Amakusaichthys benammii, Alvarado-Ortega, 2024

Amakusaichthys benammii sp. nov.

zoobank.org/ F7C9689F-D7D1-4182-95D9-23EDE83E74DE

Holotype. IGM 14027 View Materials , a nearly complete specimen preserved in part ( IGM 14027 View Materials a) and counterpart ( IGM 14027 View Materials b), both transferred to resin ( Figure 2 View FIGURE 2 ).

Paratypes. IGM 14028 View Materials , a complete and articulated specimen preserved in part ( IGM 14028 View Materials a) and counterpart ( IGM 14028 View Materials b) . IGM 14029 View Materials , an incomplete specimen with the head dorsally titled and lacking the caudal part. IGM 14030 View Materials , an incomplete specimen without the post pelvic part of the trunk, preserved in part ( IGM 14030 View Materials a) and counterpart ( IGM 14030 View Materials b), both transferred to resin . IGM 14031 View Materials , head and part of the abdominal region with bones somewhat scattered. IGM 14032 View Materials , head fish with both limbs of the lower jaw disarticulated. IGM 14033 View Materials , head lacking the ethmoid anterior tip, preserved in part ( IGM 14033 View Materials a) and counterpart ( IGM 14033 View Materials b) . IGM 14034 View Materials , a vertebral column nearly complete, transferred to resin. IGM 14035 View Materials , a fragment of the posterior part of the trunk with the caudal skeleton. IGM14036 View Materials , head and part of the abdominal region.

Etymology. The species epithet honors my friend and colleague, Dr. Mouloud Benammi, for his valuable contributions to Mexican paleontology and paleomagnetostratigraphy ( Figure 3 View FIGURE 3 ).

Age and distribution. All specimens come from the Campanian marly deposits of the Angostura Formation extracted in the Tzimol Quarry outside the Ochuxhob town, Comitán Municipality, Chiapas, southeastern Mexico.

Diagnosis. Compared to its sister species Amakusaichthys goshouraensis , A. benammii is distinguished by the skull strongly ornamented with numerous pores plus sinuous and short ridges, mainly present on the frontal and parietal bones (in A. goshouraensis the skull is relatively smooth); small frontal-parietal fontanel (unknown in A. goshouraensis ); two masticatory-like processes present in the parasphenoid bones behind the basipterygoid process (unknown in A. goshouraensis ); ethmopalatine with a dorsal extension bordering the nasal capsule dorsoanteriorly ( A. goshouraensis lacks such ethmopalatine dorsal extension); the maxilla has a dorsal small palatine notch ( A. goshouraensis lacks this maxillary palatine notch); the hypural 2 is 1.5 times longer than hypural 3 (the hypural 3 only marginally shorter than the hypural 2 in A. goshouraensis ); the hypural 3 with a small folded corner ridge similar to that of the hypural 2 but projected ventrodorsally and unpierced (in A. goshouraensis ) this ridge is unknown); the vertebral column has 78 total centra including 54 abdominals, 22 preurals, and two urals ( A. goshouraensis has nearly 76 total centra but among these 50 are abdominals, 24 preurals, and two urals).

Description

General features and proportions. Table 1 summarizes the measurements and body proportions of the specimens described here as Amakusaichthys benammii sp. nov. In this elongated-bodied fish, the length and height of the triangular head represent 18.5 and 13.2% of the SL, respectively. The trunk is evenly high along its predorsal region, which is barely higher than the head and represents 14% of the SL; behind, the trunk becomes less high up to the caudal peduncle, whose height only represents 5.2% of the SL. The short mouth opens upward; the lower jaw articulates the skull below and ahead of the orbit. The paired fins are near the trunk borders. The dorsal and anal fins rise into the last fifth of the SL. The dorsal fin is short and spreads between 80.5 and 85.9% of the SL; on the contrary, the anal fin is slightly longer and extends between 81 and 90% of the SL. The pelvic fin is placed in the posterior half of the abdomen, at 70% of the SL, about 4.5 times farther from the pectoral fin than from the anal fin. The caudal fin is deeply forked and consists of two triangular opposed and noticeably uneven-sized lobes, with the ventral lobe nearly two times longer than the dorsal one.

Skull. In lateral view, the skull is roughly triangular, tapered, and rounded anteriorly ( Figures 2 View FIGURE 2 , 4 View FIGURE 4 , 5 View FIGURE 5 ). From the anterior tip of the ethmoid region to the occipital condyle, it is nearly 1.6 times longer than high. The postorbital region is relatively small and hardly represents 25% of the skull length; in contrast, the orbit and the ethmoid region occupy the rest of the skull almost equally, with the orbit only slightly larger. The anterior projection of the principal axis of the hyomandibular fossa is anteroventrally oblique to the orbital section of the parasphenoid bone and passes below the skull ethmoid region. The inflection angle of the parasphenoid is between 120 and 135° (see below).

