J. 2001. (1965) 146, 150-174 The comparative morphology of the osteocranium and the Weberian apparatus of Tachysuridae (Pisces : Siluroidei) RAJ TILAK~ Department of Zoology, Birla College, Pilani, India (Accepted 9 February 1965) (With 77 figures in the text) The morphology of the osteocranium and the Weberian apparatus of 14 representa- tives of Tachysuridae (= Ariidae) has been described and compared. It has been shown that these features are characteristic of the family Tachysuridae (- Ariidae) and can also distinguish the genera and the species of this family. A key to the intergeneric and inter- specific characters has been provided. A few characters of morphological interest have been recorded. It has been shown that the erection of different genera for some species of the genus Tuchysurus (= Arius) by Munro (1955) is arbitrary and lacks support from morphology. The features of the oesteocranium and the Weberian apparatus support the fact that Tachysuridae are not primitive siluroids; they are at least more specialized than Siluridae, Plotosidae, and Bagridae. The advanced features of Tachysuridae are described. The morphological relationship of the superficial bone and its associated trough-like bony structure with the swim-bladder show that these bony formations develop from the tunica extema of the latter. Introduction ...... .. Material and methods . . .. The osteocranium . . .. .. The cranium .. .. .. The visceral skeleton . . .. The Weberian apparatus . . .. The pars sustentaculum . . The pars auditum . . .. Taxonomic considerations .... A key to the genera and species . . Contents .. .. .... .. .. .. .. .. .. .. .. .... .. .. .. .. .. .. The phylogenetic position of Tachysuridae . . The homology of the superficial bone . . .. Summary .. .. .. .. .. .. References . . .. .. .. .. .. Key to abbreviations on text figures . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. Page .. 1.50 .. 151 .. 151 .. 151 .. 155 .. 158 .. 158 .. 165 .. 167 .. 168 .. 169 .. 170 .. 171 .. 172 .. 173 Introduction According to Regan (1911) and Berg (1940), the family Tachysuridae (= Ariidae) represents a group of primitive siluroids, although Shelden (1937), based on a study of t Present address: Zoological Survey of India, Calcutta, India. 150
Transcript
J. 2001. (1965) 146, 150-174
The comparative morphology of the osteocranium and the Weberian apparatus of Tachysuridae (Pisces : Siluroidei)
R A J T I L A K ~ Department of Zoology, Birla College, Pilani, India
(Accepted 9 February 1965)
(With 77 figures in the text)
The morphology of the osteocranium and the Weberian apparatus of 14 representa- tives of Tachysuridae (= Ariidae) has been described and compared. It has been shown that these features are characteristic of the family Tachysuridae (- Ariidae) and can also distinguish the genera and the species of this family. A key to the intergeneric and inter- specific characters has been provided. A few characters of morphological interest have been recorded. It has been shown that the erection of different genera for some species of the genus Tuchysurus (= Arius) by Munro (1955) is arbitrary and lacks support from morphology. The features of the oesteocranium and the Weberian apparatus support the fact that Tachysuridae are not primitive siluroids; they are a t least more specialized than Siluridae, Plotosidae, and Bagridae. The advanced features of Tachysuridae are described. The morphological relationship of the superficial bone and its associated trough-like bony structure with the swim-bladder show that these bony formations develop from the tunica extema of the latter.
Introduction . . . . . . .. Material and methods . . . . The osteocranium . . . . . .
The cranium . . .. .. The visceral skeleton . . . .
The Weberian apparatus . . . . The pars sustentaculum . . The pars auditum . . . .
Taxonomic considerations . . . . A key to the genera and species . .
Contents
.. ..
. . . .
. . . .
.. ..
. . . .
. . . .
. . . .
.. ..
.. ..
.. .. The phylogenetic position of Tachysuridae . . The homology of the superficial bone . . .. Summary . . .. .. .. .. .. References . . .. .. .. .. . . Key to abbreviations on text figures . . ..
Introduction According to Regan (1911) and Berg (1940), the family Tachysuridae (= Ariidae)
represents a group of primitive siluroids, although Shelden (1937), based on a study of t Present address: Zoological Survey of India, Calcutta, India.
150
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 151
the pelvic girdle of Hexanematichthys (Galeichthys) felis, expressed doubt over this hypo- thesis. Recently the present author has shown that Tachysuridae possess more advanced type of pectoral and pelvic girdles than those of many other siluroids (Tilak, 1963d, 1965). However, evidence for remarks could not then be made on their relationships with other siluroids because the available descriptions of other morphological features, such as the osteocranium and the Weberian apparatus, were too sketchy to allow careful comparisons. In recent times, it has been demonstrated that the osteocranium and the Weberian apparatus conjointly form an important tool in assessing the phylogenetic relationships amongst fishes (Krumholz, 1943; Nelson, 1948; Robins & Raney, 1956; Tilak, 1961, 1963a,b,c, 1964a,b; Weitzman, 1954, 1962; Woolcott, 1957). Nevertheless, owing to the lack of adequate knowledge of the morphology of the osteocranium and the Weberian apparatus to the fishes of the family Tachysuridae, no such assessment of these fishes could be made with exactitude. Hence in the present work an attempt has been made to make a detailed comparative study of the osteocranium and the Weberian apparatus of fourteen representa- tives of Tachysuridae, together with a record of a few important observations of morpho- logical interest, so as to have a better insight into the phylogeny of this group.
Material and methods The following fishes were collected from the esturine and coastal waters of India for the present
The methods used for the present study are in general those of Tilak (1964a).
3. The osteocranium
The cranium
The bones of the skull in these fishes are suturally united to each other but a cephalic shield as observed in other siluroids by Gregory (1933), Merriman (1940) and Nawar (1954) is not formed.
The ethmoid (or supraethmoid, SETH) is well developed, broad, and with primitive lateral processes. The anterior tip of the bone in Tachysurus (Figs 1 , 3, 4, 6 ) is notched. In Osteogeniosus militaris the anterior tip is almost flat whereas in Batrachocephalus mino (Fig. 7 ) it is slightly convex. In T. sagor and 0. militaris the posterior part of the bone forms nearly half of the anterior fontanelle. In the rest of the species of Tachysurus only one-third of the anterior fontanelle is formed by this bone (Figs 1, 3 , 4 , 6) . In B. mino the ethmoid forms only the anterior tip of the anterior fontanelle.
Each lateral ethmoid (or ectethmoid, ECTH) articulates with the frontal by two facets (Figs 1 to 7) except in T. sagor where it articulates with one facet. A small cartilaginous
152 R. T l L A K
FIG. I . Skull of T. serrutusdorsal view. FIG. 2. Skull of T. serrutrrs-ventral view (infraorbitals removed). FIG. 3. Skull of T. coelutus4orsal view. FIG. 4. Skull of T. gugoru4orsal view.
C O M P A R A T I V E MORPHOLOGY O F T H E O S T E O C R A N I U M I53
piece is present between these bones in all the species studied here except in 0. militaris and B. mino.
