A previously unrecognized group of Middle Jurassic triconodontan mammals from Central Asia
- First Online:
- Cite this article as:
- Martin, T. & Averianov, A.O. Naturwissenschaften (2007) 94: 43. doi:10.1007/s00114-006-0155-5
- 77 Views
Ferganodonnarynensis gen. et sp. nov. is represented by a lower molariform tooth from the Middle Jurassic (Callovian) Balabansai Svita in Kyrgyzstan. The new genus is allied with Klamelia zhaopengi Chow and Rich 1984 from the Middle Jurassic Shishugou Formation in Xinjiang, northwest China to the new family Klameliidae based on parallelogram-shaped lower molariforms, imbricating rather than interlocking of cusps e-d-f, by a peculiar distolabial cingulid cusp, and by vertical folding of the enamel on the labial crown side. The new family Klameliidae fam. nov. is most similar to Gobiconodontidae by the structure of the molariform teeth and represents a previously unrecognized radiation of eutriconodontan mammals possibly endemic to Central Asia.
There are three vertebrate localities within the Middle Jurassic Balabansai Svita in the northern Fergana Valley, Kyrgyzstan that have produced remains of mammals: (1) Sarykamyshsai 1 (site FTA-30), 3–4 km east of the town of Tashkumyr (lower, gray to greenish colored part of the Balabansai Svita, Callovian); (2) Tashkumyr 1 (site FTA-131), left bank of the Naryn River close to the town of Tashkumyr (bonebed within a calcareous sandstone of the lower part of the Balabansai Svita, Callovian); and (3) Dzhiddasai (site FBX-23), 5 km west of the town of Tashkumyr (upper, red colored part of the Balabansai Svita, Callovian; see Averianov et al. 2005 for a more detailed description of localities). More than 20 mostly fragmentary mammalian teeth were recovered so far from these localities. The mammal fauna is dominated by docodonts, represented by an isolated molar of Tashkumyrodon desideratus Martin and Averianov 2004 from Sarykamyshsai 1 and by a lower molar and several upper molar fragments of a new docodont from the Tashkumyr 1 bonebed that is similar to Dsungarodon zuoi Pfretzschner and Martin, 2005 from the Late Jurassic (Oxfordian) Qigu Formation in the Junggar Basin, Xinjiang, northwestern China. This locality produced also one almost complete and several fragmentary teeth referable to one or two taxa of Eutriconodonta. The most complete eutriconodontan tooth is described here and attributed to a new taxon within the newly established family Klameliidae fam. nov.
Institute abbreviations IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing; ZIN, Zoological Institute, Russian Academy of Sciences, St. Petersburg.
Mammalia Linnaeus 1758
Eutriconodonta Kermack et al. 1973
Klameliidae fam. nov.
Type genus: Klamelia Chow and Rich 1984.
Triconodontan mammals with linearly arranged cusps on lower molariforms referable to the Eutriconodonta rather than to the Morganucodonta by lack of a cingular lingual cusp g (kühneocone). Shares with the Gobiconodontidae Chow and Rich 1984 the following derived characters on lower molariforms: crown height approximately equivalent to the crown length (reconstructed), cusps b (when present) and c diverging from rather than parallel to the central cusp a (in lateral view), and cusp c taller than cusp b (reconstructed), with the base of cusp c positioned high on the crown, well above the level of the cingulid. Differs from Gobiconodontidae by parallelogram-shaped lower molariform crowns (in occlusal view), indicating an imbrication of the molariforms rather than e-d-f interlocking, a peculiar distolabial cingulid cusp, and vertical folding of the enamel on the labial crown side (all derived characters).
Ferganodon gen. nov. and Klamelia show a tendency to form a functionally bicuspid lower molariform pattern, with cusp b reduced in the former and completely lost on the known teeth of the latter (see “Discussion” for the interpretation of the cusps in Klamelia). Some Early Cretaceous gobiconodontids show a similar trend, but have cusp b reduced only on the most anterior molariform tooth (m1; see, e.g., Jenkins and Schaff 1988; Kielan-Jaworowska and Dashzeveg 1998; Meng et al. 2005; Minjin et al. 2003, 2004). This similar tendency and the shared derived features outlined above suggest a close relationship between Gobiconodontidae and Klameliidae fam. nov.
Type genus and Ferganodon gen. nov., Middle Jurassic of northwest China and Kyrgyzstan.
Ferganodon gen. nov.
Type species: Ferganodon narynensis sp. nov.
