Morphology and spatial constraints in a dorso‐ventrally flattened skull, with a revised species description of Platyallabes tihoni (Poll, 1944)

This study details the morphology of Platyallabes tihoni (Poll, 1944) as part of a complete revision of the anguilliform clariids. The overall body form of air‐breathing clariids ranges from fusiform to anguilliform genera (Boulenger 1911; Pellegrin 1927). Although P. tihoni has the typical external morphological features of other elongate clariids, this study shows that it occupies an intermediate position between fusiform and anguilliform taxa with regards to its cranial and postcranial morphology. The main morphological similarities of anguilliform species are a narrow skull roof with a high level of interdigitation between the bones, the reduced canal bones, a connection between the neurocranium and suspensorium via several processes, an extended tooth patch on the lower jaw, and the hypertrophied jaw muscle complex. Shared features typical for the fusiform species are a low coronoid process on the lower jaw and the posteroventral orientation of the opercular process on the hyomandibula. Platyallabes tihoni shows a series of unique features: an anterior fontanel situated entirely between the frontals, two tooth plates on the prevomer, a reduced height of the suspensorium, the absence of an anterior bony plate on the hyomandibula, a horizontal position of the sphenotic and pterotic, and a toothed entopterygoid. Many of these unique characters are linked to the spatial constraints associated with an extremely flattened skull in this species.


External morphology
Platyallabes tihoni has a long, dorsoventrally flattened body, with only caudal tip laterally compressed. The specimens show an even light-brownish colour (Figure 1).
One of the most striking features is the extremely flattened head with bulging jaw muscles. The mouth is surrounded by fleshy lips, which are fused at the corners of the mouth. Platyallabes tihoni shows a distinct prognation of the lower jaw ( Figure 2) and the eyes are clearly visible. The dorsal fin originates very close to the tip of the supraoccipital process and forms a continuous fin-fold with the anal and caudal fins. Both pectoral and pelvic fins are typically present (pelvic fins were absent however, in one specimen; MCZ 50239). The pectoral fin has a large spine anteriorly.

Cranial skeleton
Neurocranium. Platyallabes tihoni has a remarkably narrow skull roof (orbito-temporal region), though the total skull width is distinctly broad (Figure 2).  Ethmoid region: the tubular nasal bone of P. tihoni is the anteriormost bone of the supraorbital portion of the laterosensory canal system. The nasal bears small, lateral platelike extensions, as observed in some other clariids (Cabuy et al. 1999).
The large, forked mesethmoid ( Figure 2) is connected to the frontals posteriorly via large interdigitations. There is a limited constriction of the mesethmoid behind the rounded, anterior wings. Further, the posterior border does not reach the anterior limit of the anterior fontanel, which as a consequence is completely surrounded by the frontals. The partially open supraorbital canal passes between the lateral ethmoid and the mesethmoid and forms a distinct boundary between the bones. The lateral ethmoid ( Figure 2) bears a large, laterally pointed process but lacks any articulation with the infraorbitals and as a consequence also lacks an articular ridge. The arrow-shaped prevomer runs along the mesethmoid ventrally and interdigitates posteriorly with the parasphenoid via a pointed process. The prevomer bears contralateral tooth plates (Figure 3b).
Orbital region (Figure 2): the circumorbital series involves four infraorbitals and an antorbital. The antorbital is a small bone located at the base of the nasal barbel and situated dorsal to the anterior tip of the autopalatine. The tubular lacrimal (first infraorbital) lies at the level of the olfactory organ. The reduced, tubular second through fourth infraorbitals surround the eye. The fourth infraorbital lies dorsal to the adductor mandibulae complex posterolateral of the lateral ethmoid and is sometimes divided into two parts. The infraorbital canal splits within the fourth infraorbital and ends in a lateral pore (I 6 in Adriaens et al. 1997).