The skull is triangular in dorsal and ventral views ( Figures 4–6 View FIGURE 4 View FIGURE 5 View FIGURE 6 ). Its ethmoid region is triangular, almost 1.5 times longer than it is wide, and tapered and rounded anteriorly. In contrast, its post-ethmoid region is rectangular, barely 1.3 times longer than wide, and has sinuous lateral edges in which the frontals (= parietal sensu Schultze, 2008) have a long and shallow concave curvature above the orbital. Behind the orbital, the roof of the skull is pierced medially by a small frontal-parietal fontanel.

The rostrodermethmoid is a complex bone dorsally extended in the anterior quarter of the skull length ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). This triangular bone is not constrained laterally in the dorsal view. In the dorsal view, this broad triangular bone covers most of the ethmoid region and a large part of the olfactory cavity. This bone has numerous zigzagging sutures that attach its entire posterior edge to both frontals. Three small processes are present in the anterior part of this bone; a narrow and rounded anterior medial process forms the skull tip, and two small lateral descending processes suture the ethmopalatine bones. The supraethmoid is almost entirely covered by the rostrodermethmoid, except for two small regions: one exposed below the junction between the rostrodermethmoid and mesethmoid in the anterior inner region of the nasal capsule in IGM 14027; the other forming part of the crescent maxillary facet of the ethmoid region ( Figure 5 View FIGURE 5 ). In its anterior tip and between its lateral processes, the rostrodermethmoid is superficially smooth; however, further back, it is intensely ornamented by numerous pores and sinuous ridges of irregular size and arranged in a radiant pattern from the tip of the bone ( Figures 5–6 View FIGURE 5 View FIGURE 6 ).

The ethmopalatine is a paired elongated complex bone in the ventral part of the skull ethmoid region that borders the nasal capsule and laterally has three processes that contact the frontal, supraethmoid, rosterodermethmoid and lateroethmoid bones ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). In lateral view, the posterior limb of this bone sutures in a deep zigzagging pattern with the anterior descendent process of the lateroethmoid bone. The dorsal limb of the ethmopalatine forms the anterior and dorsal edges of the nasal capsule below the respective frontal bone. It reaches the level of the ascending limb of the lateroethmoid. The anterior ethmopalatine limb precedes a deep ventral notch, showing an irregular surface and outgrowths suturing with the lateral descending process of the rostrodermethmoid. The anterior ethmopalatine limb forms part of two ventral facets, lying ahead and close to the anterior end of this part of the head, far from the nasal capsule. The anterior of these facets articulates the most anterior dorsal maxillary process and is formed by the ethmopalatine and the supraethmoid; in contrast, the posterior facet is formed only by the ethmopalatine and joints with the head of the palatine. The ethmopalatine is smooth except for part of its dorsal limb, which shows the same pattern of pores and ridges in the posterior part of the rostrodermethmoid and frontal.

The lateroethmoid consists of a triradiate wing laterally exposed plus a thin medial concave wing forming the anterior wall of the orbit ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). The posterodorsal limb of this bone is broad, forms its anterior third, sutures the frontals, and borders the posterior nasal capsule posteriorly. The posteroventral limb of this bone extends ventrally beyond the parasphenoid. The anterior process is as long as the nasal capsule and sutures the ethmopalatine bone, following a deep zigzagging pattern. The foramen for the olfactory nerve is exposed in a laminar and rounded medial component of this bone extended into the nasal capsule. In IGM 14027, the nasal bone is an elongated rod-like structure preserved in the nasal capsule. In the base of the skull ethmoid region, the vomer is an elongated toothless bone with a broad anterior expansion and a long and sharp posterior limb that sutures with the parasphenoid.

The frontals are roughly rectangular bones that cover a little less than half the length of the skull above the orbit. These are anteriorly broad, posteriorly expanded, and joined together by a slightly sinuous middle suture that extends along their entire length except at the most posterior end, where these separate to form the anterior third of the frontal-parietal fontanel. The posterior end of each frontal sutures to the mesoparietal, pterotic, and sphenotic posteriorly while anteriorly attaching to the rostrodermethmoid and lateroethmoid. The lateral edge of the frontal that roofs the nasal capsule is straight, not notched ( Figures 5–7 View FIGURE 5 View FIGURE 6 View FIGURE 7 ). Numerous pits and ridges showing two patterns cover the dorsal surfaces of the frontals: 1) around the middle suture, these bones exhibit an oval zone in which these ornaments radiate from the point furthest from the side edges; 2) around this oval zone, such ornaments are posteroanteriorly aligned.

The parietals (= postparietal sensu Schultze, 2008) are fused, forming a single mesoparietal ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). This huge bone is far from the posterior skull edge, roofs nearly half of the skull behind the orbit, and sutures the frontal, pterotic, supraoccipital, and epioccipital. Although this bone has a slight medial elevation before suturing to the anterior tip of the supraoccipital, it definitively does not form part of the supraoccipital crest. Posteriorly, the mesoparietal has two posterior lateral projections that barely border the anterior end of such crest. Also, the mesoparietal borders the posterior two-thirds of the narrow frontal-parietal fontanel. Numerous pits of irregular size and random distribution in the mesoparietal surface.