The prevomer (VO) bears a pair of tooth bands in T. serratus, T. thalassinus, T. coelatus, T. sagor and T. sona but in 0. militaris, B. mino, T. gagora, T. subrostratus, T. malabaricus, T. fukarius, T. arius and T. platystomus it is edentulous. In T. serratus, T. thalassinus and T. gagora a flat toothed plate is attached on either side of the prevomerine band of teeth (Fig. 14). The prevomerine dentition shows differences among other siluroids also (Nawar, 1954; Srinivasachar, 1958; Tilak, l961,1963a, 1964a). The reduction in size of the prevomer is an advanced feature according to Bhimachar (1933).
c 6
FIG. 5. Skull of 0. millfaris-ventral aspect (infra-orbitals removed). FIG. 6. Skull of T. fhalassinus-dorsal view.
A small portion of each frontal takes part in the formation of the dorsal side of the orbit. The fontanelles on the dorsal side of the skull have been used as taxonomic characters by David (1935), Merriman (1940), Taranetz (1938), and Tilak (1961, 1963a,b,c, 1964~). Segemehl (1891) did not attach much importance to the presence of fontanelles. The disposition of the fontanelles is an important taxonomic feature in tachysurid fishes studied. In 0. militaris and T. sagor the frontals enclose half and in the rest of the species of Tachysurus studied here two-thirds of the anterior fontanelle (Figs 1, 3, 4, 6, 7, AF). In B. mino the frontals enclose both the anterior and the posterior fontanellesalmost completely (Fig. 7). In T. serratus (Fig. l), T. malabaricus, and T. thalassinus (Fig. 6) the posterior fontanelle is rather small and is completely enclosed by the frontals. The posterior fontanelle is formed by both the frontals and the ethmoid.
The supratemporal is uniformly present in the members of this family (Figs 1, 3 ,4 ,6 , 7, ST). It is quadrangular, thin, covers the epiotic, and articulates with the supraoccipital, pterotic, and posttemporal. Sub-temporals are absent in these fishes. The posttemporal
154 R. T I L A K
(PT) has only a ligamentous connection with the cleithrum while it has acquired a firm sutural union with the cranium. In most of the cases it bears a foramen between it and the pterotic. In the families Siluridae, Plotosidae and Bagridae, the post-temporal has only a ligamentous connection with the pterotic and the epiotic. The elimination of the post- temporal from the pectoral series and its gradual inclusion into the skull in siluroids has been taken as advanced features (Tilak, 19636).
There has been a considerable amount of confusion in the terminology of the lachyrmal and the infraorbital bones. Weitzman (1962) has reviewed and discussed the important publications in connection with the terminology of these bones and concluded that the bones, so far named adnasal or lachrymal, be designated as antorbital, while the rest of the bones around the orbit, bearing the infraorbital sensory canal, as infraorbitals. It may be stated here that the antorbital of characins (Weitzman, 1962), like the supraorbital, does not have a portion of the latero-sensory canal and hence it cannot be similar to the bone named lachrymal in siluroids, where the latero-sensory canal extends into it. The nature and disposition of the antorbital of Brycon meeki (Characidae) (Weitzman, 1962) suggests that it is a part of the supraorbital series. Kindred (1919) in IctaZurus and Gosline (1961) in cyprinoids have interpreted that the antorbital has fused with lachrymal. The supraorbital series is absent in the siluroids studied here as well as elsewhere (Tilak, 1961, 1963a,b,c, 1964a,b) and therefore the present author is not sure whether any extra element (the antorbital) has fused with the lachrymal. The lachrymal described here, lies much anterior to the orbit, although it is posteriorly connected with the infraorbital bone through latero-sensory canal. The shape and size of lachrymal also differentiates it from the infra- orbital bones. Considering the nature and relationships of lachrymal (containing a part of the latero-sensory canal) in Tachysuridae and other siluroids studied by Tilak (1 961, 1963a,b,c, 1964a,b) the present author is of the opinion that this bone is not similar to the one named antorbital in characins by Weitzman (1962). Hence this bone has been called lachrymal in the present study. The rest of the bones following behind the lachrymal and enclosing the infraorbital laterosensory canal, have been identified as infraorbitals, as was done by Lekander (1949) and Weitzman (1962) because homologization of these bones is difficult in the present state of knowledge. The infraorbitals have been earlier differentiated into antorbitals, suborbitals and postorbitals by Tilak (1961, 1963a,b,c, 1964a,b). The infraorbitals in Tachysuridae studied here (lo1, lo,, lo,) are three long and stout bones. The third infraorbital has a curved posterior end which receives the sensory canal from the sphenotics while the first infraorbital conveys the sensory canal to the lachrymal.
The sphenotic (SPH) and the pterotic (SQPT) form an articular facet for the hyomandi- bular; the former contributing a major part. The prootics (PRO) and the pterotics form an auditory bulla which contains a big lapillus.
Regan (191 1) stated that the epiotics are rarely prominent in siluroids and Nawar (1954) reported them absent in Clarias lazera. However, the epiotics are exceptional in the fishes studied here in being well developed and in having the posterior lamellar extensions (EPL) which interdigitate with the diagonal ridges of the complex vertebra (DRTP).
Although, the supraoccipital (SOC) is a prominent bone, it takes no part in the formation of the foramen magnum. The foramen magnum is completely enclosed by the exoccipitals. The horizontal plates of the exoccipitals cover the lagenar and the saccular recesses and also the cavum sinus imparis. The occipital region of all these fishes is variously excavated to form an occipital fossa which is a sign of advancement (Bhimachar, 1933).
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 155
The paired ventral articulating processes of the basioccipital, independent in other siluroids (Joseph, 1960; Tilak, 1963c), appear to have fused medially into a cone-like midventral sub-pharyngeal process. The pterosphenoids take part in the formation of the lateral part of the foramina for the second to fifth and the seventh cranial nerves. They exclude the sphenotics from the nerve foramina. The cranial cavity does not extend beyond the anterior part of the orbitosphenoid and thus, the skull is not platybasic. The skull is platybasic in other siluroids (Kindred, 1919; Bhimachar, 1933; Srinivasachar, 1956, 1957), except Horabagrus brachysoma (Gunther) of the family Bagridae (Tilak, 1964b).
The visceral skeleton
The mandible (Figs 8, 10, 11, 12) is composed of the dentary (D), the articular (ART), and the coronomeckelian (the splenial of Bhimachar, 1933, SP). On the medial aspect of the mandible the coronomeckelian is suturally attached on the anterior end of the articular.
SETH
y s o c s
8 R T
D' ARTF
SCART
MCKCAR~ sP 1 ,ART AYTC ' 11 *
ARTC
FIG. 7. Skull of ktruchocephalus minodorsal view. FIG. 8. The mandible of B. mino-medial view. FIG. 9. The upper jaw bones of 0. milituris-medial aspect. FIG. 10. The mandible of T. serrutus-medial aspect. FIG. I 1. The mandible of T. gugoru-medial aspect.
