Beishanichthys brevicaudalis, Xu & Gao, 2011
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publication ID |
https://doi.org/ 10.1111/j.1096-3642.2010.00645.x |
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persistent identifier |
https://treatment.plazi.org/id/03B78799-FFAA-FFFE-FC88-DD1A00F9F989 |
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treatment provided by |
Valdenar |
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scientific name |
Beishanichthys brevicaudalis |
| status |
gen. et sp. nov. |
SPECIES BEISHANICHTHYS BREVICAUDALIS SP. NOV. ( FIGS 2–6 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 )
Holotype: PKUP V4881, a laterally compressed specimen split into part and counterpart slabs, with pectoral fins missing.
Referred specimens: PKUP V4882–4886, all topotypic specimens exposed in part and counter-part slabs.
598 G.-H. XU and K.-Q. GAO
Type locality and horizon: Quarry 3 in the Beishan
Hills, Subei County, Gansu Province, northern China; Lower Triassic Hongyanjing Formation.
Diagnosis: A scanilepiform differing from other members of the group by having three supraorbitals, two suborbitals, ten pectoral fin rays, 65 dorsal fin rays (most fin rays branched), 24 anal fin rays, 18 caudal fin rays, and a squamation formula of D13/P11,A22,C39/T42.
Etymology: Species epithet derived from brevi plus caudalis (Latin for short tail), referring to the short caudal peduncle.
DESCRIPTION
GENERAL MORPHOLOGY
Similar to other scanilepiforms, B. brevicaudalis gen. et sp. nov. has a fusiform body outline, a long-based dorsal fin, and a slightly convex caudal fin ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ).
The largest specimen (PKUP V4886) reaches a total length of 700 mm. The measurements of three nearly complete specimens are shown in Table 1.
SNOUT
The rostral is about one-third of the length of the frontal, with a curved anterodorsal margin ( Fig. 3 View Figure 3 ). This bone is located anterior to the frontal and anteromedial to the nasal, showing a condition different from crown-group neopterygians that have a reduced rostral not contacting the frontal.
The nasal is a deep, slightly curved bone, forming the anterior border of the orbit ( Fig. 3 View Figure 3 ). It contacts the premaxilla ventrally, and the frontal and supraorbitals posterodorsally. The anteromedial and posterolateral margins of the nasal are notched for the anterior and posterior nares, respectively. Although the sensory pores on the surface of the nasal are difficult to identify, the underlying sensory canal is clearly shown, as that bone was highly compressed during fossilization. The supraorbital sensory canal runs longitudinally through the nasal, and terminates at the posterior part of the frontal.
The small premaxilla is located anterior to the maxilla, and bears four conical teeth ( Fig. 3 View Figure 3 ). A portion of the ethmoid commissal sensory canal is clearly preserved in this bone, as in other scanilepiforms. Considering this character state, the new taxon Beishanichthys is different from perleidiforms and crown-group neopterygians that have a premaxilla lacking the sensory canal. Phylogenetic analysis indicates that the loss of the sensory canal in the premaxilla is a derived condition for Perleidiformes + crown-group neopterygians.
SKULL ROOF
The frontal is the longest bone of the skull roof, and is more than twice the length of the rectangular- shaped parietal ( Figs 2 View Figure 2 , 3 View Figure 3 ). The frontal is slightly wider posteriorly than anteriorly, and contacts the parietal along a transverse suture.
The dermopterotic (= intertemporal plus supratemporal in more primitive actinopterygians; Gardiner & Schaeffer, 1989) is elongate, flanking the parietal and the posterior part of the frontal. The supratemporal sensory canal runs longitudinally through this bone.
The extrascapular series consists of a trapezoidal lateral extrascapular and a subtriangular median extrascapular ( Fig. 3 View Figure 3 ). The supratemporal canal enters the lateral extrascapular from the dermopterotic, and has a branch extended to enter the median extrascapular, becoming the supratemporal commissural canal.
PALATE
Because of taphonomic compression, the palatal elements can only be observed through the orbit ( Fig. 3 View Figure 3 ). The parasphenoid runs through the orbit, and has an elongate orbital portion. The pterygoid bones are difficult to identify because of poor preservation.
INFRAORBITAL AND SUPRAORBITAL BONES
The infraorbital bone series includes two elements (a lachrymal and a jugal), in contrast to the four or more elements commonly seen in crown-group neopterygians. An increase in the number of infraorbital elements to three or more has been recognized as a derived condition in actinopterygian evolution ( Gardiner & Schaeffer, 1989).
The lachrymal is an elongated bone, extending between the premaxilla and the jugal to form the ventral border of the orbit. The jugal is slightly wider than the lachrymal, and forms the posterior border of the orbit. The infraorbital sensory canal runs through the jugal, and branches into four short canals that terminate in the posterior portion of the jugal ( Fig. 4 View Figure 4 ).