The posterior half of the dorsal postorbital region of the skull is occupied chiefly by three elongated bones that form conspicuous crests ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). In the middle, the supraoccipital displays a thin and triangular supraoccipital crest with a concave posterior border, culminating in an acute dorsoposterior tip that projects up to the level of the occipital condyle. In a lateral position to this medial crest, the epioccipital bones rise, forming a couple of relatively shallow robust epioccipital crests that are straight and project posteriorly up to the base of the posterior border of the supraoccipital crest. These bones are smooth superficially.

The autosphenotic is a triangular bone that borders the posterodorsal part of the orbit and sutures the frontal dorsally and the pterotic posteriorly ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). A medial extension of this bone behind the orbit forms part of the anterior end of the hyomandibular fossa. The ventral opening of the supraorbital canal is so large and deep in this bone that it separates the superficial plane of this bone, giving it an inverted Y appearance.

The pterotic is a somewhat trapezoidal bone exposed in the dorsolateral surface of the skull postorbital region ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). This bone joins the intercalar ventroposteriorly, the autosphenotic ventroanteriorly, and the frontal, parietal, and epioccipital medially. The anterior tip of this bone sutures the frontal ahead of the autosphenotic. Posteriorly, it joins the exoccipital bone and roofs the posttemporal fossa. Additionally, it forms part of the posterior edge of the skull, shows an anterodorsally inclined fold that forms the roof of the lateral temporal fossa, and its ventral border covers the temporal region of the skull that houses the hyomandibular and supratemporal fossae. The pterotic has a deep groove in which the otic sensory canal opens. Numerous pits aligned radially from its center cover the surface of this bone.

The intercalar bone is elongated and occupies the posterolateral part of the postorbital region of the skull, suturing the basioccipital ventrally, the protic anteriorly, and the pterotic dorsally ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). The ventral part of this bone becomes a noticeable lateral protruding massive structure behind the orbit. This massive bone extends forward, forming the posterior end of the hyomandibular fossa and ventrally roofs the jugal canal (where the jugular vein runs and the foramina for the glossopharyngeal and vagus nerves open).

The bones of the ventral part of the skull are exposed laterally in IGM 14027 and ventrally in IGM 14030 ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). The prootic is a broad rectangular bone occupying most of the skull postorbital region behind the orbit. Dorsally, the prootic forms the large and middle part of the hyomandibular fossa and forms most of the subtemporal fossa. Near its dorsoanterior end, the prootic has two large foramina corresponding to the hyomandibular and palatine branches of the cranial nerve VII. The basioccipital bone forms the basal half of the skull’s posterior height. This bone is rectangular in both views and shows deep depressions around the occipital condyle. The sharp and deeply zigzagging projections of the parasphenoid extend almost along the entire length of the basioccipital, providing a tight suture between them. The occipital condyle is an articular surface formed only by the basioccipital, in which its rounded shape has a deep concave cavity.

The parasphenoid is an elongated complex and toothless bone with two differentiable parts ( Figures 5–6 View FIGURE 5 View FIGURE 6 ). Anteriorly, the orbital parasphenoid section is a long bar extended along the orbit and most of the base of the ethmoid region below the vomer. This parasphenoid section is inverted Tshaped in cross section and consists of a high dorsal lamina plus a ventral horizontal lamina expanded in its anterior and posterior ends. In ventral view, this horizontal lamina has a deep anterior notch to allocate the posterior limb of the vomer; in contrast, its posterior end expands laterally and forms a T-shaped structure together with the pair of basipterygoid processes that are long, straight, and tilted laterally, ventrally, and anteriorly. The postorbital parasphenoid expands dorsally to form the basal lateral quarter of the skull. It has an elongated sharp posterior projection to suture the basioccipital bone, almost reaching the occipital condyle. Behind and below the basipterygoid process, two large foramina are present. One corresponds to the afferent pseudo-branchial artery, and the other to the internal carotid artery. Beyond the orbit, the postorbital section of the parasphenoid has an anterior longitudinal expansion with two ventral processes, which are projected downward and end in broad masticatory-like plates ( Figure 7 View FIGURE 7 ). The anterior of these ventral processes, also the widest and the most ventrally projected, has a heart-shaped plate that resembles those masticatory processes of basioccipital bones present in cypriniform fish (e.g., Ramaswami, 1955, figure 4); in contrast, the posterior process is arrow-shaped and points posteriorly.

The posterior longitudinal expansion of the postorbital section of the parasphenoid causes the inflection point between the orbital and postobital sections of this bone to be placed far behind the basipterygoid process, forming an angle near 120°. In general, in other ichthyodectiforms, the inflection point between the two sections of the parasphenoid occurs just behind the basipterygoid process because this bone does not have longitudinal anterior expansion in the postorbital section. Amakusaichthys benammii sp. nov. has an angle of about 135° formed at the base of the posterior border of the pterygoid process, between the ventral border of the orbital section of the parasphenoid and the straight line formed from the pterygoid process to the ventroposterior termination of the basicranium (as the angle of inflection of the parasphenoid in other ichthyodectiforms measures it).