The coronomeckelian has been reported to be absent in Heteropneustes fossilis (Bloch) and Clarias batrachus by Srinivasachar (1958), although it is present in Clarias lazera according to Nawar (1954) and in Clarias batrachus according to Tilak (19636).
Regan (191 1) stated that the pterygoid (ectopterygoid) has disappeared in Tachysuridae (Ariidae) (except Aelurichthys) and many other siluroids because the metapterygoid has moved dorsal to the quadrate. Gregory (1933) pointed out that such a condition does not
11
156 R . T I L A K
exist in the chondrocranium of 10 mm Ameiurus figured by Kindred (1919) and the ptery- goid (ectopterygoid) was in its normal position. In the present study of the members of the family Tachysuridae, the three pterygoid bones, i.e. the metapterygoid, the ectopterygoid and the endopterygoid, have been observed in their usual positions. The endopterygoid (the mesopterygoid of Regan, 1911, and Gregory, 1933) lies dorsal and medial to the ectopterygoid (pterygoid of Regan, 1911 and Gregory, 1933). The relationships of the ectopterygoid and the endopterygoid in Tachysuridae are as in Characidae (Harrington, 1955; Weitzman, 1962) except that the two bones lie closely applied in the latter while in the former they lie away from each other because of their reduced size. David (1935) in Clariidae, Merriman (1940) in Galeichthys felis and Bagre marinus, and Nawar (1954)
ARTF
LG
EC
FIG. 12. The upper jaw bones of T. mulaburicus-medial aspect. FIG. 13. The upper jaw bones of B. mino-medial aspect. FIG. 14. The opercular and upper jaw bones of T. gagora-lateral view. FIG. 15. Urohyal of 0. milituris-antero-lateral aspect. FIG. 16. The urohydal of T, serrutusdorso-lateral aspect. FIG. 17. The urohyal of B. mino-antero-lateral view. FIG. 18. The urohyal of T. .qayora-antero-lateral aspect.
in Clarias Iazera have shown the presence of only metapterygoid and ectopterygoid elements (pterygoid of Regan, 191 1, and Gregory, 1933). In Heteropneustes fossilis there are three pterygoid bones according to Srinivasachar (1958). The presence of all three pterygoid bones is a primitive feature according to Bhimachar (1933) while the reduction in their size and number is a specialized feature according to McMurrich (1884), Nawar (1954), Srinivasachar (1958) and Joseph (1960).
The ectopterygoid (ECTPT) lies anterior to the metapterygoid and ventral to the endo- pterygoid. The teeth on the ectopterygoid are either villiform as in T. sona, T. serratus, T. coelatrrs, T. sagor, T. thalassinus, and T. subrostratus or they may be molariform as in
C O M P A R A T I V E M O R P H O L O G Y OF T H E O S T E O C R A N I U M 157
T. fulcarius, T. gagora, T. jella, T. malabaricus, T. platystomus, 0. militaris, and B. mino. In T. serratus, T. thalassinus, and T. gagora, in addition to the three pterygoid bones, there is an extra toothed bony plate (A") attached on the ventral aspect of the palatine and the lateral ethmoid of each side. The nature of the teeth on these plates resembles those present on the ectopterygoid. The homology of these bones is not clear; possibly they could be dis- placed and modified autogenous dermal toothed elements.
AUA /u
F I ~ . 19. The upper jaw bones of T. serrutus-lateral view. FIG. 20. The branchial basket of 0. rnilitaris. FIG. 21. Operculum and interoperculum of T. thalassinus. FIG. 22. Operculum and interoperculum of T. muluburicus.
The endopterygoid (ENTPT) is a bent rod which is connected by a ligament to the posterior end of the palatine, the anterior end of the metapterygoid, and the ventral side of the lateral ethmoid. The metapterygoid is large and flat. It articulates ,suturally with the quadrate and the hyomandibular (Figs 9, 13, 19), while its medial process articulates with the orbitosphenoid by a ligament. The quadrate is quadrangular and gives articulation to the mandibles. The anterior and the posterior processes of the hyomandibular are prominent. The preopercular (PREOP) is attached to the posterior side of the hyomandi- bular and the quadrate and takes part in the formation of a foramen between it, the hyomandibular and the quadrate.
There is a strip of cartilage between the hyomandibular, the quadrate, and the pre- operculum like that of other siluroids (Bhimachar, 1933; Tilak, 1961). The cartilage has been considered as a remnant of the symplectic by McMurrich (1 884), Kindred (1919), and Bhimachar (1933), although Srinivasachar (1958) and Joseph (1960) do not agree with this view. However, the constant presence of this cartilage in all the siluroids, so far studied, may be of significance.
158 R. T I L A K
The hyobranchial apparatus (Fig. 20) has the same general pattern as observed in other siluroids (Tilak, 1961, 1963a,c, 1964a). The members of this family differ from one another in the number of the branchiostegal rays (BR) attached on the ceratohyal and the epihyal (Table I). The urohyal (Figs 15, 16, 17) has the usuaLshape found in other siluroids. In
TABLE I
Branchiostegal rays on Ceratohyal Epihyal
A. serratus A . tlialassinus A. coelatrrs A. sagor A. sona A. subrostratus A. arius A. fulcarius A. gagora A. jella A. malabaricus A. platystomiis B. mino 0. militaris
3 4 3 5 4 5 5 4 4 5 4 6 4 3
2 1 2 1 1 I 1 1 1 1 1 1 1 2
T. gagor (Fig. 18) the anterior dorsal concavity (DOC) is absent and the anterior surface of the urohyal forms an oblique plate (OPL). The shape and the size of the operculum differ in the different species studied here (Table 11). Bhimachar (1933) reported the presence of a reduced suboperculum in 0. militaris. However, no suboperculum could be observed in any of the siluroids studied here. The last branchiostegal ray, which is well developed, lies in the position of the sub-operculum and serves the same purpose.
The Weberian apparatus
The Weberian apparatus of the different species of Tachysurus, Osteogeniosus, and Batrachocephalus resembles those of Tachysurus pidada (Arius pidada, Bridge & Haddon, 1893) and Tachysurus platystomus (Day) (Arius platystomus Day, Krandikar & Masurekar, 1954). There are, however, differences in detailed morphology and these are presented here under two heads.
The pars sustentaculuni
An intercalated cartilaginous piece is embedded into the anterior grooved end of the third neural spine (Bridge & Haddon, 1893). This cartilaginous piece represents the neural spine of the first vertebra on the basis of its morphological relationships with the related parts (Tilak, 1964a). There is another small bony piece (Figs 34, 38, 58, 61, NS,) lying embedded between the supraoccipital and the third neural spine and visible from the dorsal
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 159
side. This piece, which is not mentioned by Bridge & Haddon (1893), represents the neural spine of the second vertebra and it is present in all the tachysurid fishes studied. In the rest of the siluroids, the second and the third neural spines have fused to form a compound neural spine (Tilak, 1964a) except in PZotosus canius (Tilak, 1963~). The cartilaginous neural spine of the first vertebra and the neural spine of the second vertebra have not been reported by earlier authors.
