Cynoclupea nelsoni, Malabarba & Di Dario, 2017

CYNOCLUPEA NELSONI SP. NOV.

Holotype: MCP 3447 View Materials -PV, fossil specimen corresponding to the anterior part of a fish including the head and the first 11 vertebrae.

Etymology: The species is named after Gareth J. Nelson (University of Melbourne, Australia). In a series of now-classic papers ( Nelson, 1967, 1970, 1973), Gary Nelson established the basis for understanding clupeomorph intrarelationships from a modern phylogenetic (cladistic) perspective.

Locality and age: The holotype and single specimen known was collected in the shales of the Morro do Chaves Formation of the Sergipe-Alagoas Basin exposed at the Cimpor quarry, municipality of S ao ~ Miguel dos Campos, Alagoas State, Brazil. The locality is correlated with the Barremian, Lower Cretaceous ( Campos Neto et al., 2007) .

Diagnosis: Cynoclupea nelsoni is a medium-sized, herring-like clupeiform distinguished from all other members of the order by the following combination of characters: head triangular in lateral view, acuminated snout; long and straight lower jaws with a conspicuous dentition consisting anteriorly of enlarged and backward curved teeth followed by smaller teeth that extend well back on the dentary border (vs. absent or inconspicuous dentary teeth restricted to the anterior region of the bone); presence of three massive conical teeth slightly orientated backwards on the palatine; suspensorium slightly inclined posteriorly, being almost vertically orientated (vs. clearly posteriorly or anteriorly orientated); dorsal margins of the symplectic and quadrate in line; quadrate with a laminar outgrowth on the anterior margin; two laminar postcleithra (vs. rod-like postcleithra in clupeids; postcleithra absent in chirocentrids, engraulids, and most pristigasterids).

DESCRIPTION

The type specimen consists of the anterior part of an articulated fish including the head and the first 11 vertebrae. The head is relatively large, longer (62 mm) than deep (44 mm), triangular in lateral view, with a somewhat acuminated snout. The dorsal and ventral profiles are almost straight, diverging from the terminal mouth towards the origin of the dorsal and pectoral fins, respectively. Based on head length and height, C. nelsoni is assumed to be moderately elongated, with an estimated standard length of approximately 240 mm.

Neurocranium

The somewhat distorted nature of the fossil precludes precise observation of some features and anatomical details. The skull roof is represented by the frontals, which are ornamented with parallel grooves and extend between the parietal and the anterior tip of the head ( Fig. 3 View Figure 3 ). Anteriorly, the frontals are poorly preserved, represented by a portion of the bone associated with the supraorbital sensory canal. A small and tubular nasal is preserved at the front margin of the frontals ( Fig. 6 View Figure 6 ). The frontal forms the roof of the orbit laterally, and a lateral wing seems to meet the sphenotic ventrally, limiting the dilatator fossa. Posteriorly, the frontals are more extensively preserved, diverging slightly and meeting the parietals. Even though the precise borders of the bones in this region of the cranium could not be defined, a large parietal, bearing a distinct backward-curved tube, which is herein interpreted as the supratemporal commissural sensory canal (a section of the extrascapular canal; Di Dario, 2004), is identified in the right side of the skull. An upward compression of the ventral region of the posterior third of the neurocranium and some displacement of the bones at the back of the skull, posterior to the parietal, seem to have occurred during fossilization. An epioccipital is tentatively identified in the right side of the skull ( Fig. 4 View Figure 4 ). In lateral view, the epioccipital has a typical tripartite structure and forms the posterodorsal corner of the skull in modern clupeiforms ( Ridewood, 1904; Di Dario & de Pinna, 2006: fig. 3). The shape of the dorsal third of the bone is apparently intact, but its ventral arm, which usually, in modern clupeoids, articulates with the exoccipital and the pterotic inside the pre-epiotic fossa, seems to be dorsally dislodged in such a way that its ventral tip inserts more deeply in the pre-epiotic fossa ( Fig. 4 View Figure 4 ). This condition might be a consequence of compression during fossilization. The pre-epiotic fossa is low in comparison to the typical condition in modern clupeiforms, also probably as a consequence of the compression of the skull. The exoccipitals are partially fragmented and displaced (mainly the left one) in relation to the epioccipital, again probably as a consequence of the distortion at the back of the skull. A relatively large, but fragmented, supraoccipital seems to medially separate the parietals, as in the typical clupeiform condition ( Fig. 4 View Figure 4 ; Grande, 1985). A temporal foramen, usually located between the parietal and frontal in clupeiforms ( Lauder & Liem, 1983; Di Dario & de Pinna, 2006), was not identified in the specimen. Its apparent absence in the fossil, however, may be a result of the distortion of the cranium that displaced the right parietal and epioccipital into a more lateroventral position. If this is the case, then the shallow depression at the ventral margin of the parietal, anterior to the epioccipital and between the parietal and the lateral wing of the frontal, is a remnant of the structure.

