Osteology of some catfishes of the genus Glyptothorax (Teleostei: Siluriformes) of northeastern India

Blyth (1860) described the genus Glyptothorax, for which Bleeker (186263) designated Glyptosternon striatus McClelland as the type species. Glyptothorax is the most species-rich and widely distributed genus of the family Sisoridae. The genus has as many as 81 valid species (Ferraris 2007; Gopi 2007; Vishwanath & Linthoingambi 2007; Ng & Freyhof 2008; Ng & Kottelat 2008; Ng & Rainboth 2008). de Pinna (1996) casts doubt on the monophyly of the genus Glyptothorax, citing the lack of unambiguous synapomorphies to diagnose it. However, his phylogenetic analysis of the Sisoroidea still recovered a monophyletic Sisoridae. Although there are reports on the osteological studies of Glyptothorax, viz., Gauba (1966) on G. cavia (Hamilton) and Diogo et al. (2002) on G. fukiensis (Rendahl), there are no reports of the comparative accounts on the osteology of the genus. This study attempts to identify the variation of bones i.e., premaxilla, dentary, infraorbital series, frontal, vomer and articulation of complex centrum with remaining vertebral column in eight species of Glyptothorax of north east India, viz. Glyptothorax botius (Hamilton), G. cavia (Hamilton), G. chindwinica Vishwanath & Linthoingambi. G. granulus Vishwanath &


Author Contribution:
The study: AD -survey, collection, morphometric and anatomic study of catfishes of northeastern India and their phylogenetics; NA -survey, collection, morphometric and anatomic study of catfishes of northeastern India and their phylogenetics; WV -supervision of taxonomy and phylogeny of freshwater fishes of northeastern India. Current paper: AD -detailed examination of collections, comparision with specimens in ZSI and preparation of drawings; NA -detailed examination of the collections of Glyptothorax in MUMF and osteological studies; WVsupervision of the work, interpretation of the results and comparison with available literature.