The dermosphenotic seems to be trapezoidal in shape, forming the posterodorsal border of the orbit.
Three small rectangular supraorbital bones are located along the lateral margin of the frontal, preventing the nasal from contacting the dermosphenotic above the orbit ( Fig. 3 View Figure 3 ). This condition is different from primitive actinopterygians that commonly lack supraorbital bones, or have only a single supraorbital bone (e.g. the so-called adnasal in Cheirolepis ).
CHEEK BONES
The suborbital bones, preopercular, and dermohyal form the dorsal margin of the cheek ( Figs 2 View Figure 2 , 3 View Figure 3 ). Two suborbital bones are present anterior to the preopercular. They are roughly quadrangular in shape, although the upper ossification is slightly larger than the lower one. The presence of two suborbitals is also known in Fukangichtys, whereas other scanilepiforms ( Scanilepis and Evenkia ) have three or more suborbitals.
The preopercular has an enlarged anterodorsal part located above the posterior extension of the maxilla, and a narrow posteroventral part between the maxilla and the subopercular. The preopercular canal runs upwards along the posterior margin, and has a branch that extends forwards at the median portion of this bone ( Figs 2 View Figure 2 , 3 View Figure 3 ). The angle between the posterior margin of the preopercular and the tooth-bearing margin of the maxilla is about 50°, indicating an oblique suspensorium angle. In this condition, the new taxon is clearly different from crown-group neopterygians that have a vertical suspensorium ( Gardiner, Schaeffer & Masserie, 2005).
The dermohyal is a deep, triangular bone, contacting the opercular anteriorly ( Figs 2 View Figure 2 , 3 View Figure 3 ). A similar configuration is commonly seen in other primitive actinopterygians, but crown-group neopterygians have lost this bone entirely.
The toothed maxilla and the subtriangular quadratojugal form the lower margin of the cheek ( Figs 2 View Figure 2 , 3 View Figure 3 ). The maxilla is of the usual ‘palaeoniscoid’ type, having an elongate suborbital portion and a dorsoventrally expanded cheek portion. Dense striae are present on the external surface of the posterior expansion. These striae radiate posteriorly at the level close to the posterior border of the orbit. The maxilla bears at least 21 simple, conical teeth. These homodont teeth are almost equal in size and have an acrodin cap, as generally seen in other actinopterygians.
The subtriangular quadratojugal is relatively large, contacting the maxilla anteriorly and the preopercular dorsally ( Fig. 4B View Figure 4 ). A similar plate-like quadratojugal is also seen in other scanilepiforms (e.g. Scanilepis and Fukangichthys ). In this character, the scanilepiforms are different from the perleidiforms and crown-group neopterygians that have the quadratojugal greatly reduced or entirely lost.
MANDIBLE
The dentary is the largest bone of the wedge-shaped mandible, and is sutured with the angular posteriorly ( Figs 2–4 View Figure 2 View Figure 3 View Figure 4 ). It lacks a prominent coronoid process that is commonly present in crown-group neopterygians. The lower dentition is similar to that on the maxilla, with at least 23 pointed teeth that are roughly equal in height. The mandibular sensory canal is partly preserved along the dentary ( Fig. 3 View Figure 3 ). The enclosed pattern of this canal shows no difference in comparison with other actinopterygians.
OPERCULO–GULAR SERIES
The opercular is high, trapeziform, and inclined anteriorly. The rectangular subopercular is as deep as the opercular. The external surfaces of the opercular and the subopercular are ornamented with dense striae. The interopercular is absent, as in all primitive actinopterygians, whereas crown-group neopterygians commonly have an interoptercular, with the exception of Lepisosteus that lost this bone secondarily ( Olsen & McCune, 1991).
Seven branchiostegal rays are preserved, and these are elongated as usual, as in other actinopterygians ( Figs 2 View Figure 2 , 4 View Figure 4 ). Scanilepis has between nine and 11 pairs of branchiostegal rays ( Lehman, 1979), representing the largest number of this group. The branchiostegal rays are incompletely preserved in other known scanilepiforms, but there are at least three pairs in Fukangichthys ( Su, 1978) , six pairs in Evenkia ( Sytchevskaya, 1999) , and eight pairs in Tanaocrossus ( Schaeffer, 1967) .
An elongate gular element is well preserved in the holotype, with its enlarged posterior portion being
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56.9 46.4 52.5
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12.3 10.9 12.5
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72.3 81.8 78.1
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PAL 47 90 125
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41.5 47.3 41.3
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abbreviations
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65 21 110 37
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160 58 definition for of
Table.