The right hyomandibula is ventrally dislocated, revealing substantial portions of the prootic, pterotic, exoccipitals, and basioccipital ( Fig. 5 View Figure 5 ). The prootic is large and somewhat expanded, with ridges and grooves crossing its surface. The ‘strawberry’ appearance, which typically indicates the presence of the prootic bulla in modern clupeiforms (Grande, 1985), could not be detected in C. nelsoni . However, the slightly inflated morphology of the prootic and the radial fibrous structure of its marginal bony surface suggest the presence of a bulla. A slender and curved lateral commissure for the passage of the hyomandibular trunk nerve is identified in the prootic surface, near the orbit. The pterotic is only partially visible, with a posteriorly directed spine and bearing three clearly distinct openings ( Fig. 5 View Figure 5 ), which are herein interpreted as the following openings of sensory canals to the chamber of the recessus lateralis ( Di Dario, 2004): the posterior opening of the recessus (por), which in living clupeiforms connects with the extrascapular and post-temporal canals; the middle opening (mor), which probably connected the recessus of C. nelsoni with the preopercular canal; and the anterior opening (aor), which externally would have connected the recessus chamber with the infraorbital canal, even though only traces of one or more infraorbital bones, not connected or in close proximity with this opening, were identified ( Fig. 2B View Figure 2 ). A vertically orientated slit can also be seen on the region of the pterotic that supposedly housed the recessus chamber, between the middle (mor) and anterior (aor) openings. If this slit was indeed originally present in C. nelsoni , it might represent the opening of the recessus for the accessory temporal sensory canal (acr), typically present in clupeoids ( Di Dario, 2004), or the posteroventral opening of the recessus (pvor), which so far has been identified only in D. clupeoides ( Di Dario & de Pinna, 2006) . The preserved walls of the pterotic have the same fibrous structure as the prootic, suggesting that a bulla was also present in this bone. The right exoccipital ( Fig. 5 View Figure 5 ) closes posteriorly the occipital region of the neurocranium. Its bulbous basal region bears two openings: the anterior, somewhat funnel-shaped opening, possibly representing the foramen through which the right anterior diverticulum of the swimbladder penetrated the neurocranium; and the posterior opening, which is probably the foramen shared by the glossopharyngeal and vagus nerves (IX + X). The exoccipital expands anteriorly, contacting the prootic and forming the auditory fenestra ( Fig. 5 View Figure 5 ). The intercalar was also tentatively identified, lying between the exoccipital and the prootic, dorsal to the auditory fenestra. The posterior region of the basioccipital distinctly shows a circular articular surface for the first vertebra ( Fig. 5 View Figure 5 ).

The parasphenoid is preserved as an edentulous and straight bone for most of its length ( Figs 2 View Figure 2 , 6 View Figure 6 ); its anterior tip is slightly curved upwards, meeting the vomer. An ascending arm in its posterior region contacts the prootic. A foramen for the internal carotid artery is also preserved in the parasphenoid, in the region of contact with the prootic. Owing to the state of preservation, it is not clear if the parasphenoid projects posteriorly beyond the basioccipital, as in most modern clupeoids. Bony remains tentatively identified as parts of the lateral ethmoid contact the anterior portion of the parasphenoid shaft.