MATerIAl AND MeThoDS
Fresh specimens of Glyptothorax were collected from different areas of northeastern India. Measurements of antero-posterior length and lateral-extent of upper jaw tooth band follow Vishwanath & Linthoingambi (2007). Specimens were cleared and stained following Hollister (1934). Terminology and nomenclature of bones follow Chen &Lundberg (1995) andde Pinna (1996). Components of premaxillary bones are named as proximal, distal and posterior elements based on their positions.
Visible jaw structures of holotype of Glyptothorax burmanicus in ZSI Kolkata was also observed and compared.
reSulT Premaxilla: In six species, viz., Glyptothorax botius,G. granulus,G. manipurensis,G. ngapang,G. striatus,and G. ventrolineatus,the premaxilla (Figs. 1A & 1B) consists of four bony elements, i.e., a pair of medially located proximal elements (prp) and another pair of distal elements (dp), located lateral to the proximal element. The convex anterior margin of the distal element articulates with the corresponding concavity along the posterolateral margin of the proximal element. The distal element is firmly united to the proximal end by rigid connective tissue. However, in G. cavia and G. chindwinica, the premaxilla comprises of numerous distal ( Fig. 1D: dp) and posterior elements ( Fig. 1D: pop) in addition to a pair of proximal elements ( Fig. 1D: prp). The distal and posterior elements are attached respectively to the lateral and the posterior parts of the proximal elements. All the subunits of the proximal, distal and posterior elements are tightly fastened by connective tissue to form the premaxilla. The sutures among the elements that comprise the premaxilla are not visible ventrally because the presence of numerous villiform teeth ( Fig. 1C: vt) on this surface obscures it. Ventrally the premaxilla is seen as a single structure on the roof of the oral cavity (Fig. 1C).
Dentary: In the six species, viz., Glyptothorax botius, G. granulus, G. manipurensis, G. ngapang, G. striatus, and G. ventrolineatus, the anterior portion of the dentary is slender, with villiform teeth on its dorsal surface (Figs. 2A & 2B). However in G. cavia and G. chindwinica, the dentary is a stout, long, curved bone with a very broad anterior two-third portion. This broadness is manifested by a posterior extension of the tooth-bearing surfaces on the dentary ( Fig. 2C & 2D: pdtp) which bears villiform teeth at its dorsal surface. Gauba (1966) also reported the toothbearing portion of the dentary to be very much flattened in G. cavia. The dentary tooth plate of G. burmanicus is also similar to that of G. cavia and G. chindwinica.
Posterior portion of Weberian lamina: The transverse process or parapophysis of complex centrum was named the Weberian lamina by de Pinna (1996). In the present study, all the examined species of Glyptothorax  Infraorbital: The number of infraorbital bones is variable. Glyptothorax botius has six infraorbital bones; of which the sixth is longest while the fifth, the shortest. Both G. ngapang and G. chindwinica have nine infra-orbital bones, while G. cavia has ten. In the remaining species, there are eight bones in the series. The third infra-orbital in G. ngapang (Fig. 4B: io 3 ) bears a broad ventral laminar process. Both G. cavia and G. chindwinica have a larger and broader body of the lacrimal (Fig. 4A: lac) when compared to other species examined. orbital notch: The orbital notch of the Glyptothorax is formed at the lateral margin of frontal as a shallow depression, forming an arc smaller than a semicircle ( Fig. 5C: on). G. honghensis Li (Zhou & Zhou 2005 : Fig.  6B), G. fukiensis (Diogo et al. 2002: Figs.1 & 2) and G. major (de Pinna 1996: Fig. 13) have also shallow orbital notches. However, in G. cavia and G. chindwinica, the notch is deep and forms an arc larger than a semicircle (Figs. 5A & 5B: on).
Vomer: The head of the vomer of Glyptothorax is edentulous and extended laterally along the entire length of the articular process of lateral ethmoid, reaching the articular facet for palatine at the lateral tip of each lateral ethmoid. In all the species examined (except in G. cavia and G. chindwinica), the anterolateral margin of the head of the vomer is concave resulting in the formation of a thin lateral process and another sharply pointed medial anterior tip (Fig. 6 B & C). In G. cavia and G. chindwinica, the head of vomer is very large and broad with roughly convex anterior margin (Figs. 5A & 5B: v; 6A).

DIScuSSIoN
Among the Siluriformes, the premaxilla of Glyptothorax (Tilak 1963;de Pinna 1996) and Bagarius Bleeker (Gauba 1962) is characteristic in having separate distal elements connecting laterally to the proximal element. Diogo et al. (2002) also reported the same structure in G. fukiensis (Rendahl). A subdivided premaxilla has also been reported in Glyptosternum reticulatum McClelland (Gauba 1969), and fragmentation of the distal element of premaxilla  into tripartite or multipartite structures have also been reported in Euchiloglanis kishinouyei Kimura (de Pinna 1996). This study shows that structure of premaxilla in most of the examined species have a pair of medially located proximal elements and another pair of distal elements situated laterally. However, the premaxilla in G. cavia and G. chindwinica is markedly different, consisting of numerous posterior elements and distal elements in addition to proximal elements. Gauba (1966) recorded the premaxilla of G. cavia as being generally segmented or fused to form an enormously broad band that extends a considerable distance posteriorly across the palate. However, he failed to notice the numerous individual tooth plates tightly attached by connective tissue. The holotype of Glyptothorax burmanicus Prashad & Mukherji (Image 1A) has been examined. It has a central depression in the thoracic adhesive apparatus (Image 1B) and a premaxilla in the form of a broad patch with minute villiform sharp teeth and a dentary with broad teeth-bearing plate (Image 1C), the characters, similar to those of G. cavia and G. chindwinica.
Large variations in osteological characters of the Weberian lamina, infraorbital series, the shapes of the vomer and frontal have been observed among the members of the genus. However, pending examination of more species of the genus, it is not possible to establish the paraphyly of Glyptothorax. This study will help future workers to some extent in the study of phylogenetic relationships within Glyptothorax.