1
Specimen) (TL (V 4885 81 V (4883 135 (V4881 203 See text clearly visible ( Fig. 2 View Figure 2 ). This element is bilaterally asymmetrical, and is interpreted here as the lateral gular. The presence of this element is a different condition from that in the perleidiforms and crowngroup neopterygians, which commonly have a broad median gular, but lack lateral gulars. The median gular is unexposed in all the specimens, as a result of lateral compression of the skull; thus, the morphology of this element is unknown for the new taxon. The closely related Fukangichthys is known from threedimensionally preserved specimens that show the median gular is relatively smaller than that in the perleidiforms and crown-group neopterygians.
PECTORAL GIRDLE
The pectoral girdle includes the post-temporal, supracleithrum, postcleithrum, and cleithrum ( Figs 2–4 View Figure 2 View Figure 3 View Figure 4 ). The post-temporal is a subtriangular bone, located posterior to the extrascapular series ( Figs 3 View Figure 3 , 4 View Figure 4 ). The post-temporal sensory canal runs ventrally through the post-temporal, and enters the dorsal part of the supracleithrum. The supracleithrum is a deep element that contacts the post-temporal posteroventrally ( Fig. 4 View Figure 4 ). The postcleithrum is of a similar depth as the supracleithrum, and is located behind the supracleithrum–cleithrum junction ( Fig. 2 View Figure 2 ). The large cleithrum is, as usual, L-shaped, as seen in other actinopterygians ( Fig. 2 View Figure 2 ). The clavicle is obscured by the branchiostegal rays, and is thus unexposed in all specimens.
PAIRED FINS
The pectoral fins are preserved in PKUP V4882 and PKUP V4884. The ten rays in the pectoral fin are segmented and branched distally (PKUP V4882; Fig. 5 View Figure 5 ). The fringing fulcra are absent, as in all other scanilepiforms but Fukangichthys .
The pelvic fin originates below the 11 th vertical scale row. Eleven fin rays are preserved in the pelvic fin of PKUP V4885. The pelvic fins of the other three specimens are incompletely preserved, with ten fin rays in PKUP V4882, and six in PKUP V4883 and V4884. The fin rays are segmented and distally branched. As in the pectoral fins, fringing fulcra are absent.
MEDIAN FINS
Similar to other scanilepiforms, Beishanichthys possesses a long-based dorsal fin with a lobe-shaped posterior margin ( Figs 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 ). This has been recognized as a scanilepiform synapomorphy ( Lehman, 1979; Sytchevskaya, 1999). The dorsal fin originates at the level of the 13 th vertical scale row, bearing 65 proximally segmented fin rays, of which the anterior one or two short fin rays appear unbranched, whereas the rest are distally branched.
The anal fin originates below the 22 nd vertical scale row, and has a lobe-shaped posterior margin. Of the 24 proximally segmented anal fin rays, the anterior four are short and unbranched, and the rest are long and distally branched ( Fig. 6 View Figure 6 ).
The heterocercal caudal fin bears 18 fin rays, and has a slightly convex posterior margin ( Figs 2 View Figure 2 , 5 View Figure 5 ). The upper caudal fin rays ( Figs 5 View Figure 5 , 6A View Figure 6 ) are proportionally longer than those of chondrosteans and other primitive actinopterygians. The ventralmost caudal fin ray stems from a position below the 39 th vertical scale row. Except for one or two fin rays at the dorsal and ventral margins, all other fin rays are distally branched.
The short segments of median fin rays are ornamented with longitudinal striae. Similar ornaments are also present in other scanilepiformes, including Tanaocrossus ( Schaeffer, 1967; Schaeffer & Donald, 1978: fig. 18), Fukangichthys ( Su, 1978) , and Scanilepis ( Lehman, 1979) . Similar to other scanilepiforms, fringing fulcra are absent in the median fins.
SCALES
The body is covered by large ganoid scales. A total of 42 vertical scale rows are present between the posterior margin of the pectoral girdle and the inversion of the caudal. Scales of the anterior flank are rectangular, and are twice as deep as wide. The depth of the scales decreases dorsally, ventrally, and posteriorly, and the scales become rhomboidal shaped in the caudal region. The flank scales have a straight posterior margin lacking any serrations. The scales of the median and posterior flank regions are twice as long as they are wide, and are smooth, whereas those of the dorsal and ventral regions are ornamented with ganoine ridges ( Fig. 2 View Figure 2 ). Ridge scales anterior to the dorsal fin total 15 in number, and each is ornamented by five ganoine ridges ( Fig. 6C View Figure 6 ). The ventral scales anterior to the pelvis total about 20 in number, and each is ornamented with eight curved ganoine ridges ( Fig. 6G View Figure 6 ). As commonly seen in other lower actinopterygians, the articular pegs and anterodorsal extensions are present on the flank scales ( Fig. 6F View Figure 6 ).
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