The narrow frontals are the largest bones of the skull roof ( Figure 2). The two frontals, slightly concave in the middle of their lateral margins, are strongly fused, with a barely visible median seam. These bones are anteriorly separated by the anterior fontanel, with a short anterior area of contact between these bones behind the mesethmoid. The frontal bones have two lateral plates visible on a ventral view slightly extending beyond the orbitosphenoid bone margins (Figure 3b). The anastomosis of the infraorbital and the supraorbital canal is situated on the anterior part of the frontal. The lateral wall of the central portion of the skull is formed by the orbitosphenoid and pterosphenoid, which connect ventrally to the dermal parasphenoid. The latter bone forms the skull floor up to the temporal region and has an elongated process extending to the occipital region ( Figure 3b).
Temporal region (Figure 2): the sphenotic interdigitates anteromedially with the frontal and posteriorly with the pterotic. The latter interdigitates medially with the parietosupraoccipital and the frontals and posteriorly with the posttemporo-cleithrum. The sphenotic and pterotic are horizontally aligned, with a slight ventrolateral curvature and no lateral plate. Both bones form the only firm connection between the neurocranium and the suspensorium. The sphenotic has a plate-like and a spiny process anterolaterally that extend ventrally to interdigitate with the hyomandibula. Posterolaterally, a cartilaginous facet on the pterotic and sphenotic articulates with the hyomandibula. The pterotic also has one spiny and one plate-like process. Ventrally, the brain cavity is enclosed by the paired perichondral prootics (Figure 3b).
Occipital region (Figure 2): the complex parieto-supraoccipital is situated on the posteromedial portion of the skull roof and is characterized by a short, pointed posteromedial process. This bone encloses the posterior fontanel in the posterior part of the bone. The dermal posttemporo-supracleithrum connects to the pectoral girdle, the parapophyses of the fourth vertebra (by means of the transscapular process) and to the pterotic anteriorly. The occipital region of the skull is ventrally bordered by the unpaired basioccipital and paired exoccipitals ( Figure 3b). Platyallabes tihoni lacks epioccipitals.
Upper jaw (Figures 2, 3a, b): the premaxilla are plate-like bones supporting the nasal sac ventrally. The anterior, larger ventral portion of each premaxilla is covered with several rows of posteriorly directed teeth and the posterior half bears a small process. The maxilla forms a cup-like bone, which encloses the base of the maxillary barbel and bears two articulatory facets for articulation with the palatine.
Splanchnocranium. Lower jaw (Figure 4a, b): the long lower jaw consists of two components: the dento-splenio-mentomeckelium and the angulo-splenio-articulo-retroarticular complexes. The coronoid process on the lower jaw is indistinct and dorsal surface of the anterior part of the lower jaw is covered with a large tooth patch of posteriorly curved teeth that extends almost to the coronoid process. The two processes on the retroarticular process are highly asymmetrical and lie completely medial to the quadrate ( Figure 4). Suspensorium (Figure 4a, b): as in most siluriforms, the suspensorium consists of the hyomandibula, quadrate, preopercle, metapterygoid, and entopterygoid. The hyomandibula suspends the suspensorium from the neurocranium in the area of the sphenotic and pterotic. The hyomandibula connects to the sphenotic through one broad, anterior process and two pointed, posterior processes and to the pterotic by one pointed and one broader process. These secure the neurocranial-hyomandibula articulation. In between these two sets of processes lies a short articulatory facet is. No bony process is present at the anterior margin of the hyomandibula. The opercular process of the hyomandibula is posteroventrally orientated. Medially, the hyomandibula bears a ridge for the attachment of the hyomandibula-ceratohyal ligament. The quadrate interdigitates with the metapterygoid, but no direct contact is present between the quadrate and the entopterygoid. Anteroventrally, the quadrate has a well-developed articulatory facet, articulating with the angular complex. The small metapterygoid connects with the quadrate through a synchondrosis and a bony interdigitation, and is dorsally bordered by the quadrate and the large entopterygoid. The plate-like entopterygoid lies anterodorsally to the metapterygoid. The entopterygoid bears several teeth on its ventral margin ( Figure 4a). Anteriorly, the entopterygoid is connected ligamentously to the prevomer, the palatine and the lateral ethmoid, thus corresponding to a sesamoid ''entopterygoid type 4'' (Arratia 1992). The cylindrical autopalatine (Figure 3b) extends ventrally from the lateral ethmoid, with which it articulates through a well-developed articular facet. Both ends of the autopalatine are cartilaginous and it articulates anteriorly with the maxillary, thus being part of the palatinemaxillary mechanism.
