Redescription of <span class="genus-species">Santanichthys diasii</span> (Otophysi, Characiformes) from the Albian of the Santana Formation and Comments on Its Implications for Otophysan Relationships

Ethmoid Region:

A few specimens show a relatively well-preserved ethmoid region, but the mesethmoid is rarely visible. The bones of the snout are sometimes connected forming a small ossification of uncertain shape (e.g., AMNH 19439, 20050). This strongly suggests that the mesethmoid is reduced to a very small bone and was probably partially cartilaginous (fig. 1). By contrast, the frontals are long and extend very far anteriorly. The ethmoid region is moderately long and limited posteriorly by the lateral ethmoid (well preserved in AMNH 19439, 20068). This bone is characteristically triangular, with its upper arm long, slender, and backwardly directed. There is sometimes a rather square ossification behind the lateral ethmoid (e.g., AMNH 20068). It is not yet possible to determine whether this element is unpaired, but its position and shape recall those of a rhinosphenoid. A comparable ossification was not found in AMNH 19439. Other bones of the ethmoid region are not observable.

Dermal Bones of the Skull Roof:

Much of the skull is roofed by the frontals (figs. 1, 2). These bones are tapered anteriorly, where they meet the small mesethmoid, but become wider posteriorly and form the sides of a large oval frontooccipital fontanelle, best seen in AMNH 20050 (fig. 2). This fontanelle is present in many extant characiforms as well as in some other otophysans. A similar fontanelle is also reported in three fossil taxa, Clupavus maroccanus (Taverne, 1977, 1995), Salminops ibericus (Gayet, 1985), and Lusitanichthys characiformis (Gayet, 1981). The shape of the fontanelle in Santanichthys, Salminops, and Lusitanichthys differs from that of extant characiforms, where the fontanelle is less wide but much longer.

Silva Santos (1991) identified an additional frontomesethmoid fontanelle in Santanichthys, situated anteriorly between the frontals. A frontomesethmoid fontanelle is reported in Clupavus maroccanus and Lusitanichtys characiformis as well as in various otophysans. In our Santanichthys material, however, the contact between the frontals anterior to the epiphyseal canal is almost straight, although in specimens where the frontals have moved out their original position the displaced bones can sometimes appear to diverge anteriorly (e.g., AMNH 20068–20070). In specimens of Santanichthys where the frontals are still in contact with each other anteriorly, no frontomesethmoid fontanelle is evident. We conclude that no such fontanelle was originally present in this form.

The small and subrectangular parietals are separated by the frontooccipital fontanelle in Santanichthys, but the extent to which the parietals contribute to the margin of the fontanelle is variable. In AMNH 19439 the parietals reach the fontanelle and form a significant part of its margin, but in AMNH 20050 the supraoccipital extends anteriorly to contact the frontal, just occluding the parietal from the fontanelle margin.

The supraorbital canal runs through the entire length of the frontals and continues into the parietals. The parietal also contains part of the supratemporal commissure (AMNH 20065). The extrascapulars are not fused with the parietals, however, and are clearly visible in AMNH 19439. These elongate bones bear the sensory canal from the posttemporal to the parietals and are loosely applied to the posterolateral part of the cranium.

Endocranium:

The endochondral bones of the neurocranium are very loosely connected in Santanichthys and are rarely preserved in their original location except in the unusually well-preserved specimen AMNH 19439. Much of the endocranium was clearly cartilaginous, and chondral bones are difficult to recognize and may have been unossified. In most of the specimens we observed the ventral part of the neurocranium has become displaced, so the basioccipital is not in contact with the first vertebra, but lies more ventrally, and often the parasphenoid is also displaced with its anterior extremity meeting the frontals.

The orbitosphenoid is a small ossification forming the anterodorsal part of the endocranium. It is not well preserved in our material, and all that can be determined is that it lacks an interorbital septum and seems to project far anteriorly.

The autosphenotic is often preserved (e.g., AMNH 20050, 20055, 20062) and typically defines part of the orbital margin. The bone is subrectangular and bears a thin and spinous process directed slightly anteriorly (e.g., AMNH 20050). Its posterior portion is covered by the anterior part of the dermopterotic.

The dermopterotic is also commonly preserved (figs. 3, 4), even though it is not sutured to any surrounding bones and presumably was held in place by soft tissues in life. It is very elongated anteroposteriorly and bears a strong posterior process, which recalls the condition seen in characiforms (e.g., Brycon meeki; Weitzman, 1962). This process seems to project well behind the posterior wall of the skull (e.g., AMNH 20062, 20063, 20068). The dermopterotic carried its sensory canal within a bony tube, with a ventral tubule extending from the middle part of the bone. Anteriorly, the dermopterotic meets the posterolateral corner of the frontal.