Palatoquadrate and suspensorium

Only portions of the palatoquadrate and the suspensorium can be identified in the fossil, with the quadrate – symplectic complex as one of the most distinctly preserved features ( Fig. 3 View Figure 3 ). The shape of the The proximal region of the hyomandibula is expanded in a fan-shaped structure, forming a straight margin with no differentiated articular heads for the sphenotic and pterotic ( Fig. 5 View Figure 5 ). Ventrally, the hyomandibula narrows abruptly into a vertical limb directed towards the posterior margin of the quadrate and the symplectic, but its length could not be determined because its distal third is broken ( Figs 2A View Figure 2 , 3 View Figure 3 ). The hyomandibula is slightly inclined posteriorly, being almost vertically orientated. Parts of a toothless endopterygoid ( Fig. 6 View Figure 6 ) and possibly the metapterygoid are visible in the orbit region ( Fig. 2B View Figure 2 ). However, the anterior portion of the metapterygoid and the posterior region of the endopterygoid are not preserved and, therefore, their topographical relationships with other bones of the suspensorium could not be determined. Only the anterior tip of the palatine is visible, with three massive conical teeth slightly orientated backwards ( Figs 2B View Figure 2 , 6 View Figure 6 ).

endochondral portion of the quadrate is similar to that of an equilateral triangle, with a roughly straight dorsal margin. The anterior margin of the quadrate has a narrow laminar outgrowth, where a fragment of the slender ectopterygoid is still firmly attached ( Fig. 3 View Figure 3 ). A short, splint-like posterior process into which the symplectic inserts is also clearly discernible in the quadrate, as in the typical condition present in teleosts in general, including modern clupeiforms ( Arratia & Schultze, 1991; Di Dario, 2009). The condyle of articulation of the quadrate with the lower jaw is practically ventrally orientated. The symplectic is elongated and nearly straight. Its dorsal margin is in line with the dorsal margin of the quadrate.

Jaws

The jaws are long and relatively straight. Teeth are present on the premaxillae, maxillae, and dentaries. A substantial portion of what is herein identified as the right and left premaxillae is preserved, but the bones are displaced from their original position. Overall, the preserved fragments of the premaxillae indicate that the bone was nearly triangular and slightly curved in lateral view. The left premaxilla is turned over the mesethmoid, in such a way that its ventral margin is facing up ( Fig. 6 View Figure 6 ). The right premaxilla, by contrast, is nearly totally preserved, and bears a dorsally orientated protuberance that seems to be a rudimentary ascending process ( Fig. 6 View Figure 6 ). The preserved dentition of the bone is made up of a few needle-shaped, irregularly sized teeth distributed along the oral border, apparently in a single row.

The maxilla is slender and very long, covering laterally the posterior region of the dentary ( Fig. 2 View Figure 2 ). The bone has a compressed baseball bat shape, deeper posteriorly and narrowing anteriorly to form a distinct head. Its oral margin is covered with a row of small, closely placed, and equally sized teeth, resulting in a comb-shaped structure as in the typical condition amongst modern clupeiforms. A fragment of bone attached to the dorsal margin of the maxilla is herein interpreted as part of the supramaxilla ( Fig. 3 View Figure 3 ). Another fragment of a flat, somewhat elongated, structure with a rounded posterior margin is also present on the counterpart of the fossil approximately in the region where the posterior end of the upper maxilla was probably located ( Fig. 2B View Figure 2 ). This fragment is also interpreted as a supramaxilla. If this identification is correct, and given the relatively well-developed degree of development of the structure, it is presumed that C. nelsoni had two supramaxillae.