Hyoid arches (Figure 3a): the hyoid arch consists of paired anterior and posterior ceratohyals and paired ventral and dorsal hypohyals. The hyoid arch articulates ventrally with nine branchiostegal rays. The parurohyal lies in between the two hyoid arches and bears two lateral processes and one medial process. The parurohyal is connected to the hypohyal by means of two separate paruro-hypohyal ligaments ( Figure 6c).
Branchial arches: the branchial morphology resembles that of Clarias gariepinus (Adriaens and Verraes 1998) with the exception of the low number of gill rakers (nine or fewer).
Opercular series (Figure 4a, b): the opercle is a pointed, slender, somewhat triangular, dorsoventrally compressed bone, bearing a large articulatory facet for its articulation with the hyomandibular. The posterior part of the bone bears a horizontal ridge for the attachment of the opercular levator muscle. The opercle is ligamentously attached to the anteriorly positioned interopercle. The interopercle is a long, flat bone, ligamentously attached to the angular complex of the lower jaw. The preopercular bone is incorporated in the suspensorium and surrounds the dorsal part of the preoperculo-mandibular canal. The

Cranial myology
Muscles of the lower jaw ( Figure 5). Adductor mandibulae muscle: this complex forms an enormous, jaw-closing muscle covering most of the lateral surface of the head of Platyallabes tihoni. It consists, as in other clariids, of an external A 2 A 3 9-part and an internal A 3 0, that are separated by the levator arcus palatini (Adriaens and Verraes 1996).
The bipennate A 2 A 3 9 forms the largest part of the complex (Figure 5a-c). It is divided into a dorsal A 2 A 3 9a-part and a ventral A 2 A 3 9b-part, both of which are attached to an aponeurosis connected to the angular complex of the lower jaw close to the coronoid process. The directions of the muscle fibres of the A 2 A 3 9 has a range of almost 160u. The A 2 A 3 9a is attached to the frontal, the sphenotic, the pterotic, the parieto-supraoccipital, and the suprapreopercular series, and is covered by the fourth infraorbital. The ventral A 2 A 3 9b is posteriorly connected to the hyomandibula, the quadrate and the preopercle. The A 2 A 3 9 covers the levator arcus palatini, the dilatator and adductor operculi, and part of the levator operculi.
The A 3 0 lies between the levator arcus palatini and the medially situated retractor tentaculi ( Figure 5c). The horizontally orientated muscle fibres originate on the medial side of the suspensorium, more specifically on the hyomandibula and the anterodorsal rim of the quadrate, on the frontal, on the sphenotic, and on the pterosphenoid, and insert on the angular complex of the lower jaw medial to the A 2 A 3 9 tendons.
Intermandibular muscle: this muscle is a short, solid muscle with transverse fibres, lying over the mandibular symphysis connecting the contralateral lower jaws (Figure 6a, b). The intermandibularis is bordered posteriorly by the interconnecting cartilage of the left and right bases of the internal-mandibular barbels, thus separating it from the protractor hyoidei muscle.
Suspensorial muscles. Levator arcus palatini muscle (Figure 5c): this thin muscle sheet connects the ventrolateral side of the skull roof with the suspensorium. This muscle complex consists of two separate parts (pars rostralis and pars caudalis) and a large complex of aponeuroses. The muscle complex originates on the posterolateral surface of the lateral ethmoid, the ventrolateral side of the frontal up to the sphenotic and inserts mostly tendinously on the anterodorsal border of the interdigitation between the quadrate and the hyomandibula. The levator arcus palatini runs medially from the eye. The levator arcus palatini pars rostralis is located in the anterodorsal part of the complex. It runs from the lateral ethmoid and the frontal and attaches through a medial aponeurosis on the suspensorium. The levator arcus palatini pars caudalis lies more posteriorly and runs between the medial and lateral aponeuroses. The posterior part of the pars caudalis muscle inserts directly on the suspensorium. The predominant fibre direction in the muscle is dorsoventrally orientated, with the anterior fibres showing a more horizontal radiation anteriorly. This complex morphology appears to be two separate muscles.