The representatives of the genera Tachysurus, Batrachocephalus, and Osteogeniosus show differences among themselves in the detailed morphology of the pars sustentaculum and the pars auditum.
TABLE I1
Operculum Shape of Shape of Angle of posterior dorsal inferior
border
Opercular Maximum length Maximum width arm border border
A. serralus A. tholassinus
(Fig. 21) A. coelatus
(Fig. 29) A. sagor
(Fig. 26) A. sona
(Fig. 25) A. subrostratus
(Fig. 24) A. ariiis
(Fig. 30) A. fiilcariiis
(Fig. 33) A. gagora
(Fig. 23) A. jella
(Fig. 31) A. nialabaricus
(Fig. 22) A. platystornus
(Fig. 28) 0. militaris
(Fig. 27) B. niino
(Fig. 32)
Concave Concave
Concave
Concave
Wavy and straight Wavy and straight Concave
Concave
Concave
Wavy
Convex
Diagonal
Convex
Straight
Convex Convex
Concave
Concave
Diagonal
Concave
Concave
Concave
Convex
Concave
Concave
Concave
Concave
Concave
Prominent Prominent
Prominent
Prominent
Medium
Medium
Medium
Mild
Prominent
Prominent
Prominent
Prominent
Prominent
Prominent
25.0" 21.0"
20.0"
25.0'
32.5"
23.5"
21.0"
20.0"
22.5"
23.0'
27.0
25.0"
32.5"
19.0"
1.8 1.9
1.85
1.4
1 .5
1.43
1 6 4
1-7
1.57
1.71
1.41
1.47
1.58
2.7
Tachysuridae is the family with the largest number of vertebrae suturally united behind the complex centrum. Only one vertebra is united behind the complex centrum in Sisoridae (Tilak, 1963a), Clariidae (Tilak, 19633), Siluridae and Plotosidae (Tilak, 1963c) and Hetero- pneustidae (Tilak, unpublished) and two in Schilbeidae (Tilak, 1964a) and Bagridae (Tilak, 19643). Among the Tachysuridae, the lowest number of vertebrae added to the
R. T I L A K
FIG. 23. Operculum and interoperculum of T. gagoru. FIG. 24. Operculum and interoperculum of T. subrostrutus. FIG. 25. Operculum and interoperculum of T. sona. FIG. 26. Operculum and interoperculum of T. sagor. FIG. 27. Operculum and interoperculum of 0. mifiraris. FIG. 28. Operculum and interoperculum of T. platystomus.
FIG. 29. Operculum and interoperculum of 7. coelutus. FIG. 30. Operculum and interoperculum of T. urius. FIG. 31. Operculum and interoperculum of T. iellu. FIG. 32. Operculum and interoperculum of B. mino. FIG. 33. Operculum and interoperculum of T. fulcariits.
C O M P A R A T I V E M O R P H O L O G Y OF T H E O S T E O C R A N I U M 161
complex vertebra is two in T. jella and the highest four in T. thalmsinus and T. arius. Commonly three vertebrae are added to the complex vertebra in the remaining species of Tachysurus, Batrachocephalus, and Osteogenioms (Figs 34 to 41). The eighth centrum in T. thalassinus and T. arius is only partly invested by the superficial bone. The fifth to eighth vertebrae are united to the complex vertebra partly by sutures and partly by the underlying superficial bone. The great number of vertebrae which have united with one another in this region help to support the well developed swim-bladder.
FIG. 34. The pars sustentaculum of T. arius-dorsal view. FIG. 35. The pars sustentaculum of T. rhalassinus-dorsal view. FIG. 36. The pars sustentaculum of 0. milirarisdorsal aspect. FIG. 37. The pars sustentaculum of T. gagoradorsal view.
The amphicoelous centrum of the first vertebra (Figs 49 to 52,55,58,59), which remains completely embedded in the superficial bone, bears a pair of rudimentary dorso-lateral parapophyses (P)’ and also ventral processes (VP) which may be fused into one. The ventral processes are paired and separate in T. serratus (Fig. 55), T. subrostratus (Fig. 49). T. fulcarius (Fig. 52), T. arius, T. jella, T. sona, T. malbaricus (Fig. 59), T. platystonius, and 0. militaris. They are very close to each other at their bases in T. thalassinus (Fig. 51) but in T. gagora (Fig. 50) they are still further amalgamated into a single median ventral process which is bifid at its tip. In T. sagor and T. coelatus the ventral processes
162 R . T I L A K
are absent. In B. mino there is a single median ventral process (Fig. 58). Bridge & Haddon (1893) have made no mention regarding the ventral processes of the first vertebra.
The distinction between the anterior and the posterior parts of the fourth parapophysis is almost lost in tachysurid fishes and they cover the swim-bladder on the dorsal and anterior sides (Bridge & Haddon, 1893; Krandikar & Masurekar, 1954). Such a type of fourth parapophysis should be more advanced than that of Siluridae, Plotosidae (Tilak,
41 SPP
40
FIG. 38. The pars sutentaculum of T. serrarus-dorsal view. FIG. 39. The pars sustentaculum of T. sagor-dorsal view. FIG. 40. The pars sustentaculum of B. mino-dorsal view. FIG. 41. The pars sustentaculum of B. mino-ventral view.