Based on the preserved parts, the lower jaw is long and robust, high posteriorly, and tapering towards the symphysis. Differing from the condition described for the maxilla, the dentary dentition is very conspicuous and well developed: two enlarged and backward-curved teeth at the anterior tip of the dentary are followed by at least five smaller and fully preserved teeth of similar shapes. In addition, a series of bony protuberances in the dentary border suggests that regularly spaced and decreasing in size teeth were present along most, if not all, the oral margin of the bone ( Fig. 7 View Figure 7 ). The anterior portion of the mandibular sensory canal is included in the ventral third of the dentary ( Figs 3 View Figure 3 , 6 View Figure 6 ). A fragment of the posterior portion of the anguloarticular is recognized in the region of articulation of the lower jaw with the quadrate ( Fig. 2 View Figure 2 ).

Opercular and hyobranchial apparatuses

Only fragments of the opercular and hyobranchial apparatuses are preserved, and most components of these systems are displaced from their presumed original positions. The preserved margins and surfaces of the opercular apparatus are smooth. The preopercle is probably the best-preserved bone of the series ( Fig. 3 View Figure 3 ). It has a crescent-shaped structure, as in the typical condition amongst modern members of the Clupeoidei . The bone is longitudinally crossed by the preopercular sensory canal, which bears some distinct openings. The angle of the preopercle is conspicuous, and caudally projected. A small remnant of what is apparently a flat interopercle is located along the ventral border of the preopercle ( Fig. 3 View Figure 3 ). The largest fragment of the opercle ( Fig. 2B View Figure 2 ) is roughly shaped like an equilateral triangle. However, other opercle fragments, more dorsally located ( Figs 2A View Figure 2 , 3 View Figure 3 ), indicate that the opercle of C. nelsoni was large, approximately square-shaped, slightly wider than deep, and with a somewhat concave ventral margin. The subopercle is also preserved, but was displaced dorsally during fossilization ( Fig. 3 View Figure 3 ). The bone has a straight dorsal margin and a well projected and sharp anterodorsal ascending process, where the adductor mandibular probably attached during life.

Scattered remains of well-ossified gill arches and rakers are preserved in abundance. However, precise description of the structures and assessment of homologies are prevented by the state of preservation. Overall, elements identified as gill rakers ( Fig. 3 View Figure 3 ) are relatively numerous and well ossified, somewhat slender, closely packed, and apparently toothless; these last two features might result from the fossilization. An approximately triangle-shaped and relatively well-developed structure located in the posterior region of the branchial chamber is tentatively identified as an edentulous fifth ceratobranchial ( Fig. 3 View Figure 3 ). No evidence of epibranchial organs could be detected. Five branchiostegal rays are preserved, the two anterior ones being tubular, and the three more posterior, broad and smooth.

Pectoral girdle and fins

Parts of the pectoral girdles are damaged or concealed by vertebral elements, but the main components of the left and right sides are visible. Both post-temporals are near to their original position, and can be observed in medial (left post-temporal) and lateral (right post-temporal) views ( Figs 2A View Figure 2 , 7 View Figure 7 ). They are well-developed, flat bones, with a roughly rounded posterior outline and a large anterior arm that in life would have articulated with the epioccipital. A fracture in the left post-temporal reveals a segment of the lateral line extending through its posterior third. The proximal portion of the right supracleithrum is preserved, although partially concealed by postcranial remains. The bone seems to be relatively long and plate-like, with its dorsal region articulating with the corresponding post-temporal.

The portion of the left and right pectoral girdles ventral to the post-temporal and supracleithral elements is slightly dislodged into a more posteroventral position than would have occurred in life ( Fig. 3 View Figure 3 ). Both cleithra are preserved, but the left one is partially concealed by its right counterpart ( Fig. 3 View Figure 3 ), which is therefore described herein. The cleithrum is developed, being the largest element in the pectoral girdle of C. nelsoni . The proximal region of the cleithrum is approximately vertical, but the main vertical shaft of the bone curves anteriorly as it descends towards the ventral region of the specimen. The cleithrum seems to be well expanded posteriorly above the level of insertion of the pectoral fin. The scapula, which should be located in this region, could not be identified in the fossil. The lateral margin of the ventral third of the cleithrum projects laterally and partially covers the coracoid, forming the concavity where the pectoral-fin musculature originates in extant teleosts. Both coracoids, right and left, are partially visible ( Fig. 3 View Figure 3 ); they are rectangular, flat, with straight ventral margins. The mesocoracoid could not be identified, but the bone is probably concealed behind the pectoral-fin girdle elements, especially the right cleithrum. Two laminar postcleithra are partially visible behind the dorsal third of the cleithrum, the posterior one being more elongated ( Fig. 3 View Figure 3 ).