Adductor arcus palatini muscle (Figure 5f): the adductor arcus palatini connects the skull floor and the dorsal rim of the suspensorium. This muscle has the most medial position of all cranial muscles, lining the mouth cavity dorsolaterally. On the neurocranium, it inserts primarily on the parasphenoid, but also on the orbitosphenoid and pterosphenoid. The adductor arcus palatini attaches on the suspensorium (dorsal rim of the hyomandibula, the quadrate and the entopterygoid). The fibres run in a transverse plane with the anterior fibres more obliquely orientated.
Opercular muscles. Three opercular muscles could be discerned. The dilatator operculi is situated most anteriorly, partially covering the adductor operculi, with the levator operculi most posteriorly.
Dilatator operculi muscle (Figure 5c): the flattened dilatator operculi starts from halfway up the ventrolateral surface of the frontal between the two aponeuroses of the levator arcus palatini and the ventrolateral part of the sphenotic and the pterotic (Figure 5c). It inserts through a large tendon on the lateral surface of the dorsal process of the opercle close to the articulation with the hyomandibula. This tendon runs along the anteroventral side of the dilatator operculi.
Adductor operculi muscle (Figure 5d, e): this shorter and more robust muscle connects the posterior side of the suspensorium with the opercle. The adductor operculi originates on the posterodorsal part of the hyomandibula and the posteroventral part of the pterotic and inserts on the dorsal process of the opercle and thus lateral to the opercular articulation with the hyomandibula, in the region medial to the insertion of the dilatator operculi. The predominantly dorsoventrally directed fibres also attach to the connective tissue covering the dorsolateral surface of the remnant suprabranchial cavity. The posterior part lies against the posteriorly situated levator operculi.
Levator operculi muscle (Figure 5d, e): the levator operculi is the largest and most robust of the three opercle muscles. It connects the posteriormost part of the neurocranium with the opercle. This muscle originates on the posteroventral part of the posttemporosupracleithrum, the suprapreopercles and the connective tissue covering the suprabranchial cavity. Ventrally, the levator operculi attaches to nearly all of the complete dorsal side on a ridge of the opercular bone.
Maxillary barbel muscles. Retractor tentaculi muscle (Figure 5d): the retractor is a bundle-like muscle running from the maxillary bone to the suspensorium between the A 3 0 and the adductor arcus palatini. The fibres in general all follow the same anteroposterior direction. Anteriorly, the retractor lies medial to the eye and lateral to the autopalatine. It inserts on the anterior side of the hyomandibula and the quadrate. The muscle is divided into a dorsal and ventral part, separated by an aponeurosis, and attaches to the posterodorsal side of the maxillary through a short tendon.
Extensor tentaculi muscle (Figure 5e): the extensor tentaculi extends from the ventral and ventrolateral surface of the lateroethmoid, the ventral side of the frontal and the lateral side of the orbitosphenoid to the autopalatine. The fibres, organized in two separate bundles, attach to the autopalatine posterior from the articulatory facet and enclose the posterior end of the autopalatine.
Hyoid muscles. Protractor hyoidei muscle (Figure 6a, b): the protractor hyoidei connects the hyoid bars with the lower jaw and is divided in a ventral and dorsal part, each of which originate on the ventrolateral side of the anterior ceratohyal and inserts on both complexes of the lower jaw. The ventral part is V-shaped ( Figure 6a); with the left and right halves attached medially by a fascia at the plane through the mandibular symphysis. The insertion of the muscle on the mandibula occurs over the entire antero-ventral surface of the lower jaw and on the bases of the mandibular barbels. Several fields of fibres can be distinguished interconnecting different parts of the barbel bases. The dorsal part of the protractor inserts on the medial surface of both lower jaws (Figure 6b) and is divided into a medial and a lateral part. The medial parts attach postero-medially to the dental complexes of the lower jaw through a tendon and are separate. The lateral subparts attach to the ventrolateral surface of the dental complexes through a double tendon system.