Upper jaw. Each branch of the upper jaw consists of four flat and thin bones, including the premaxilla, maxilla, and two supramaxillae ( Figures 5 View FIGURE 5 , 8 View FIGURE 8 ). The mouth opens anterodorsally. The premaxilla is an undulating laminar rhomboidal bone widely attached to the anterior surface of the maxilla. The premaxilla has slightly convex edges and a shallow notch in the dorsal half of the medial edge. Scattered small pits ornament the labial surface of this bone. The premaxillary alveolar edge exhibits a row of 10 to 12 conical, straight, hollow, and slen- der small teeth. These are projected transversally from the alveolar edge, evenly sized, closely distributed, and set into deep alveoli.

The maxilla is a sinuous bone. Its alveolar section is a laminar, flat, and curved structure superficially ornamented with scattered pits and grooves ordered in an anterior-posterior radiating pattern ( Figure 8 View FIGURE 8 ). The maxillary alveolar edge is convex and shows numerous small teeth resembling the premaxillary teeth. In contrast, its toothless articular section is rectangular and robust, as long as one-fifth of the total maxillary length and at least as high as two-thirds of the maximum height of the maxilla. These two sections are inclined to each other, forming a dorsal inflection angle of about 235°. The maxilla shows two robust articular processes that barely protrude from the dorsal margin of its articular section. The anterior or ethmoid process of the maxilla joins the anterior ventral articular facet of the skull ethmoid region formed between the ethmopalatine and supraethmoid. On the other hand, the posterior or the maxillary palatal process articulates with the ventral part of the head of the palatine. Right at the dorsal inflection point, behind its palatal process, this bone shows a small notch, here called the maxillary palatine notch, which allocates the ventral flange of the palatine bone (below, see description of the palatine bone).

The supramaxillae are laminar and smooth bones extended above the maxilla. The anterior supramaxilla is oblong and extends along most of the alveolar maxillary section. The posterior supramaxilla is oval, with the acute anterior end overlapping most of the anterior supramaxilla.

Lower jaw. Each branch of the lower jaw consists of the dentary, angular, articular, and retroarticular ( Figure 8 View FIGURE 8 ). Together, these bones form a robust and rectangular lower jaw slightly longer than the upper jaw, about two times longer than high, and with a somewhat convex ventral edge. The high and straight mandibular symphysis is lightly inclined anterodorsally, and the coronoid process is even higher than the alveolar section of the jaw, which rises near half the jaw length. The postarticular process is small and protrudes backward.

The dentary is a deep trapezoid bone with a posterior edge forked in the lingual view and scarcely forked in the labial view ( Figure 8 View FIGURE 8 ). The ventral dentary posterior limb is extended backward up to the base of the postarticular process. The dentary shows a high symphysis, nearly half the coronoid process height. This bone shows a longitudinal thickening that protrudes near its ventral border and extends along its entire length up to the base of the postarticular process. The dentary alveolar edge is sinuous and bears a single row of numerous small teeth that resemble those already described in the upper jaw bones. Numerous pits randomly distributed are in the labial surface of the anterior third part of this bone. The mandibular sensory canal is bone-enclosed, runs longitudinally, and opens in pores near the ventral edge of the dentary.

The angular bone is a roughly triangular bone exposed in both the lingual and labial surfaces of the lower jaw ( Figure 8 View FIGURE 8 ). It has an acute anterior edge that sutures the bifid posterior dentary edge, with poor penetration in the labial view but reaching the anterior third of the mandible in the lingual view. Its labial posterior extension forms most of the postarticular process, and its lingual surface participates in the articular facet of the quadrate. The postarticular process is robust, shallow, and extends forward, representing about one-sixth of the mandibular length. The retroarticular bone is small and does not form part of the articular facet for the quadrate.

Circumorbital bones. Bones of the circumorbital series are flimsy, thin, and laminar ( Figures 5–8 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 ). Unfortunately, these are poorly preserved, crushed, and broken in the referred specimens because they are usually strongly compressed against the skull. Nevertheless, these bones enclose the orbit. The antorbital is long and oblong, covering most of the skull ethmoid region. Three infraorbital bones border the orbit ventrally; the first two are rectangular, elongated, relatively small, and have ventral edges intensely fringed. The infraorbital 3 is triangular, expands over much of the cheek, and has a posterior edge intensely fringed. The infraorbital 4 is rectangular, higher than long, borders the orbit posteriorly, and has a posterior edge fringed. There is no suborbital bone behind the orbit. The dermosphenotic is a somewhat triangular and curved bone bordering the dorsoposterior part of the orbit. The single supraorbital bone is oblong and elongated and borders all the dorsal edges of the orbit.

The infraorbital sensory canal runs alongside the orbital edges of the dermosphenotic and infraorbital bones. This canal shows many branches or canaliculi that open through tiny pores at the end. A couple of large and broad sclerotic bones occupy the orbit. A thin, rounded basal sclerotic bone with serrated edges occupies the center of the orbit.

