Platyallabes tihoni (Poll, 1944)
publication ID |
https://doi.org/10.1080/00222930400020103 |
DOI |
https://doi.org/10.5281/zenodo.15601552 |
persistent identifier |
https://treatment.plazi.org/id/039FDF1F-FFB1-4F7C-5D8F-FB5FFF28E69A |
treatment provided by |
Felipe |
scientific name |
Platyallabes tihoni |
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Platyallabes tihoni View in CoL
has a long, dorsoventrally flattened body, with only caudal tip laterally compressed. The specimens show an even light-brownish colour ( Figure 1 View 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 View 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 View 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 View 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 View 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 View Figure 3 ).
Orbital region ( Figure 2 View 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 View 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 View Figure 3 ). 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 View Figure 3 ).
Temporal region ( Figure 2 View 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 View Figure 3 ).
Occipital region ( Figure 2 View 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 View Figure 3 ). Platyallabes tihoni lacks epioccipitals.
Upper jaw ( Figures 2 View Figure 2 , 3a, b View Figure 3 ): 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 View Figure 4 ): 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 View Figure 4 ).
Suspensorium ( Figure 4a, b View Figure 4 ): 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 View Figure 4 ). 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 View Figure 3 ) 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 View Figure 3 ): 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 View Figure 6 ).
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 View Figure 4 ): 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 posterior part of the canal extends through a series of four or five ‘‘open’’ tubular suprapreopercular bones ( Figure 2 View Figure 2 ).
Cranial myology
Muscles of the lower jaw ( Figure 5 View 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 39 -part and an internal A 30, that are separated by the levator arcus palatini ( Adriaens and Verraes 1996).
The bipennate A 2 A 39 forms the largest part of the complex ( Figure 5a–c View Figure 5 ). It is divided into a dorsal A 2 A 39 a -part and a ventral A 2 A 39 B -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 39 has a range of almost 160 °. The A 2 A 39 a 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 39 B is posteriorly connected to the hyomandibula, the quadrate and the preopercle. The A 2 A 39 covers the levator arcus palatini, the dilatator and adductor operculi, and part of the levator operculi.
The A 30 lies between the levator arcus palatini and the medially situated retractor tentaculi ( Figure 5c View Figure 5 ). 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 39 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 View Figure 6 ). 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 View Figure 5 ): 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 View Figure 5 ): 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 View Figure 5 ): 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 View Figure 5 ). 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 View Figure 5 ): 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 View Figure 5 ): 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 View Figure 5 ): the retractor is a bundle-like muscle running from the maxillary bone to the suspensorium between the A 30 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 View Figure 5 ): 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 View Figure 6 ): 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 View Figure 6 ); 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 View Figure 6 ) 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 View Figure 6 ): 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 View Figure 6 ): 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 View Figure 6 ): 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 View Figure 6 ): 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 View 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 View Figure 8 ). 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 View Figure 8 ). 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 View Table I . Platyallabes tihoni characterized by elongated, dorsoventrally flattened body; with only the caudal tip transversely flattened ( Figure 1 View 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.
Head length 7.4–17.2% of SL; and skull width 62.9–82.5% of head length. Skull roof width (exposed) 11.5–40.1% of orbital skull width. Skull roof appears very small, due to dorsomedial outgrowth of hypertrophied adductor mandibulae complex, but remains visible. Eyes clearly visible, in contrast to situation in some other anguilliform clariids ( Cabuy et al. 1999; Devaere et al. 2001). Mouth width almost equals interorbital distance. Fleshy lips fused at mouth corners. Lower jaw distinctly prognathous ( Figures 1 View Figure 1 , 2 View Figure 2 ).
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 View Figure 1 ). Distance only 2.2–6.6% of SL.
Both pectoral fins always present. Pectoral fin length 43.3–82.0% of head length. Pelvic fins present in all except for one specimen (MCZ 50239). Pelvic fin length 31.2–71.2% of head length. Pectoral fins always preceded by non-serrated spine (59.2–95.1% of pectoral fin length). The pectoral fins have 11 fin rays; the pelvic fins carry six fin rays. Branchiostegal rays 8–10.
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.
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