Hyohyoidei inferior muscle (Figure 6b): this muscle inserts on the ventral side of the anterior ceratohyal, medial to the insertions of the protractor hyoidei, and to the dorsal and ventral hypohyals. The contralateral fibres are medially connected to a fascia. Posteriorly the muscle also attaches to the bases of the branchiostegal rays.
Hyohyoideus abductor muscle (Figure 6c): the hyohyoideus abductor links the anterior tip of the hyoid bars with the first contralateral branchiostegal rays. The muscles originate from the anterior face of these branchiostegal rays and insert on to the ventral hypohyal of the opposite side by means of a double tendon.
Hyohyoidei adductores muscles (Figure 6c): the hyohyoidei adductores muscles lie between successive branchiostegal rays, starting from the first ray and attaching on to the medial surface of the opercle.
Sternohyoideus muscle (Figure 6d): this muscle connects the pectoral girdle to the parurohyal, which, in turn, is ligamentously connected to the ventral hypohyals of the hyoid bars. Posteriorly, the sternohyoideus inserts on the cleithrum on both its dorsal and ventral surfaces. Anteriorly, both muscle heads attach on to the double-forked parurohyal. On the anterolateral surfaces, two contralateral tendineus plates lie on top of the muscle. No myocommata were observed.

Postcranial skeleton
The total number of vertebrae in Platyallabes tihoni (58-490 mm SL) ranges from 63 to 83 (mode: 78). The numbers of precaudal vertebrae varies from 16 to 20 (mode: 17) and of caudal vertebrae from 47 to 66 (mode: 63). Platyallabes tihoni has from four to seven pairs of ribs. A notable feature of the vertebrae is the presence of a large foramen at the bases of the parapophyses of the precaudal vertebrae (Figure 7).
The pectoral fins of P. tihoni have non-serrated spines and 11 branched fin rays that articulate with two proximal radials. The pectoral girdle has a distinct fenestra between the coracoid and the cleithrum. Furthermore, the cleithrum lacks an anterior process but has a distinct anterior rim ventrally, on to which the sternohyoideus is attached. The pelvic fins each carry six, branched fin rays which articulate with the broad and flat basipterygium of the pelvic girdle. Anteriorly an internal and external anterior process can be distinguished (Figure 8a). The number of dorsal and ventral fin rays varies, respectively, from 125 to 139 and from 114 to 129.
The morphology of the caudal skeleton of P. tihoni shows some variation but always consists of the parhypural, five hypurals and an urostyl, but various fusion patterns can be discerned. Poll (1977) reported a total fusion of all hypurals and the epural; however, our observations show that hypurals 3 and 4 appear fused, as well as the urostyl and hypural 5. Dorsally of the latter complex lies the broadly tipped epural. The neural arches of the second and third preural vertebrae are spiny, not elongated and do not support fin rays. The haemal arch of the second preural vertebra is elongated, broadly tipped and supports fin rays, the haemal arch of the third preural vertebra is not elongated, spiny and does not support rays (Figure 8b). At the level of the non-supporting arches pterygiophores support the fin rays.

Species description
The proportional measurements and counts are given in Table I. Platyallabes tihoni characterized by elongated, dorsoventrally flattened body; with only the caudal tip transversely flattened (Figure 1). Degree of anguilliformity as expressed by ratio of total length to body height (Poll 1942) between 15.6 and 33.9, with average of 24.7¡4.4, with postanal length of 65.3% up to 81.4% of SL.
Unpaired fins form continuous fin fold. Dorsal fin origin very close to skull; with small distance between dorsal fin origin and occipital process on parieto-supraoccipital (Adriaens et al. 2002, Figure 1). Distance only 2.2-6.6% of SL.
Teeth present on dentary, premaxilla, prevomer and on entopterygoid, with latter unique within clariids (see below). All teeth pointed and curved. Prevomer with two separate tooth plates. Alcohol-preserved specimens show even, light-brownish colour over whole body, nostrils, barbels and fins.
Platyallabes tihoni lacks usual components of suprabranchial organ, with arborescent organs and fan-like covers completely absent. Remnant of small suprabranchial cavity apparently present.