Suspensorium. Bones of the suspensorium are exposed partially in different specimens ( Figures 5 View FIGURE 5 , 7 View FIGURE 7 , 8 View FIGURE 8 ). The hyomandibular is an axe-shaped bone about 2.5 times higher than wide. Its head is a robust, long rectangular structure that nearly represents the upper third of the bone. Two large cavities arranged longitudinally pierce the hyomandibular head. The dorsal border of the hyomandibular head is somewhat sinuous but definitively forms a single articular surface for the skull. The opercular process of this bone forms shallow rounded protuberance in the basal half of the posterior margin of the hyomandibular head. The shaft of this bone is rectangular, almost vertical, slightly bent anteriorly, and as long as the anterior two-thirds of the head. This shaft has two laminar components; the lateral one forms a narrow and thick external surface with superficial apicobasal shallow grooves and a posterior deep preopercular groove; in contrast, the medial component forms a smooth internal wing, thinner, mostly extended anteriorly, with a dorsal large foramen for the hyomandibular nerve.

The quadrate bone is flat, triangular, and strongly inclined posteriorly, with its anterior and posterior edges almost vertical and horizontal, respectively ( Figures 7–8 View FIGURE 7 View FIGURE 8 ). The quadrate articular head is short and does not overhang from the anterior edge of the bone; this joints the lower jaw a little forward of the orbit and below the lateroethmoid bone; and its articular surface is oval, vertically exposed, broad, and consists of two malleoli separated by a wide groove arranged on a lateral-dorsal to the medial-ventral axis. Just behind the articular head, the ventral border of this bone has a slight broadening with a shallow notch, in which the dorsal posterior termination of the post-articular process of the lower jaw rests. The quadrate has a slightly curved edge meeting the metapterygoid bone. The posterior process of the quadrate is nailshaped and as high as the quadrate body. The symplectic is wedge-shaped, lateromedially flat, nearly 2.5 times longer than high, and only slightly shorter than the quadrate.

The ectopterygoid is triangular, gracile, and toothless ( Figure 5 View FIGURE 5 ). This bone has a ventral horizontal limb attached to the anterior edge of the quadrate and a vertical anterior dorsal limb attached to the ventral edge of the endopterygoid and the terminal end of the palatine. The endopterygoid is a rectangular bone extending above the dorsal anterior half of the quadrate dorsal edge and below the orbital anterior half and the lateroethmoid, and its anterior termination dorsally borders the dorsoposterior half of the palatine. The lingual surface of the endopterygoid is spongy endochondral bone and bears petite small tubercular teeth.

The metapterygoid is an elongated trapezoidal bone, higher anteriorly and with curved edges ( Figure 9 View FIGURE 9 ). Its dorsal edge is slightly concave and extends below the posterior half of the orbit. Its ventral edge is concave and extends between the symplectic and the hyomandibular shaft. Its posterior edge is shallow, almost straight, and reaches the medial internal wing of the hyomandibular just behind the ventroposterior part of the orbit. Finally, its anterior edge is high, slightly concave, anterodorsally tilted, and attaches to the posterior edge of the quadrate.

The palatine is a long rectangular bone, slightly dorsoposteriorly tilted, placed below the skull ethmoid region and nearly as long as the lateroethmoid ( Figures 5 View FIGURE 5 , 7 View FIGURE 7 , 8 View FIGURE 8 ). Anteriorly, the articular end of the palatine forms a robust, rectangular head. Its articular head is sinuous anteriorly, and its articular facet for the ethmopalatine is somewhat protruding and rounded. In contrast, its articular facet for the posterior articular process of the maxilla is practically flat. Behind the articular head, this bone shows a semicircular medial wing protruding dorsally and a thick middle triangular process to attach the palatine notch of the maxilla.

Opercular bones. The opercular series of this fish is complete and includes four laminar bones ( Figures 7–8 View FIGURE 7 View FIGURE 8 ). The opercle is an oval fan-shaped bone, about 1.6 times higher than long, with a rounded and thin contour except for its straight and thickened anterior border, which only represents about half the height of the bone. The posterior and dorsal borders of the opercular are fringed, especially in the dorsoposterior part. The articular facet for the hyomandibular is at the dorsal end of the anterior border of the bone. This facet is almost flat, exposed on the anterior opercular edge, and protruding anteriorly. The ornament of this bone consists of scattered pits and short grooves ordered in radiating pattern from the articular facet for the hyomandibular.

The preopercle is an inverted-L-shaped bone with dissimilar limbs, forming an inner angle slightly higher than 90° and a posteroventral corner somewhat prominent ( Figures 7–8 View FIGURE 7 View FIGURE 8 ). The height of the preopercle barely reaches the opercular articular facet for the hyomandibular. Here, the vertical preopercular limb is relatively short, slightly broad, and has a posterior edge intensely fringed. On the contrary, the horizontal preopercular limb is two times longer than the height of the vertical preopercular limb, slender, and its edges are entirely harmonious (= not fringed). The smooth preopercular median shelf shows a sinuous dorsal edge. The preopercular sensory canal runs near the inner edge of this bone and is intensely branched, forming a peculiar superficial pattern on the horizontal limb of deep grooves that are numerous and parallel.