1963~) and Bagridae (Tilak, 19646) where the anterior and the posterior parts of this parapophysis are completely separated or show union only at their bases. The diagonal ridge (DRP4) on the dorsal side of the fourth parapophysis is formed in relation to the backward lamellar extension of the epiotic bone (Figs 34 to 40). The shape and disposition of the diagonal ridge correspond to the disposition of the lamellar extension of the epiotic. Posteriorly, the diagonal ridge starts from the dorsal part of the fourth parapophysis and runs obliquely anteriad to end on the lateral side of the neural spine of the third vertebra. A transverse ridge starts from the middle of the diagonal ridge and meets the base of the fourth neural spine. The diagonal ridge is in firm sutural union with the posterior lamellar
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 163
extension of the epiotic except in its most anterior and posterior regions. Anteriorly, where it does not unite with the lamellar extensions of epiotic, it leaves a foramen between the third neural spine and the epiotic. The characteristic shape, which the dorsal aspect of the complex vertebra shows, depends upon:
(i) The extent of the median ridge which connects the third and the fourth neural
(ii) The development of the transverse ridge; (iii) The extent of the approximation of the anterior halves of the diagonal ridge to
The pattern, which the dorsal aspect of the complex vertebra shows, differs in the differ- ent members of the family Tachysuridae studied here. A simple condition as described above is found in T. subrostratus and T. malabaricus but the median ridge is absent in the former whereas it is short in the latter. Tachysurus coelatus, T . fulcarius, T. jella, T. sona, and T. arius (Fig. 34), also have the basic pattern as in the former two species. But T. sona is distinguished from them in having a low median ridge whereas in others it is absent. Moreover, T. sona has a transverse ridge disposed dorsally and posteriorly in such a way that the two oblique ridges, in conjunction with the transverse ridge, enclose a wider space than that in the former species. Tachysurus thalassinus (Fig. 35) and T. sagor (Fig. 39) have a similar pattern of dorsal ridges because in both of them the comparative length of the vertebrae is more than the breadth as compared to the other species of this genus. Tachysurus sagor is differentiated from A. thalassinus in the respect that the fourth neural spine of the former slants more posteriorly than that of the latter. Thus the posteriorly dis- posed transverse ridges in T. sagor enclose a space which is different in shape from that of T. thalassinus. The median ridge is low in T. thalassinus but it is absent in T. sagor. In T. platystomus the diagonal ridges directly join the fourth neural spine. There is no transverse ridge. The diagonal ridges proceed anteriorly and articulate with the third neural spine close to each other. The portion of the oblique ridges posterior to the fourth neural spine has sutural union with the posterior lamellar extensions of epiotic but their portions anterior to the fourth neural spine remain independent of the epiotic lamellae. The median ridge is high. In 0. militaris (Fig. 36), two transverse ridges, arising from the diagonal ridges, fuse medially and continue with the fourth neural spine. Similarly, the anterior ends of the diagonal ridges fuse medially and continue with the third neural spine. The whole of the diagonal ridge has sutural union with the epiotic lamellae except in its anterior fused region. The median ridge is high. Thus the space enclosed by the portion of the diagonal ridges, anterior to the fourth neural spine, is lanceolate. Tachysurusgagora (Fig. 37) shows a further advance over this condition in that the anterior fused ends of the diagonal ridges, which attach on the third neural spine in 0. militaris, remain independent from the third neural spine. Thus all the diagonal ridges have sutural union with the epiotic lamellae which also fuse medially over the supraoccipital. There is a big foramen left behind the third neural spine on the anterior side and the fused anterior ends of the diagonal ridges on the posterior side. The median ridge is high. In B. mino (Fig. 40) the condition is similar to that of T. subrostratus but the transverse ridges are comparatively smaller in the former than in the latter.
The superficial bone is well developed in all the tachysurid fishes studied here. It inter- digitates with the basioccipital, although in Siluridae, Plotosidae (Tilak, 1963~)~ and
spines ;
each other.
164 R. T I L A K
APR 42 43
SPR
CPR
45 46 A PR CPR
SPR
CPR HPR 49
55 APR
PR CPR
AP
CP
59 ..I
il 60 63
DI
FIG. 42. The scaphium of T. sugor. FIG. 44. The scaphium and claustrum of T. muluburicus. FIG. 45. The claustrum, intercalarium and scaphium of 0. milituris. FIG. 46. The scaphium of T. jellu. FIG. 48. The scaphium of T. serrutus. FIG. 50. The first vertebra of T. gugoru. FIG. 52. The first vertebra of T. fulcarius. FIG. 54. The scaphium of T. coelatus. FIG. 56. The scaphium of T. gugoru. FIG. 57. The claustrum, scaphium, and intercalarium of T. thulusstirrs. FIG. 5 8 . The second neural spine and first vertebra of B. mino. FIG. 59. The first vertebra of T. muluburicus. FIG. 61. The second neural spine of T. serrulus. FIG. 63. The intercalarium of T. serrutus.
FIG. 43. The scaphium of T. subrostratus.
FIG. 47. The scaphium of T. fulcarius. FIG. 49. The first vertebra of T. subrostratus. FIG. 5 1. The first vertebra of T. thulussinus. FIG. 53. The scaphium of T. arius. FIG. 55. The first vertebra of T. serrufirs.
FIG. 60. The intercalarium of T. niulubnricus. FIG. 62. The claustrum of T. serrufus.
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 165
Bagridae (Tilak, 19643) they simply articulate and are bound together by a ligament. A “V” shaped trough of bone is attached on the ventral side of the superfkial bone (Fig. 41) which turns the aortic groove into a canal (AC). At the point of fusion of the basioccipital and the superficial bone, a bony sub-vertebral process is formed (SPP). The sub-vertebral process (Figs 2, 5, 41, SBP) is well developed and conical in T. coelatus and T. subrostratus. It is conical but of medium size in T. thalassinus, T. fulcarius, T. arius, T. jella, T. sona, T. malabaricus, B. mino and 0. militaris. In T. gagora it is well developed and bifid at the tip. In T. sagor it is blunt and flat at the tip.
The neural arches of the Mth to the seventh vertebrae meet dorsally except in T. sonu where they are partly open by foramina. The fifth to seventh vertebrae, in most of the cases, do not possess neural spines. In T. platystomus the sixth and the seventh vertebrae bear a pair of rudimentary neural spines. In 0. rnilitaris there is a pair of neural spines on the seventh vertebra. The seventh and the eighth vertebrae bear a pair of small neural spines in T. serratus.
The pars auditum
The claustrum is a very thin and long spicule of bone and lies closely adherent to the perineural arch (Figs 44, 45, 57, 62, CL). The claustrum has no contact with any part of the scaphium in these fishes. The scaphium is thick and has a normal shape. In B. rnino and T. fukarius (Fig. 47), T. malabaricus (Fig. 44), and T. coelatus (Fig. 54) the condylar process is demarcated from the ascending process as a small protuberance. The condylar process in T. arius (Fig. 53), T. thalassinus (Fig. 57), T. gagora (Fig. 56), T. subrostratus (Fig. 43), T. sona, T. sagor (Fig. 42) is demarcated but it is thick and is more or less continuous with the thickened base of the ascending. process. The thin posterior portion of the spatulate process of the scaphium is termed a handle by Krandikar & Masurekar (1954). In T. gagora, T. subrostratus and T. thalassinus the thick condylar process is separate from the ascending process by a notch. In T. serratus (Fig. 48) and T. jella (Fig. 46) the condylar process is not clearly demarcated from the rest of the scaphium.
In 0. militaris (Fig. 4 3 , T. thalassinus (Fig. 57), T. malabaricus, T. coelatus, and T. serratus (Fig. 63) a “V” shaped bony piece is present posterior to the condylar process of the scaphium. This is recorded for the first time. It has an anterior and a dorsal limb both of which are united to a thick nodular portion. The position and the structure of this piece suggests that its three parts represent the rudiments of the horizontal, the condylar, and the ascending processes of intercalarium as in Bagridae (Tilak, 19643, and unpublished work). The intercalarium disc (Fig. 60) has lost contact with these processes which are gradually disappearing.