Discussion
The anatomy and taxonomy of the more extremely elongate clariids is poorly known. This is the case for Platyallabes tihoni, which has only been cited in several papers by Poll (e.g. 1944Poll (e.g. , 1957Poll (e.g. , 1977, dealing mainly with morphometric and meristic features and some gross osteological characters. These were all based solely on the holotype. As a result, a complete description of P. tihoni was necessary, as is a comparison with several other representatives of the clariid family, in order to determine their affinity and the systematic position of P. tihoni.

Morphology and comparison with other clariids
Platyallabes tihoni has previously been described as an anguilliform clariid (Poll 1944(Poll , 1957(Poll , 1977, with this character as one of the primary reasons for its original placement within the genus Gymnallabes (Poll 1944(Poll , 1957. Our own observations show that P. tihoni has an intermediate position with characters of both anguilliform and fusiform Bauplan. The reduced skull and narrow skull roof is comparable to that of the anguilliform clariid species (Cabuy et al. 1999;Devaere et al. 2001Devaere et al. , 2004. Also typical for anguilliform species is the extreme reduction of all infraorbitals and nasal bones, as they become tubular but still enclosing limited portions of the infraorbital canal of the lateral line system. A comparable reduction occurs in the suprapreopercles and the praeoperculo-mandibular canal. Although the primitive catfish family Diplomystidae has tubular infraorbitals and suprapreopercles (Arratia 1987), many clariids possess plate-like infraorbitals and suprapreopercles. Based on the widespread assumption that Heteropneustidae are the sister group of Clariidae, the presence of plate-like bones is to be considered plesiomorphic for Clariidae (Heteropneustidae have plate-like bones). However, following the phylogeny suggested by de Pinna (1993) and Diogo (2003), Heteropneustes is to be considered a member of the Clariidae, with Uegitglanis being the sister group of Heteropneustes and all other clariids. The blind and cave-dwelling Uegitglanis bears tubular infraorbitals and suprapreopercles (David 1936). This would imply that the plesiomorphic condition of Clariidae would be tubular bones, a retained plesiomorphic character of Siluriformes in general. The problematic relation of Uegitglanis with respect to other clariids is among other things because of the absence of the synapomorphic suprabranchial organs (Chardon 1968;Teugels and Adriaens 2003). The absence of the typical arborescent organs in Heteropneustidae also makes the phylogenetic position of this genus problematic (Diogo 2003;Teugels and Adriaens 2003). Further knowledge on the true nature of the respiratory organs, as well as morphological and molecular data, might elucidate the true relationships, and thus the status of the character of tubular bones.
The higher level of interdigitation of, among others, the skull roof bones is another correspondence with the anguilliform species and contributes to the solidity of the skull; additionally P. tihoni even shows a fusion of the frontals (Figure 2). Furthermore, the infraorbital-supraorbital canal anastomosis is situated in the anterior part of the frontals ( Figure 2) and no longer in the sphenotic, the latter being the case in the fusiform species and in Diplomystis chilensis (Adriaens et al. 1997;Arratia 1987). The lateral ethmoid has no contact with the second infraorbital bone; this is also seen in Gymnallabes typus (Cabuy et al. 1999). The sphenotic and pterotic lack a lateral plate, as is the case in all elongated species, except for Platyclarias machadoi (current research). The hyomandibula has a high number of anterior and posterior processes (respectively two and three) (Figure 4), comparable to the number in anguilliform species. The lower jaw also shows osteological similarities: the robust tooth battery that runs close up to the coronoid process ( Figure 4) and the low number of gill rakers (up to nine).
Also the general muscular morphology of P. tihoni resembles that of the anguilliform species and shows the same insertion sites for the different muscles. A first clear example is the hypertrophied jaw muscle and more specifically the A 2 A 3 9-part. Compared to C. gariepinus additional insertion sites are found, the frontal, pterotic and posttemporosupracleithrum. Besides this, there is also a large increase in the range of muscle fibre directions, up to almost 160u. This is still less than that found in most extremely elongated species (C. gariepinus 105u; Clariallabes sp. 140u; G. typus 150u; C. apus 180u). As in C. apus and G. typus, the ventral part of the protractor hyoidei is V-shaped. Consequently, the fascia, connecting the two contralateral parts of that muscle, does not overlap with the anterior side of the hyohyoideus inferior. Furthermore, the two contralateral parts of the dorsal part of the protractor hyoidei do not contact each other either through direct muscle contact or through tendons (Figure 6b), comparable to the situation in G. typus (Cabuy et al. 1999).