Between 17 and 18 pectoral-fin rays are preserved in the specimen, most notably in the right side, the outermost ray being the most developed. Ventrally to these rays, a few bases of the left pectoral-fin rays can also be seen ( Fig. 3 View Figure 3 ). As only the proximal half of most rays are preserved, it is not possible to determine the full extent of the pectoral fin, but the structure is obviously well developed. Some skeletal support elements for the pectoral fin (distal and/or proximal radials) are preserved in the region ventral to the bases of the pectoral-fin rays ( Fig. 3 View Figure 3 ).

Axial skeleton and associated structures

The anterior-most 11 vertebrae are preserved, and therefore the description below refers to this portion of the axial skeleton ( Figs 3 View Figure 3 , 7 View Figure 7 ). The vertebral centrae are slightly deeper (3.10 mm) than long (2.94 mm), thick and laterally sculptured with at least four longitudinal ridges. They are completely ossified with a strongly constricted notochord, in such a way that the notochordal pit is not discernible. The neural arches are autogenous, and the corresponding pairs of neural spines are also unfused amongst themselves. A few rod-like bony structures preserved in the dorsal region of the centrae between approximately the sixth and the last preserved vertebrae are tentatively identified as epineurals, their bases apparently fused with each corresponding neural arch ( Fig. 8 View Figure 8 ). The pleural ribs are strongly developed and roughly rod-like in cross-section. Their proximal region is curved proximally, each rib having an expanded and straight head for articulation with the centrum. Bony rod-like structures tentatively identified as epipleurals are present amongst the pleural ribs, but it is not clear if these elements were fused to ribs as in the Clupeoidea ( Patterson & Johnson, 1995; Di Dario, 2002).

Seven elements of the supraneural series were identified in the fossil, but only the anterior-most four are preserved well enough to be described ( Fig. 8 View Figure 8 ). Supraneurals are well developed and rostrocaudally expanded, slightly differing in shape from each other. The endochondral, rod-like portion of the supraneurals is located at the posterior third of the bone, except in the case of the second supraneural, where this portion of the bone is located approximately in the middle of the structure. The laminar bone projects anteriorly from the endochondral core of the supraneurals, resulting in a roughly rectangular shape, except for the second supraneural (described below). The anterior-most supraneural preserved was probably the first of the series in the living specimen, as it lies close to the post-temporals, as in modern clupeiforms. The second supraneural is more triangle-shaped, expanded proximally and narrow distally, with its posterior margin contacting the anterior margin of the third supraneural. The third and fourth supraneurals are still somewhat rectangle-shaped, but the laminar portion of their proximal half is more expanded anteriorly ( Fig. 8 View Figure 8 ). The first and second supraneurals are preserved in an almost vertical orientation in relation to the vertebral series, but the subsequent ones are slightly inclined posteriorly. However, differences in the orientation amongst these bones are possibly artefacts of fossilization.

Squamation and scutes

Remains of thin cycloid scales are preserved in the lateral region, mainly over the ribs. A few abdominal scutes are preserved on both sides of the fossil specimen. Bony elements identified as abdominal scutes are attached to their presumed living position at the posteroventral margin of the pectoral girdle. More developed scute-like elements, bearing the typical clupeomorph ventral keel, are also present ventrally to the pectoral fin-base insertion ( Fig. 3 View Figure 3 ). In addition, fragments of three to four abdominal scutes with remains of ascending arms are preserved in the pre-pelvic region of the specimen ( Fig. 2B View Figure 2 ). Remains of three bony structures tentatively identified as dorsal scutes are also preserved in the occipital region ( Figs 3 View Figure 3 , 4 View Figure 4 ). These structures are keeled and not ornamented.