The tripus is normal but shows differences in structure in the different species studied. The anterior process (APT) has the outer side convex and the inner concave and smooth. In T. sona (Fig. 66) the inner side is also convex and slightly wavy. The anterior side is truncated and forms an articular facet for the ligamentous tripodes (LT). The articular process bears a very poorly developed hamulus (H). The articular tip is strong and forms the only point of attachment on the lateral side of the complex centrum. In T. thalassinus (Figs 64, 65) and 0. militaris (Figs 73, 74) the hamulus is thin but in T. subrostratus and T. arius it is a thick ridge. Bridge & Haddon (1893: 136) remarked that a heel like process (as found in Mystus = Macrones of authors) is absent in the species of Tachysurus
166 R. T I L A K
76 77 FIG. 64. Tripus of T. rhalussitius-dorsal aspect. FIG. 65. Tripus of T. rhalussinus-ventral aspect. FIG. 66. Tripus of T. sonu-ventral view. FIG. 68. Tripus of T. sagordorsal aspect. FIG. 70. Tripus of T. serrurus-ventral aspect. FIG. 71. Tripus of B. mino-ventral aspect. FIG. 72. Tripus with scaphium and intercalarium of B. m h o . FIG. 73. Tripus of 0. milituris-dorsal aspect. FIG. 75. Tripus of T. roelutus-ventral aspect. FIG. 77. Tripus of T. nialabariars--ventraI aspect.
FIG. 67. Tripus of T. sugor-ventral aspect. FIG. 69. Tripus of 7. serratrisdarsal aspect.
FIG. 74. Tripus of 0. niilitaris-ventral aspcct. FIG. 76. Tripus of T. ~riulabaricusdorsal aspect.
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 167
(= Arius) studied by them. However, it has been found here that the ventral ridge (VR) formed by the transformator process does not run along the posterior side of the articular process but it runs anteriorly or outwards. The anterior tip of the ridge points downwards and keeps away from the tripus except in T. sagor (Figs 67, 68) where it is fused with the body of the tripus. The present author designates the free anterior tip of the ventral ridge as “vertical hood” which is comparable to the heel like process of Mystus. The hood (HD) points anteriorly in T. thalassinus, T . subrostratus, T. arius, T . jella (Figs 69,70), T. serratus, and B. mino (Figs 71, 72) but in T. malabaricus and T. sona it points medially. In T. sagor, 0. militaris, and T. coelatus (Fig. 75) it points slightly laterally and forms a rudimentary hood in the former two species. The hood projects outwards. The transformator process (TPT) is narrow anteriorly but its posterior tip is much broader and forms a beak shaped structure. In T. malabaricus (Figs 76, 77) and T. sagor (Figs 67, 68) it is thin. In T. sona (Fig. 66) it is thick and broad but it is of moderate size in others.
Taxonomic considerations
The morphology of the osteocranium and the Weberian apparatus has been considered to be of taxonomic importance by Krumholz (1943), Chranilov (1929), Merriman (1940), Nelson (1948), Weitzman (1954, 1962), Robins & Raney (1956, 1957), Bailey (1959), and Tilak (1961, 1963a,b,c, 1964a). In the present study also the morphology of the osteocran- ium and the Weberian apparatus has been found to be helpful in distinguishing the genera and species. The patterns of bones of the skull and the Weberian apparatus conform to those of other siluroids (Bridge & Haddon, 1893; McMumch, 1884; Gregory, 1933; Bhimachar, 1933; Krandikar & Masurekar, 1954; Tilak, 1961, 1963a,c, 1964a) in their general plan but differ in a number of points from them (as well as among themselves) in detailed structure.
Day (1877) divided the species of the genus Tachysurus mainly into two groups; one where the palatine is covered with villiform teeth and the other where the teeth on the palate are molariform. Chandy (1953) used the number and shape of the tooth patches on the palate to distinguish the species of the genus Tachysurus. The teeth on the palate of the fishes studied here are ectopterygoid teeth which are sometimes combined with those of the prevomer. The type of teeth and the size of the tooth patches are uniform features for each species and distinguish these fishes at the species level. There are other features of taxono- mic importance in addition to those mentioned above. They are :
(i) the number and arrangement of the branchiostegal rays on the hyoid arch (Table I); (ii) the form of operculum and interoperculum (Table 11); (iii) the anterior and posterior fontanelles of the dorsal side of the cranium; (iv) the pattern of the diagonal ridges on the pars sustentaculum.
On the basis of these features the genera and species can be distinguished, as shown in the key to genera and species.
Fowler (1940), Chandy (1953), and Munro (1955) have replaced the name Arius Valenciennes 1840 by Tachysurus LacCpCde 1803, according to the law of priority. Weber & de Beaufort (1913) and Herre (1941) have used the name Arius instead of Tachysurus. In the present study the name Tachysurus has been used. Munro (1955) has followed Fowler (1940) and raised some of the species of the genus Tachysurus to generic level such
168 R . T I L A K
as T. sona to Hexanematichthys sona, T . thulassinus to Netuma thalassinus, T. serratus to Netuma serratus, T. platystomus to Pseudarius platystomus, and T. jella to Pseudarius jella. In the present study it has been found that the differences in the Weberian apparatus and the Osteocranium of the species of the genus Tachysurus are only of specific value and do not support a generic status for any of these species. The magnitude of differences between the species of the genus Tachysurus studied here is not of the same degree as that between the genera Tachysurus, Osteogeniosus and Batrachocephalus (genera and species key). Thus an arbitrary division of the genus Tachysurus as attempted by Munro (1955) would obscure the relationship between these fishes.
A key to the genera and species ( I ) Space enclosed between the diagonal ridges and the median ridge on the
dorsal side of the pars sustentaculum is lanceolate; median ridge high; anterior tip of ethmoid flat and convex; cartilage between lateral ethmoids absent.
Space enclosed by the diagonal ridges and the median ridge on the dorsal side of pars sustentaculum not lanceolate; median ridge low or absent; anterior tip of ethmoid deeply notched; cartilage between the lateral ethmoids present.
(2) Two ventral processes from first vertebra separate from each other; anterior tip of ethmoid flat; posterior tip of posterior fontanelle formed by supraoccipital; maxilla very long; half of anterior fontanelle formed by frontals; fifth and sixth neural spines absent; condylar process of scaphium less demarcated from spatulate process.
Ventral process of first vertebra single and median; anterior tip of ethmoid convex and broad; posterior fontanelle formed by frontals only; maxilla small; major part of anterior fontanelle formed by frontals; fifth and sixth neural spines present; condylar process of scaphium well demarcated.
(3) Anterior tip of the diagonal ridges on either side of pars sustentaculum widely separated; ectopterygoid with villiform teeth; prevomer mostly toothed.
Anterior tip of the diagonal ridges of both sides of pars sustentaculum close to each other or fused; ectopterygoid with villiform teeth; prevomer mostly toothed.
(4) An extra toothed piece attached on ventral side of metapterygoid. No extra toothed piece of bone attached on ventral side of metapterygoid.
( 5 ) Eight vertebrae form pars sustentaculum; ventral processes of first vertebra close together; condylar process of scaphium well demarcated.
Seven vertebrae form pars sustentaculum; ventral processes of first vertebra widely separated, condylar process of scaphium not demarcated.
(6) Sub-vertebral process blunt and flat; ethmoid forms half of anterior fontanelle; lateral ethmoid articulates with frontal by single facet and there is no foramen between them.
Sub-vertebral processconical; ethmoid forms one third of anterior fonta- nelle; lateral ethmoid articulates with frontals by two facets and thus leaves a foramen between them.