The supraoccipital is a flat bone with a deeply concave anterior margin forming the posterior limit of the frontooccipital fontanelle (e.g., AMNH 20050, 20060, 20062; fig. 3). Its posterior edge bears a very small crest as in many primitive teleosts, unlike the derived characiform condition (with a well-developed crest extending posteriorly).

Nothing can be said concerning the epiotics (which are badly preserved in all our specimens) or the prootic (which is either covered by the hyomandibular or poorly preserved; e.g., AMNH 20070). A basisphenoid has not been observed and may have been absent (it is unlikely to have been lost in all the specimens examined). Absence of a basisphenoid is regarded as a synapomorphy of ostariophysans (Fink and Fink, 1981: char. 7).

The exoccipitals and basioccipital are characteristic and form a large globular lagenar chamber that projects laterally (figs. 4, 5). This unique feature is regarded as a synapomorphy of characiforms (Fink and Fink, 1981). The basioccipital contributes to the lower part of the capsular wall (this is particularly clear in AMNH 20068 and can also be observed in AMNH 20052, 20053, 20064, 20069). In life, the lagenar chamber contained a large otolith (the astericus), but the otoliths are not preserved in acid-prepared fossils. The right and left lagenar chambers are separated posteriorly by a narrow septum (fig. 4) and open anteromedially into the saccular chamber (housing the sagitta in life). This cavity is slender compared with the posterior width of the bone. The anterior edge of the basioccipital is almost straight and vertical, and posteriorly this bone forms the entire occipital condyle. The exoccipital forms the dorsolateral parts of the capsular wall. Although this bone is often badly preserved, it clearly has concave posterior and dorsal surfaces. The lateral outer surface of this bone is subrectangular in shape and is pierced posteriorly by three foramina (e.g., AMNH 20071). The largest of these foramina is probably for the vagus nerve.

The parasphenoid differs from the generalized teleostean condition in being curved, as in characiforms. It is slender, lacks teeth, and extends posteriorly below the basioccipital as far as the lagenar chamber (a condition strongly resembling that of Brycon meeki as drawn by Weitzman, 1962).

Suspensorium:

The hyomandibular is well preserved in many specimens (fig. 3). Its head is large and has two articular facets; one facet is elongated and directed dorsally to articulate with the cranium, and the other one is rounded and directed posteriorly for the opercle. The lower arm of the hyomandibular is slender in its median portion but becomes very wide and flattened farther ventrally. The bone has a very thin anterior wing whose size and extent seem to vary betweem individuals. The interhyal has not been observed.

The symplectic is elongate and triangular. As in teleosts generally, it sits within a deep notch in the quadrate. Its posterior portion extends beyond the quadrate while its anterior extremity almost reaches the quadrate lower margin.

The quadrate is typical of teleosts generally: it is triangular in shape, with a posterior arm for the symplectic and a condyle for the articulation with the lower jaw (fig. 3). Unlike in most basal teleosts, however, the bone is slightly inclined and the articular condyle is positioned somewhat anteriorly. The quadrate lacks a depression in its dorsal margin like that found in extant characiforms (e.g., Brycon).

The ectopterygoid is frequently lacking or poorly preserved in the available specimens. It is a thin, toothless bone that meets the metapterygoid posteriorly and projects anteriorly toward the ethmoid region, where it becomes even thinner.

The endopterygoid is also usually broken. It is a very thin toothless bone, oval in shape, stretched anteroposteriorly and lies dorsally against the ectopterygoid. Its dorsal surface can be seen in AMNH 20050.

The exact shape of the palatine is uncertain, and no dermopalatine has been observed. Absence of the dermopalatine is regarded as a synapomorphy of ostariophysans (Fink and Fink, 1981).

Upper Jaw:

The premaxilla is thick, short, edentulous, and has no dorsal process. It has a condyle articulating with the small mesethmoid (AMNH 20050). The maxilla is seen best in AMNH 20050, 20065, and 20068. Its articulation is more complex than that of the premaxilla. The condyle for the mesethmoid is followed posteriorly by a rounded articular facet for the autopalatine. Although the maxilla is slender anteriorly, it becomes wider posteriorly and has an extremely deep and rounded ventral part. The maxilla is completely edentulous. Two supramaxillae are present. The anterior supramaxilla is elongated and pointed anteriorly, bordering the anterior part of the maxilla. The posterior one has a thin pointed arm projecting anteriorly above the posterior margin of the anterior supramaxilla. An unusually large oval posterior portion of the posterior supramaxilla lies against the maxilla.