Besides these character states similar to the anguilliform clariids, Platyallabes tihoni shares a list of symplesiomorphies with the fusiform representatives.

Special traits of Platyallabes tihoni
The intermediate position of anguilliformity of P. tihoni is reflected in the number of vertebrae, as this number can give an indication of the degree of anguilliformity. The mode of the total number of vertebrae of P. tihoni is 78. The vertebral counts for the extremely anguilliform representatives are higher, e.g. Dolichallabes microphthalmus (n511): 106, Channallabes apus (n598): 104 and Gymnallabes typus (n526): 87, while the number of vertebrae found in a fusiform member of the family, Clarias gariepinus (n547): 56-63 (Teugels 1986), is clearly lower.
Besides the above-mentioned characters that exhibit a resemblance with the anguilliform or fusiform representatives, P. tihoni shows a large number of unique features. The fused frontals show a restricted concavity along their lateral sides (Figure 2), whereas the width remains the same along the whole length in anguilliform species (Cabuy et al. 1999;Devaere et al. 2001) or broadens in the middle in most fusiform representatives (Adriaens and Verraes 1998). These lateral plates slightly pass the orbitosphenoid outlines ( Figure 3b); a situation otherwise only seen in Diplomystis chilensis (Arratia 1987) but that does not correspond with the broader plates in the fusiform clariids or the smaller extensions in the anguilliform species. The suprapreopercular bones are reduced to a set of tubular bones. This lack of any plate-like extension on the dorsal suprapreopercular is unique within the clariids, as is the higher number of bones (up to five). Both the sphenotic and pterotic, although lacking a lateral plate, exhibit a more or less horizontal position. Also, on the low suspensorium some unique features can be observed: on the anterior margin of the hyomandibula there is no bony outgrowth present, an absence only viewed elsewhere in Heteropneustis fossilis (sister group of the Clariidae) (Srinivasachar 1958) and Platyclarias machadoi (current research). The prevomer shows in all specimens two tooth plates, clearly separated from each other, a situation seen in other juvenile clariids, but never in adults (the strongly fused frontals, with a hardly visible seam, indicate that we are dealing with adult specimens of P. tihoni) (Cabuy et al. 1999). The feature most remarkable in P. tihoni is the dentigerous entopterygoid (Figure 4). This characteristic has not been observed elsewhere in the Clariidae, except on one side in one G. typus specimen. This was at that point considered as an aberrant condition (Cabuy et al. 1999). Moreover, a toothed entopterygoid generally does not occur in catfishes, with the exception of some ariids (Arratia 1992). These teeth could possibly be linked to a more specialized diet. The retroarticular process on the lower jaw is extremely asymmetric, lying completely on the medial side of the angular bone. Although this process is never fully symmetrical in the other clariids, it is never asymmetrical to this degree. A last feature of P. tihoni is the absence of a suprabranchial organ, together with a reduced suprabranchial cavity. Such a situation can also be seen in Uegitglanis, Bathyclarias and Xenoclarias. In Xenoclarias and Bathyclarias, the reduction is considered to be related to living in deep waters (Chardon 1968;Poll 1977;Anseaume and Teugels 1999;Agnèse and Teugels 2001;Teugels and Adriaens 2003). Phylogenetically, this should consequently be considered as a secondary reduction, a hypothesis also supported by genetic evidence (Agnèse and Teugels 2001).
On the muscular level, we see that there is a unique system of two tendons for the insertion of the lateral parts of the pars dorsalis of the protractor hyoidei on to the lower jaw. A similar system with two tendons is seen on the hyohyoideus abductor (Figure 6b, c). A last remarkable myological difference is the very complex morphology of the levator arcus palatini (see Results) ( Figure 5c).
