonisi n.
|
publication ID |
https://doi.org/ 10.5252/g2009n4a973 |
|
persistent identifier |
https://treatment.plazi.org/id/03FDC109-FFC0-AE5A-9592-FA88FF1F66DA |
|
treatment provided by |
Felipe |
|
scientific name |
Siamosorex debonisi n. |
| status |
n. |
HOLOTYPE. — SHM-CP 677, incomplete right hemimandible with i2, partial p3, p4-m1, isolated i1, i3, m2, and isolated left m3 located very near the mandible in the same block of sediment and probably belonging to the same individual ( Fig. 2 View FIG A-C; 3; 4A-C).
PARATYPES. — SHM-CP 327, fragmentary left hemimandible with p4-m2; SHM-CP 234, fragmentary right hemimandible with p3-m3 ( Fig. 2 View FIG D-F; 4D-F).
TYPE LOCALITY. — Cha Prong pit of Nong Ya Plong coal mine, Province of Phetchaburi, Thailand.
DIAGNOSIS. — As for the genus.
ETYMOLOGY. — Named for Louis de Bonis (IPHEP, University of Poitiers) in recognition of his many contributions to the paleontology of carnivorous fossil mammals.
DISTRIBUTION AND AGE. — As for the genus.
DESCRIPTION
Mandible ( Fig. 2 View FIG )
The mandibular body is slender and has approxi-
mately the same depth throughout its length. It is shallow, with a maximum depth similar to or lower than the height of the protoconid of m1. The symphysis is poorly marked, which suggests that the hemimandibles were not closely attached together. A shallow but distinct longitudinal groove is present on the lingual face of the corpus, from the level of m3 to the symphysis; it is slightly buccally positioned posteriorly, then ventral beneath p4-m1, and lingual throughout its anterior part. There are two mental foramina. A small one is visible beneath the anteriormost border of p2 and a larger one, which opens forwards, is located beneath the main cuspid of p4. In dorsal view, the root of the coronoid process appears to be laterally located relatively to the tooth row; it has its anterior border at the level of the distal rim of the distal alveolus of m3. A prominent crest is present on the lingual surface of the mandible, posterior to the m3. There is a distinct retromolar space. SHM-CP 234 (paratype) preserves most of the coronoid process, which is tall and oriented slightly forwards, making an acute angle with the ventral border of the mandible. The masseteric fossa is very shallow. The angular process is completely preserved in the holotype. It is slender, elongated, and weakly hooked dorsally. The insertion area for the M. pterygoideus lateralis is developed in the form of a shallow and elongated depression that faces mediodorsally. The internal crest that separates this area from that for the M. pterygoideus medialis is strong. The ventral surface of the angular process is horizontally oriented and displays an expanded surface for the insertion of the M. masseter superficialis .
Dentition ( Figs 3 View FIG ; 4 View FIG )
The dentitions are almost unworn and represent young adults with permanent dentition. The i1 ( Fig. 3 View FIG A-C) is laterally compressed; the main cuspid is lanceolate and slightly procumbent, with a curved lateral ridge. No accessory cuspids are preserved, but the concave distal margin of the main cuspid suggests that a distal accessory cuspid may have been present and separated from the main cuspid by a deep notch. The incisor root is about twice the length of the crown. In plesiosoricids (except in Ernosorex Wang & Li, 1990 ), the enlarged incisor is the i2. In the Thai species too, this tooth is very large, caniniform, and slender, but it is not procumbent and has a slightly trenchant distal rim ( Fig. 4 View FIG A-C). A deep groove, triangular in cross-section, and covered by enamel, is present in a mesiolingual position ( Fig. 4A View FIG ). An isolated antemolar present near the mandible SHM-CP 677 in the lignite block ( Fig. 3 View FIG D-F) is here identified as an i3, based on its great resemblance to the i3 of the type species of the genus Plesiosorex , P. soricinoides ( Blainville, 1840) from the mid-Oligocene of Saint-André (e.g., Viret 1946: fig. 1). This is a small and transversely compressed tooth, with a somewhat trenchant longitudinal crest; its crown is asymmetrical and markedly projected forwards relative to the root. The tooth row is straight and there is no diastema between the teeth. The alveolar margin between the i3 and p3 is poorly preserved in the material ( Fig. 4C View FIG ). We distinguish 3 or 4 alveoli, but it is not possible to determine whether c, p1, and p2 were all present and how many roots they had. The p3 and p4 are double-rooted and morphologically similar ( Fig. 4 View FIG ). They are tall, slender, and have a piercing protoconid, trenchant rims, with the mesial rim slightly convex and the distal rim slightly concave, a reduced paraconid, a short trenchant distal talonid, and a distinct buccal cingulid. These teeth are much taller than the m1 paraconid. The p4 differs from p 3 in being slightly larger and semimolariform: it has a slightly larger paraconid and a highly placed, small metaconid closely pressed against the mesiolingual surface of the protoconid. The m1 is marginally longer than p4 ( Fig. 4 View FIG ; Table 1). The trigonid is composed of a tall protoconid, a much lower paraconid, and a conical metaconid, reduced but taller than the paraconid. The paraconid and protoconid shearing blades make an obtuse angle; the carnassial notch is moderately deep. The metaconid is separated from the paraconid by a deep valley and is placed lingually to the protoconid. The distal face of the trigonid is oriented slightly forwards. The talonid is much lower, narrower, and shorter than the trigonid. There are no individualized cuspids. The oblique cristid is trenchant, taller than the entocristid, and meets the distal surface of the trigonid at the protocristid notch. The talonid basin is shallow. The ectocingulid is distinct and especially well developed at the contact with p4 (precingulid). The m2 ( Figs 3 View FIG G-I; 4D-F) is double-rooted and much more reduced (especially much lower) than m1. Its overall morphology is similar, however, except for some details. Thus, it differs from m 1 in being less elongated and in having a less open trigonid, a shallower carnassial notch, a metaconid as tall as the protoconid, a longer talonid, and a more developed precingulid. The m3 ( Figs 3 View FIG J-L; 4D-F) is double-rooted in the holotype, but may have partly fused roots in SHM-CP 234 (see Fig. 4F View FIG ) and has the same basic structure as the other molars. The trigonid is tall, the metaconid is as tall as the protoconid, the carnassial notch is weakly pronounced; the talonid is lower and narrower than the trigonid, and has an oblique cristid taller than the entocristid. The ectocingulid and the precingulid are strongly developed.
The molars decrease in dimensions from m1 to m3, the more so between m1 and m2 than between m2 and m3. Furthermore, from m1 to m3, the molars shorten, the molar trigonid becomes less dominant relative to the talonid (the ratio of trigonid length relative to total length decreases from m1 to m3; Table 1), trigonid cuspids form a more and more closed triangle and tend to be less prominent and less well individualized from one another.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.