Ferganodon gen. nov. differs from Klamelia by presence of cusp b (plesiomorphic character), higher cusp d (?derived character), better developed distolabial cingulid cusp (?plesiomorphic character), reduced mesiolingual cingulid cusp e and enlarged mesiolabial cingulid cusp f (character of uncertain polarity), and poorly developed and incomplete lingual cingulid (character of uncertain polarity).
It cannot be completely ruled out that in Klamelia, cusp b is lost only in the known anterior molariforms (m1–4) and was still present in the unknown posterior molariforms (m5–6).
Included species: Type species only
Etymology: After the Fergana Valley in Central Asia and oδoν, stem of oδoυς (Greek), tooth
Ferganodon narynensis sp. nov.
Holotype: ZIN 94214, right lower molariform tooth
Type locality and horizon: Tashkumyr 1 (site FTA-131), left bank of the Naryn River near the town of Tashkumyr; 36–37 m above the base of the Balabansai Svita, Middle Jurassic (Callovian)
Diagnosis: As for the genus
The height of the crown in ZIN 94214 is about 70% of the crown length. However, the height of cusp a is significantly reduced by wear and the unworn crown possibly was as high as long.
Etymology: After the Naryn River in Kyrgyzstan
K. zhaopengi Chow and Rich 1984 is known from a single specimen, a left dentary fragment with five teeth preserved (Fig. 2), from the basal section of the Shishugou Formation at Jianshan Wash in the Junggar Basin, Xinjiang-Uygur Autonomous Region of China (Chow and Rich 1984; Kielan-Jaworowska et al. 2004; Zhang 1984). The lower part of the Shishugou Formation was originally determined as late Middle Jurassic to Late Jurassic in age, then as Late Jurassic (Dong 1994; Kielan-Jaworowska et al. 2004), but according to more recent data is now regarded as Middle Jurassic (Clark et al. 2004; Eberth et al. 2001).
Interpretation of the known dentition and systematic position of Klamelia is difficult. Chow and Rich (1984, p. 227) combined Klamelia with the Early Cretaceous Gobiconodon in the Gobiconodontinae (within Amphilestidae) based on “foreshortening of the mandible and correlative reduction of the number of antemolar teeth to six or less.” These authors considered a procumbent incompletely preserved anterior two-rooted tooth to be the last premolar and the following teeth, gradually increasing in size posteriorly, as molars. This would give, together with the preserved tooth roots, at least two premolars and six molars for Klamelia. That interpretation was criticized by an anonymous reviewer of Chow and Rich’s paper, who suggested that the anterior procumbent tooth is a double-rooted canine and the following preserved teeth are p1–4 (Chow and Rich 1984, p. 230). The opinion of the anonymous reviewer was upheld by Rougier et al. (2001, p. 16), who considered Klamelia not being closely related to Gobiconodon and referred it to Mammaliaformes incertae sedis. Kielan-Jaworowska et al. (2004, p. 239) regarded Klamelia as “an eutriconodont of “amphilestid” grade” noting its difference to the gobiconodontids in molariform structure and imbrication in the molariform series.
We agree with Rougier et al. (2001) that the original arguments by Chow and Rich (1984) for assessing a p1+?, m1-6+? dental formula for Klamelia are not sound (although their tooth count is correct). For the canine nature of the anteriormost preserved tooth in the K. zhaopengi holotype, with two roots and accessory distal cusp, advocates its relatively anterior position on the jaw and its projection above the level of the following teeth. A similar double-rooted canine with distal accessory cusp is present in some Mesozoic mammals, such as docodonts, symmetrodonts, and dryolestids. However, in Klamelia the crown base of this tooth is remarkably expanded transversely. Such a transverse expansion is not found in the canine of any Mesozoic mammal but can occur in premolariforms. In the majority of Mesozoic mammals, the premolariforms are much simpler than the cheek teeth present in Klamelia and are dominated by the principal cusp with small mesial and distal accessory cusps. Originally (Chow and Rich 1984; Kielan-Jaworowska et al. 2004), the cusps of the teeth of Klamelia were interpreted as central cusps a and c and cingulid cusps b and d. In this interpretation, the functionally bicuspid teeth of Klamelia with cingulid cusp b superficially resemble the lower molariforms of morganucodonts (which was apparently a reason for referring Klamelia to the Morganucodontidae by Zhang 1984). However, discovery of Ferganodon gen. nov., which is structurally ancestral to Klamelia, demonstrates that cusp b, already diminished in Ferganodon gen. nov., is completely lost in Klamelia. The mesial cingulid cusp of Klamelia is not cusp b, but a cingulid cusp apparently homologous with cingulid cusp e of amphilestids and gobiconodontids. In spite of the distinct imbrication of the molariforms in Klamelia, at least a pair of molariforms (m2 and m3) exhibit a rudimentary e-d-f interlocking pattern (Fig. 2d). A similar rudimentary interlocking can be inferred for ZIN 94214, where there is a mesial embayment at the base of cusp b between the mesially prominent cingular cusp (f) and mesiolingual cingulid (rudimentary cusp e; Fig. 1a,c). Thus, we consider the imbrication pattern of the molariforms in Ferganodon gen. nov. and Klamelia to be derived from a primitive e-d-f interlocking pattern of amphilestids and gobiconodontids. This interlocking is characteristic for the molariforms, but not for the premolariforms, which further suggests a molariform nature of the teeth in Klamelia. Therefore, we think that the original interpretation of dental formula of Klamelia as p1+?, m1-6+? by Chow and Rich (1984) is correct.