These unique features are not limited to cranial characteristics; post-cranially there are also some characters that are typical for P. tihoni. The cleithral bone has no anterior process; once more a condition only previously observed in H. fossilis. The same is true for the very large parapophyseal foramina present in the precaudal vertebrae (Figure 7). Platyallabes tihoni has a very low number of ribs (four to seven, mode: five), which is the lowest number observed among examined clariids: C. apus 10-17; G. typus 7-14; D. microphthalmus 6-9; C. gariepinus 12 (Cabuy et al. 1999).
This list of above-mentioned autapomorphies can help to indicate the validity of the species and genus, while the synapomorphies help to interpret the phylogenetic situation within the clariids and especially within the anguilliform genera.

Functional implications of the extremely depressed skull
The flattened skull can be expected to be related to spatial changes in the surrounding structures, in all three dimensions of the Bauplan framework, since the spatial constraints within an integrated design imply that trade-offs will occur (Barel 1984).
Most of these characters are situated in the splanchnocranium and the articulation with the neurocranium. As mentioned above, the sphenotic and pterotic bones are in a more or less horizontal orientation. Consequently, the interdigitation with and position of the suspensorium will also be in a more horizontal, lateral plane. This results in a dorso-ventral reduction in skull height. The orientation of these above-mentioned bones is in contrast with the tendency within the anguilliform clariids, in which the sphenotic and pterotic are in a more vertical position, which could be beneficial to accommodate the hypertrophic jaw muscles (Cabuy et al. 1999;Devaere et al. 2001). Despite this different orientation, P. tihoni exhibits large jaw muscles and this with similar insertion sites as within the anguilliform species (Cabuy et al. 1999;Devaere et al. 2001). The consequently more elevated position of the adductor mandibulae complex could explain the extensive bulging of these jaw muscles. More posterior on the suspensorium, the opercular process of the hyomandibular has a more posterior orientation. This means that the dorso-ventrally compressed opercle is positioned more posterior to the suspensorium instead of ventral to it. All this leads to a decrease of the branchial cavity volume, which can have some consequences for feeding and respiratory efficiency (Adriaens and Verraes 1997d). Also the absence of the suprabranchial organ can be considered as a consequence of arising spatial constraints in the trend towards a miniaturization of the skull in the anguilliform clariids (Devaere et al. 2001). Additionally, the substantial dorso-ventral compression of the skull of P. tihoni will have generated a competition for space for both gill-apparatus and the suprabranchial apparatus. The combination of both constraints apparently favoured the loss of the suprabranchial organ.
The medial position of the retroarticular process in P. tihoni, with respect to the articulation head of the quadrate, is unique for clariids. This condition presumably is a simple consequence of the topographic relation between lower jaw and suspensorium, the plane of mandibular movements and the fact that this suspensorium has become tilted extremely, to an almost horizontal position (Figure 9a). Because the axis of rotation of the lower jaw has to run through both lower jaw articulations (in order to avoid dislocations), a horizontal tilting of the suspensorium will result in a medial shift of the articulation on the quadrate. Consequently, the post-articulation part of the lower jaw, i.e. the retroarticular process, becomes situated ventro-medially to the quadrate head ( Figure 9b).

Comparative material examined
Platyallabes tihoni (Poll, 1944) . Schematic 3D reconstruction of the lower jaw-suspensorium articulation area. (a) Overview of the lower jaw, suspensorium and part of skull at level of articulation with suspensorium. Solid lower jaw indicates closed mouth, transparent lower jaw indicates open mouth. Black arrow shows displacement of lower jaw according to the black axis indicated at the jaw articulation. The grey arrows indicate the movements of the lower jaw halves if they were rotated along the grey axis at the articulation. (b) Detail from a postero-ventral view of the lower jaw articulation, showing the two rotation axis: black rod represents the axis which allows a proper mouth opening, grey rod represents the axis of rotation which has shifted together with the suspensorium. Rotation along the black axis results in the medial position of the retroarticular process; with respect to the suspensorium (transparent lower jaw indicates the closed mouth, solid lower jaw the open mouth).