The subopercle is a semi-oval bone as long as the opercle, with a rounded contour and a slightly concave dorsal border ( Figures 7–8 View FIGURE 7 View FIGURE 8 ). The subopercular anterior ascending process is triangular and shallow. Numerous parallel and shallow radiating grooves ornament the subopercle bone laterally, from its anterior region to its ventroposterior fringed edge. Below the horizontal preopercular limb, the interopercle is an elongated and somewhat sinuous triangular bone that is acute anteriorly, nearly four times longer than high, and has a straight posterior edge.

Branchiostegal rays and branchial arch. Elements of the branchial arch are partially covered. Both hypohyal bones are rectangular, thick, lateromedially flat, smooth, and subequal-sized. The dorsal hypohyal is slightly higher than the long, and the ventral hypohyal is about 1.5 times higher.

The ceratohyal bones are flat ( Figure 8 View FIGURE 8 ). The anterior ceratohyal is rectangular, long, nearly two times longer than high, and has no beryciform foramen. The posterior ceratohyal is triangular, tapered posteriorly, broad anteriorly, and has a straight edge to join the anterior ceratohyal. Ceratohyal bones have a medial longitudinal groove for the afferent hyoidean artery. The interhyal is a short dorsal rod-like bone joining the posterior tip of the posterior ceratohyal. The urohyal is a complex elongated bone with an anterior small head, a lateral basal flat wing, and a rectangular medial dorsal flange.

A series of at least 15 elongated and dorsally bent branchiostegal rays are articulated proximally to the lateral surfaces of the ceratohyal bones; the most anterior ones are short bones and thin as threads; the subsequent branchiostegal rays become more prolonged, broader, and the last three spathiform.

Branchial bones are covered by more superficial or strongly disarticulated and dispersed in the available specimens. Therefore, the structure of the gill arch is unknown. Despite this, at least some of the pharyngobranchial bones have minute teeth. Axial skeleton. The vertebral column consists of 78 total centra, including 54 abdominals, 22 preurals, and two urals ( Figures 1 View FIGURE 1 , 10 View FIGURE 10 ; Table 1). The centra are cylindrical, well ossified, and have deep concave intervertebral surfaces perforated by a tiny central notochordal foramen. Along the abdominal and the anterior half caudal region, the centra are slightly longer than high in lateral view. Subsequent caudal centra become progressively smaller and shorter; the preural centra 1–5 are slightly higher than long. A couple of elongated oval perforations carve the lateral surfaces of all these centra except for the first abdominal and at least the preural 1, which are superficially smooth.

The ribs are long, curved, and evenly widened bones enclosing most of the abdominal cavity ( Figure 10 View FIGURE 10 ). The ribs are somewhat laterally flat bones laterally carved by a shallow apicobasal groove and have robust articular heads. The first abdominal centrum lacks ribs; the abdominal centra 2–42 have a pair of associated ribs directly in the lateroventral facets of the centra; the ribs and abdominal centra 43–54 joint with sharp autogenous parapophyses that attach with the lateroventral facets of the respective centra. These parapophyses are ventrally projected and increase in size in the anteroposterior order; the height of the posterior parapophyses equals the length of one and a half abdominal centra. The posterior eight or nine pairs of ribs become more petite and articulate with small parapophyses of the respective centra.

All the neural and hemal arches are autogenous and tightly attached to the centra ( Figures 2 View FIGURE 2 , 10 View FIGURE 10 ). These arches are shallow, almost as long as the respective centrum, and have noticeably large processes to attach the centra. Along the vertebral column, the elongated articular processes of each neural and hemal arches joint the deep cavities or facets on the dorsal and ventral surfaces of the respective centrum. The spines are long, sharp, inclined, and curved posteriorly; among these, the most anterior 35–40 are deeply bifid. In the anterior two parts of the caudal region (up to the vertebrae 60–62), the neural spines are slightly curved and tilted posteriorly, forming an arc two times higher than the respective centrum and as long as the next two posterior centra. Beyond that, the neural spines become shorter, straighter, and more inclined. Preural neural arches 2–7 have thick neural spines and dorsoanterior saddle-shaped processes that extend anterodorsally, reaching and supporting the neural spine of the adjacent anterior preural centrum. These saddle processes become smaller in these centra, following the posterioranterior order, whereas the respective neural spines become gradually more vertical. At least the neural spines of preural arches 2 and 3 present a complex articulation with their corresponding saddle-shaped processes; these spines have a callosal process allocated within a middle cavity in the dorsoanterior surface of the respective saddle process. This cavity has not yet been directly observed; however, the saddle process becomes broad and protrudes laterally around its contact with the callosal process region. Although less inclined backward, the hemal spines are almost symmetrical images opposite the hemal spines. The hemal spines of preural 2–5 are noticeably thicker than the preceding ones.

An indeterminate number of flat, long, and straight-ended supraneural bones are along the predorsal region. These bones appear to be placed in relation 1 to 1 with the neural spines without penetrating the interneural spaces. The epineurals are elongated thread-like bones that rise from the lateral surface of the neural arches, are as long as 8– 12 centra in the anterior abdominal centra, and become progressively shorter in subsequent abdominal centra. There are no epipleural nor epicentral ossified.