The imbrication pattern in Klamelia and Ferganodon gen. nov. may have independently evolved. In Ferganodon gen. nov., cusp d of the preceding molariform most probably was overlapped labially by mesiolabial cusp f; consequently, mesiolingual cusp e is considerably reduced. In Klamelia the situation is the opposite: mesiolabial cusp f is greatly reduced or lost, while mesiolingual cusp e is enlarged and overlaps cusp d of the preceding tooth lingually (Fig. 2d). Complementary to the peculiar parallelogram-shaped outline of the molariforms, Ferganodon gen. nov. and Klamelia can be united by the presence of a unique distolabial cusp and vertical folding of the enamel on the labial flank, which are not present in other eutriconodontans including gobiconodontids. Although the distolabial cingulid cusp is reduced in Klamelia compared to the condition of Ferganodon gen. nov., it is still clearly evident on m3–4 in the holotype of. K. zhaopengi (Fig. 2a).
The structure of dentary and dentition of Klamelia is remarkably convergently similar to those in Late Cretaceous stagodontid marsupials (Fox and Naylor 2006): the dentary is robust and high anteriorly, with large mental foramina, shortened premolars/premolariforms, with the ultimate premolar/premolariform very large, transversely expanded, and approximating the mandibular symphysis. This convergence may indicate a similar diet in both taxa, possibly associated with the crushing of shelly invertebrates.
Klamelia and Ferganodon gen. nov. document a previously unknown radiation of gobiconodontid-like eutriconodontan mammals. This radiation occurred much earlier than the Early Cretaceous radiation of gobicondodontids and possibly was confined to a narrow geographic area in Central Asia incorporating present-day Xinjiang and Kyrgyzstan, in contrast to the much wider, pan-Laurasian distribution of the gobiconodontids (Cuenca-Bescos and Canudo 2003; Jenkins and Schaff 1988; Kielan-Jaworowska and Dashzeveg 1998; Li et al. 2003; Maschenko and Lopatin 1998; Meng et al. 2005; Minjin et al. 2003; Rougier et al. 2001; Trofimov 1978; Wang et al. 2001).
The Middle Jurassic mammal fauna of the Balabansai Svita in Kyrgyzstan is only at the beginning of its exploration and study, and is currently represented by three to four species of Docodonta and Eutriconodonta (Martin and Averianov 2004 and this report). Roughly coeval late Middle Jurassic to early Late Jurassic strata in the Junggar Basin, northwest China so far produced remains of Haramiyida and Docodonta (Maisch et al. 2005; Pfretzschner et al. 2005). Ongoing investigations will show if the currently observed differences between these regional faunas reflect a temporal and/or geographic or environmental disparity or are mostly affected by a sampling bias.
Field work in Kyrgyzstan in 2000–2003 was carried out in cooperation with the M. M. Adyshev Institute of Geology of the National Academy of Sciences of the Kyrgyz Republic. The former director of this institute, Prof. Apas A. Bakirov (Bishkek) is thanked for his support. Zhe-Xi Luo and two anonymous reviewers provided valuable comments. Wolfgang Müller (Freie Universität Berlin) and Katrin Krohmann (Forschungsinstitut Senckenberg, Frankfurt am Main) assisted in the SEM. This project was funded by the Deutsche Forschungsgemeinschaft (DFG; Ma 1643/8 and 436 RUS 113/602/0-1-2). The work of TM was additionally supported by a Heisenberg grant (DFG) and the work of AA by a Russian Fund of Basic Research (RFBR) grant 04-04-49113, President’s of Russia grant MD-255.2003.04, and the Russian Science Support Foundation.