(7) Ventral processes of first vertebra absent; condylar and ascending processes of intercalarium present as a separate single piece; condylar process of scaphium well demarcated; vertical ridge connecting compound neural spine with fourth neural spine absent.
Genera Osteogeniosus and Batraclro- cephalus . . . (2) Genus Tachysurus . . . (3)
Genus Osteogeniosus Species 0. mifitaris
Genus Batrachocepliakts Species B. mino
. . . (4)
. . . (9)
. . . ( 5 )
. . . (6) . . . T. thalassinics
. . . T. serrafus
. . . T. sagor
. . . (7)
. . . T. coelatus
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M
Ventral processes of first vertebra paired and independent; condylar and ascending processes of intercalarium absent; condylar process of scaphium nearly differentiated; vertical ridge connecting compound neural spine and fourth neural spine present.
(8) Neural arches fifth to seventh meet dorsally; vertical ridge connecting . . . 7. subrosrrarus compound neural spine with fourth neural spine very low.
Neural arches fifth to seventh do not meet dorsally; vertical ridge con- . . . T. sonu necting compound neural spine with fourth neural spine low.
(9) Ventral processes of first vertebra fused at the base and separate at tips; . . . T.gagoru ventral ridge high; sub-vertebral process large and bifid at tip.
Ventral processes of first vertebra paired and independent; vertical ridge connecting the compound and the fourth neural spines absent; sub- vertebral process conical and single.
(10) Sixth to eighth vertebrae form pars sustentaculum; condylar process of scaphium not well demarcated.
Seven vertebrae form pars sustentaculum; condylar process of scaphium well demarcated.
( I 1) Six vertebrae form pars sustentaculum; condylar process of scaphium not . . . T. jellu demarcated; vertical ridge connecting the compound and the fourth neural spines low.
Eight vertebrae form pars sustentaculum; condylar process of scaphium . . . T. urius less demarcated; vertical ridge connecting the compound and the fourth neural spines absent.
(12) Medial ridge connecting the diagonal ridge with the fourth neural spine . . . T. malabaricus small; ascending and condylar processes of intercalarium present as a separate piece, vertical ridge connecting the compound and the fourth neural spines very low; posterior fontanelle small and enclosed by frontals.
Median ridge connecting the diagonal ridge and the fourth neural spine absent; vertical ridge connecting the compound and the fourth neural spines absent; posterior fontanelle large and formed by frontals and supraoccipi tal.
(13) The space enclosed between the anterior ends of the diagonal ridge and the . . . T. fulcarius fourth neural spines is broad and triangular.
The space enclosed between the anterior ends of the diagonal ridge and . . . T. pluytsromus the fourth neural spine is narrow and cone shaped.
. . . (8)
. . . (10)
. . . (11)
. . . (12)
. .. (13)
169
The phylogenetic position of Tachysuridae
Regan (1911) and Berg (1940) considered the Tachysuridae (= Ariidae) to be more primitive than the Siluridae and the Plotosidae and placed them next to Diplomystidae, the most primitive family among siluroids. On the basis of the present study as well as earlier ones (Tilak, 1963d, 1965), the status of the Tachysuridae as shown by Regan (1911) and Berg (1940) appears inappropriate. Tachysuridae have a number of advanced features in the osteocranium and the Weberian apparatus when compared with those of Siluridae, Plotosidae (Tilak, 1963c) and Bagridae (Tilak, 19643); hence the Tachysuridae should be recognized as a group of higher organization. The advanced features of Tachysuridae are:
(1) the bones of the skull are strong and united to one another by f-irm sutures; (2) the cranial cavity does not extend anteriorly further than the orbitosphenoid and
(3) the prevomer is small and edentulous; (4) the posttemporal has a sutural union with the skull and its connection with the
the skull is not platybasic;
pectoral girdle is only ligamentous (Tilak, 1963d, 1965).
170 K. T l L A K
(5) there is a complete fusion between the basioccipital and the superficial bone; (6) the lateral ethmoid articulates with the frontals by two facets except in T. sugor; (7) there is a large tympanic bulla formed by the prootic, the pterotic, and the exoc-
(8) the ectopterygoid is always toothed; the teeth may be villiform or molariform; (9) a major portion of the articular facet for the hyomandibular is formed by the
sphenotic alone; (10) the number of vertebrae involved in the formation of the pars sustentaculum is
six to eight; (1 1) the fourth parapophysis is much expanded so as to cover the swim-bladder com-
pletely on the dorsal and partly on the anterior sides. The distinction between the anterior and the posterior divisions of the parapophysis is lost;
(12) the transformator process of the tripus is well developed and forms a prominent ventral ridge;
(13) the sub-vertebral processes of the superficial bone have been replaced by a single mid-ventral cone shaped process;
(14) the pectoral symphysis is strong and the ridges of the cleithrum and the caoracoid are much developed (Tilak, 19634;
(1 5) the posterior processes of the basipterygium are bony (Tilak, 1965).
cipital ;
The above mentioned characters of the Tachysuridae clearly suggest that this family comes in the higher grade of specialization next to Siluridae, Plotosidae, and Bagridae. In addition to these characters, supporting evidence could be gathered from certain aberrant characters possessed by Tachysuridae alone, such as :
(i) the development of a “V” shaped trough of bone ventral to the enormously developed superficial bone. The aortic groove of Siluridae, Plotosidae, and Bagridae has become a canal ;
(ii) the epiotics develop posterior lamellae which help further in effecting an additional contact between the skull and the anterior vertebrae;
(iii) diagonal ridges are formed on the dorsal side of the pars sustentaculum; (iv) the mesocoracoid arch is absent in the pectoral girdle (Regan, 191 1 ; Tilak, 1963d). The features of the osterocranium and the Weberian apparatus of Tachysuridae show
a lesser grade of specialization when compared with those of Schilbeidae, Clariidae, Heteropneustidae, Loricariidae, Callichthyidae, and Sisoridae (Bridge & Haddon, 1893 ; Tilak, 1961, 1963b, 1964a,h, and unpublished work).