Pectoral girdle and fins. On each side of the body, this fish has a large and flat triangular medial supratemporal or extrascapular bone (sensu Patterson and Rosen, 1977, p. 93) that lies over the occiput and the dorsal part of the pectoral girdle. This bone expands posteriorly, has a deeply fringed rear, and carries the supratemporal canal alongside its anteroventral edge ( Figures 5–9 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 ).

The posttemporal is a roughly Y-shaped bone with broad limbs ( Figure 5 View FIGURE 5 ). The supracleithrum is rectangular, tilted, and slightly curved dorsoanteriorly. In these bones, the posttemporal sensory canal opens in large pores near the tip of the posttemporal limbs and in and alongside the dorsal half of the supracleithrum. Two large scale-like postcleithra are present behind the dorsal limb of the cleithrum.

The cleithrum is a crescent-shaped bone in lateral view ( Figure 9 View FIGURE 9 ). Its horizontal and vertical limbs are similarly slender, elongated, and acuteended. The horizontal limb has a poorly expanded lateroventral surface and barely convex ventral edge; on the contrary, the vertical limb has a larger lateroposterior surface and a sinuous posterior edge with a dorsal concavity to allocate the postcleithra. The intersection of these limbs forms an anterior barely obtuse angle nearly to 95–100°. The coracoid is an elongated triangular bone, broad posteriorly, almost four times higher than long, and its anterior end is ahead of the horizontal cleithrum limb. Probably, the ventral parts of both coracoids meet in the midline. The scapula is a small bone located in the ventroposterior corner of the cleithrum. The mesocoracoid is a triangular thick bone covered by the posterior part of the cleithrum just above the pectoral fin base.

The large trapezoidal pectoral fin rises near the ventral edge of the trunk and extends in the anterior fifth of the abdomen ( Figure 9 View FIGURE 9 ). The fin consists of about 12 rays with robust heads joining the distal ends of at least five short, robust pectoral radials that join the scapula. The most anterior pectoral rays are saber-shaped bones with a broad laminar body distally branched and segmented. Peculiarly, the sutures between the segments are intensely zigzagging or step-like and strongly tilted backward. The first ray is the largest, and its base is about two times wider than the second ray; subsequent rays become shorter and have less width. Pelvic girdle and fins. This fin is probably tiny, triangular, and rises in the posterior half of the trunk, at 70% of SL, below the abdominal centrum 50 ( Figures 2 View FIGURE 2 , 11 View FIGURE 11 , 12 View FIGURE 12 ; Table 1). The fin has eight broad rays extending below four to five abdominal centra (abdominal centra 51–55). Among these rays, the first one is the widest, is segmented, and branched distally as the first pectoral fin ray. The proximal ends of all pelvic rays articulate directly with the posterior border of the pelvic bone, except for the first two rays that articulate two intermediate robust and short radials.

The pelvic girdle consists of two elongated pelvic bones, laying beneath three abdominal centra (abdominal centra 47–50) and united only by their stout rectangular median processes ( Figure 11 View FIGURE 11 ). In the dorsoventral view, each pelvic bone is a complex axe-shaped structure that is expanded posteriorly and has an anterior handle that is thin, short, and sharp-ended. The central part of the pelvic bone consists of a longitudinal rod-like structure that is straight and thick. A rectangular lateral wing extends laterally from the rear to two-thirds of the central part; however, this bone has no medial wing developed in the counterpart. The ventral external wing is a thickened and shallow trapezoidal structure projected ventrally from the posterior third of the central part of the pelvic bone.

The posterior end of the pelvic bone is remarkably thick ( Figure 11 View FIGURE 11 ). A rectangular medial process occupies the fifth posterior part of the medial border. The bone has no posterior process. In opposition, the posterolateral corner of the lateral wing presents a lateral process forming two anteroposteriorly compressed triangular prominences protruding ventral and dorsally. The posterior border of the pelvic bone is sinuous and has a conspicuous posterior projection at its medial end.

Dorsal fin. This fin is triangular, short, and located far in the back of the trunk and slightly behind the origin of the anal fin, opposing the acuminate lobe of the anal fin ( Figures 2 View FIGURE 2 , 4 View FIGURE 4 , 11 View FIGURE 11 ). Although none of the specimens studied show a complete dorsal fin, the specimen IGM 14028 has 13 rays forming part of this fin, including a short unbranched ray plus 12 branched and segmented rays. The dorsal pterygiophores series that supports this fin includes 13 proximal pterygiophores and uncertain numbers of distal and medial pterygiophores. The anterior proximal pterygiophores are so long that they penetrate the spaces between the neural spines located below; behind, the posterior proximal pterygiophores become progressively shorter. The first dorsal proximal pterygiophore is forked at the base and has a long anterior projected anteroventrally.

Anal fin. This fin is long, acuminated, and located far in the back of the trunk and slightly ahead of the dorsal fin ( Figures 2 View FIGURE 2 , 4 View FIGURE 4 , 12 View FIGURE 12 ). The specimen IGM 14028 has 19 rays forming part of this fin, including a short unbranched ray plus 18 branched and segmented rays. The anal pterygiophores series that support this fin includes 18 proximal pterygiophores and uncertain numbers of distal and medial pterygiophores. All the proximal pterygiophores are long rod-like structures that are shorter in the anterior to posterior order; among these, the anterior two or three anal proximal pterygiophores are so long that they penetrate the interhemal spaces located above.

Caudal fin and skeleton. The caudal fin is deeply forked and consists of two triangular lobes opposed, similarly shaped, and markedly dissimilar in size ( Figures 2 View FIGURE 2 , 4 View FIGURE 4 ). Considering the length of the principal axes of both lobes, the ventral lobe is nearly 1.5 times longer than the dorsal one.

The caudal skeleton involves the spines of the preural centra 1–4, the parhypural, two ural centra, five uroneurals, seven hypurals, and probably only two epurals ( Figures 12–13 View FIGURE 12 View FIGURE 13 ). Neural spines of preurals 1 and 2 and the ural 1 support the anterior ends of the caudal dorsal rays; these spines are progressively enlarged and thick, somewhat curved, and tilted nearly 45°, contrasting with those of previous preurals that are shorter, thinner, and tilted more horizontally. The hemal spines of preural centra 1–4 (including the parhypural) support the anterior end of ventral caudal rays. These somewhat sinuous hemal spines are thicker, longer, and more horizontally inclined than previous preurals. Among these, the hemal spine of preural centra 2 and the parhypural lie nearly horizontal below the caudal fin rays and have noticeably robust articular heads that protrude laterally.

There are five flat uroneurals, almost straight, parallel, and inclined, about 45° covering the lateral surface of the urals and preurals 1 and 2 ( Figure 13 View FIGURE 13 ). The first uroneural is the most robust; its anterior end is round, expanded, and sloping anteriorly, while its dorsal part becomes slender, covers much of the neural spine of ural 1, and is shorter than the neural spine of preural 1. The posterior four uroneurals are thinner, equally long, and progressively located a little more dorsally. Between uroneurals 2 and 3, there are two robust and needle-like epurals dorsally extended beyond the distal end of the neural spine of ural 1. There are no urodermals. Both urals and preural 1 are almost entirely covered.

The anterior ends of the caudal fin rays cover most of the seven hypurals ( Figure 13 View FIGURE 13 ). Hypurals 1–4 have robust heads protruding laterally, as the parhypural; however, the heads of these hypural are so broad that they are in contact with each other and enclose part of the corresponding ural. It is impossible to determine whether the head of hypural 1 also has a ball-and-socket joint with ural 1, as in other ichthyodectiforms. The hypurals 1 and 2 attach the ural 1 and project posteroventrally, whereas the hypurals 3–7 joint the ural 2 and project posterodorsally, so there is a narrow and deep caudal diastema between the hypurals 2 and 3. The main axes of the bones of both hypural groups are poorly tilted posteriorly below and above the longitudinal axis of the vertebral column.

All the hypurals are elongated trapezoidal bones slightly expanded posteriorly, except for hypural 2, a large triangular or fan-like structure with a noticeably expanded rear and the dorsal and posterior edges sinuous ( Figures 12 View FIGURE 12 , 14 View FIGURE 14 ). The dorsal edge of hypural 2 lies horizontally, and its articular broad rounded head protrudes laterally and contacts the articular heads of hypurals 1 and 3. Additionally, the hypural 2 has a peculiar ridge extended in the anterior four-fifths of its dorsal edge. It extends laterally and folds ventrally, forming a triangular structure resembling the folded corner of a blanket on the bed. This blanket-corner-folded ridge has a sinuous ventral edge, is pierced laterally by three oval holes, and roofs an elongated cavity that houses the proximal ends of the ventral caudal rays.

The length of hypural 3 is nearly three-quarters of hypural 2 ( Figure 13 View FIGURE 13 ). The articular head of this hypural is also robust but quadrangular shaped and as broad as that of hypural 2. Its ventral edge is sinuous and shows a blanket-corner-folded ridge like that of hypural 2 but less developed and extended along the entire bone. The articular head of hypural 4 is broad but smaller than the head of hypural 3. The edges of hypurals 5–7 are between and below the dorsal caudal rays.

Scales. Thin cycloid scales strongly imbricated cover the entire trunk, the posterior part of the head, and the base of the caudal fin rays. The scales are circular near the ventral and dorsal edges of the trunk, but toward the flanks, these become oval, about 1.5 times higher than long. The ornaments of the scales include numerous concentric circulii, punctae in the anterior half, and straight and parallel anterior radii, long on the anterior edge and short on the posterior one ( Figure 15 View FIGURE 15 ).

Most scales are disarticulated and scattered around the skeleton in the fossils studied here. This preservation avoids the recognition of the scale lines that cover the body. Some scales longitudinally perforated by the lateral line canal are present near the vertebrae, revealing that this sensory canal may extend along the trunk near the vertebral column.