The homology of the superficial bone The homology of the superficial bone of the complex vertebra has been interpreted by
different workers. Wright (1 884) and Bridge & Haddon (1 893) claimed that it is an ossifica- tion from the wall of the swim-bladder. It has been considered as an ossification from the wall of the dorsal aorta by Sorenson (1 890). Krandikar & Masurekar (1 954) considered it as an extraordinary modification from the basapophyses of the complex vertebra. The basapophyses, according to Krandikar & Masurekar (1954), have joined mid-ventrally to form a trough like structure, which encloses the dorsal aorta in Tachysuridae. They further mentioned that the superficial bone is not in intimate connection with the fifth to seventh vertebrae. However, in the present study, it has been observed that the superficial
C O M P A R A T I V E M O R P H O L O G Y O F T H E O S T E O C R A N I U M 171
bone is present in all those vertebrae in contact with the swim-bladder. The ventro-lateral plates of the superficial bone have developed around the dorsal aorta only in the region of the swim-bladder. The posterior vertebrae of the abdominal or the post-abdominal region, where the swim-bladder does not extend, do not have the development of the super- ficial bone. The superficial bone does not develop prominently in those fishes where the swim-bladder is reduced and is enclosed in bony capsules (Clariidae, Heteropneustidae, and Sisoridae, Bridge & Haddon, 1893; Tilak, 1963b, 1964a,b, 1965). It is clear, therefore, that the superficial bone is formed only in relation to the swim-bladder. The ventro-lateral plates of the superficial bones are formed around the dorsal aorta, presumably to save the latter from being strangled by the pressure exerted by the swim-bladder. Moreover, in most of these fishes the swim-bladder has a very close contact with the superficial bone and when the former is removed, a part of the latter is also removed along with it. It is morpho- logically evident, then, that the superficial bone has been developed by an ossification from the outer hard covering of the swim-bladder (i.e. tunica externa) as opined by Bridge & Haddon (1893). In Tachysuridae an extra “V” shaped plate is formed ventral to the superficial bone. This makes the aortic groove a complete canal. This structure is an additional development from the tunica externa of the swim-bladder.
Summary
(1) The morphology of the osteocranium and Weberian apparatus of fourteen repre- sentatives of Tachysuridae has been described and compared.
(2) Some morphologically important structures have been recorded. They are: (a) the modified exogenous dermal plates bearing teeth attached to the ventral side of the palatines, and the lateral ethmoids in some tachysurid fishes; (b) the epiotics form posterior plate- like lamellae which articulate with the diagonal ridges on the dorsal side of the pars sustentaculum ; (c) the superficial bone and the basioccipital have fused indistinguishably with each other and at the point of fusion a cone-shaped sub-vertebral process is formed; (d) A trough like extra bony structure is developed on the ventral side of the superficial bone and makes the aortic groove a canal; (e) A small “V” shaped bony piece has been seen in close contact with the condylar process of scaphium in some of these fishes. This piece is comparable to the horizontal, the condylar, and the ascending processes of inter- calarium as already observed in Schilbeidae and Bagridae (Tilak, 1964a,b).
(3) The osteocranium and the Weberian apparatus are characteristic of this family and readily distinguish these fishes from other siluroids.
(4) There are intergeneric and interspecific differences which are presented in the form of a comprehensive key.
( 5 ) On the basis of structural differences in the osteocranium and the Weberian apparatus among these fishes, it is argued that the division of the genus Tachysurus (= Arius) into different genera (as done by Munro (1955)) is not well founded.
(6) Evidence has been enumerated in favour of the idea that Tachysuridae are not so primitive as to lie at the base of the series of siluroid families, but that they are at least more specialized than the Siluridae, Plotosidae, and Bagridae.
(7) Arguments have been put forward in favour of the view that the superficial bone and the “V” shaped trough of bone in connection with the former, are formed from the tunica externa of the swim-bladder.
12
172 R . T I L A K
The author is thankful to Professor A. K. Datta Gupta for the facilities in the department of Zoology, Birla College, Pilani. He expresses his appreciation for the suggestions received from Dr Stanley H. Weitzman, Smithsonian Institution, Washington, U.S.A.
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K E Y T O ABBREVIATIONS O N T E X T F I G U R E S
AF, anterior fontanelle; AH, a foramen for blood vessel; AIL, articular facet for interossicular ligament; ALSPH, pterosphenoid; ANARTF, anterior articular facet; AOG, aortic groove, APHYO, anterior process of hyomandibular; APPSPH, anterior process of parasphenoid; APR, ascending process; APT, anterior process of tripus; ARL, articular facet for radial ligament; ARP., ridge on posterior part of fourth parapophysis; ART, articular; ARTC, articular concavity; ARTF, articular facet ; ARTHYO, articular facet for hyomanbular; ARTI, articular tip; ATP, additional toothed plate; AUA, auricular arm; AVAF, anterior ventral articular facet; BK, beak; BNS4, bifid fourth neural spine; BOC, basioccipital; CART, cartilage; CERB first to fourth cerato- branchials; CL, claustrum; COP first and second copulae; CPR, condylar process; D, dentary; DOC, dorsal concavity; DG, dorsal groove; DI, body of intercalarium; DR, dorsal ridge; DRP,, diagonal ridge of dorsal side of the fourth parapophysis; ECTH, ectethmoid; ECTPT; ectopterygoid; ENTPT, endopterygoid; EPB first to fourth epibranchials; EPL, cpiotic lamella; ER, elevated ridge; EXO, exoccipital; FPH, foramen between preoperculum and hyomandibular; GECTF, gap between lateral ethmoid and frontal; GPS, foramen between posttemporal, pterotic and epiotic; H, hamulus; HD, Hood; HPR, horizontal process of exoccipital; HYB first and second hypobranchials; HYO, hyomandibular; IOP, interoperculam; IPT, inferior limb of posttemporal ;
174 R . T I L A K
LAC, lacrymal; LG, ligament; LPECT, lateral process of lateral ethrnoid; LPSETH, lateral process of ethmoid; LPSPH, lateral extension of anterior process of parasphenoid; LS, ligamenturn scaphii; LT, ligamentum tripodes; MCKCART, meckel’s cartilage; MPT, metapterygoid; MX, maxilla; N, nasal; N F 2-6r second to fifth cranial nerve foramen; NF, seventh cranial nerve foramen; NS2, second neural spine; NS,, third neural spine, OA; opercular arm; OP, operculum; OPHI, 0s pharyngeus inferior; OPHS, 0s pharyngeus superior; OPL, oblique plate; ORSPH, orbitosphenoid ; P’, parasphenoid; P4A, anterior part of the fourth parapophysis; PIP, posterior part of fourth parapophysis; P,, Pa, P,, Pa, sixth to eighth parapophyses; PAL, palatine; PEPB,, posterior process of epibranchial; PHBI, PHB,, first and second pharyngobranchial; PHB r4, compound third and fourth pharyngo- branchial; PLP, posterior lateral process; PMP, posterior medial process; PMX, premaxilla; POF, posterior fontanelle; POR, post-orbital; POSTR, posterior ridge; PPECT, posterior process of lateral ethmoid; PPVO, posterior process of prevomer ; PREOP, preopercdum ; PRO, prootic; PSPH. parasphenoid; PT, posttemporal; PTI, inner limb of posttemporal; PTO, outer limb of posttemporal; QD, quadrate; R1, RJ, R,, first to third ribs; RF, radial fibres; RN, radial nodule; SCART, symplectic cartilage; SETH, ethmoid; SOC, supraoccipital; SOCS, supraoccipital spine: SOR, sub-orbital; SP, coronorneckelian; SPH, sphenotic; SPP, sub-vertebral process; SPR, spatulate process; SQPT, pterotic; ST, supratemporal; TB, tooth band; TPT, transformator process of tripus; TR, trough like investment on the ventral side of superficial bone; VOBT, prevomerine band of teeth; VP, ventral process; VR, ventral ridge.