Lower Jaw:

It was not possible to determine exactly which bones articulate with the quadrate, since the pattern of fusion between the angular, articular, and retroarticular remains unclear. However, the socket for the quadrate condyle is often preserved and shows a short postarticular process. The limit between the angular and the dentary is visible in AMNH 20055, where the angulars have been dislodged, leaving the two preserved dentaries side by side. The angular penetrates the dentary very deeply, with the latter forming a long ventral arm that bears the mandibular canal and borders the ventral part of the angular. The dentary is edentulous and extremely deep, with a strong coronoid process (e.g., AMNH 20062; fig. 3). The mandibular canal is wide, with several progressively larger oval openings posteriorly.

Branchial Skeleton:

The branchial skeleton is incompletely preserved in many specimens, but parts are visible in AMNH 20058, 20059, 20064, 20068, and 20069. Unfortunately, the bones are never connected and a complete description of the branchial skeleton cannot be presented. Some ceratobranchials and epibranchials are recognizable, although most cannot be referred to a particular arch (the fourth epibranchials are both identifiable in AMNH 20059, thanks to their peculiar shape with two tubular arms projecting dorsally). The basihyal may be represented in AMNH 20064 by a small unpaired bone with a larger anterior portion bearing three processes on each side. In the same specimen, two tooth-bearing ceratobranchials are visible (fig. 6). In AMNH 20068, at least 12 gill rakers are preserved, inserted on their ceratobranchial. These rakers do not show any spicules.

Opercular Series:

The opercular series is perfectly preserved in AMNH 19439. The opercle is a fairly large bone, with a straight and almost vertical anterior margin. Its posterior margin is almost as straight but is more oblique, and the dorsal margin of the bone is rounded. The ventral part of the bone is the widest, with a sligthly rounded lower margin.

The preopercle comprises two branches. The dorsal branch (the longest) meets the ventral one at a slightly obtuse angle. The preopercular canal runs down the entire bone, along the anterior edge of the dorsal branch and the dorsal edge of the ventral one. This canal ramifies into five large tubules in the median and ventral parts of the bone, with the largest tubules in the median portion of the bone where the two main branches meet.

The interopercle lies beneath the ventral branch of the preopercle, but the interopercle is longer and its lower and posterior portions clearly extend beyond the preopercle. Its posterior margin is slightly rounded. Its depth equals that of the subopercle posteriorly but decreases anteriorly.

The subopercle is large and usually well exposed, since most of its surface is not covered by the opercle. Its anterior margin is almost straight and slightly oblique, and its ventral edge is strongly rounded posteriorly.

Circumorbital Series:

The circumorbital series (fig. 1) consists of several extremely delicate bones that would not normally be preserved in fossils, but they are perfectly preserved in AMNH 19439. The first infraorbital is triangular but elongated anteroposteriorly, and it is positioned above the first supramaxilla and also extends above part of the second one. The bone encloses the anterior termination of the infraorbital sensory canal and has two short ventral tubules extending from the main canal. The second infraorbital is elongate and narrow, with an unbranched canal. The third infraorbital is shorter than the second but is much deeper, with a deep rounded ventral wing reaching the preopercle. Its canal is confined to the upper portion of the bone and is unbranched. The fourth infraorbital is located at the posteroventral edge of the orbit and is also expanded ventrally, with its entire ventral margin meeting the preopercle. Its canal is confined to the margin bordering the orbit. The fifth infraorbital is almost square and contains the vertically oriented part of the sensory canal (again unbranched) behind the orbit. The dermosphenotic is a small ossification, consisting of little more than a bony tube enclosing the sensory canal, connecting the infraorbital supraorbital canals at the posterodorsal corner of the orbit. There is a single elongate supraorbital that edges the frontal all along the dorsal portion of the orbit. Nasals and antorbitals were not recognized in any of the material examined.

Pectoral Girdle:

The dorsal surface of the posttemporal is often preserved, but the inner concave surface of the bone is rarely observed (e.g., AMNH 20052). A long, well-developed arm projects perpendicular from the inner surface (fig. 5) and undoubtedly met the neurocranium to provide a firm attachment for the pectoral girdle. The posttemporal sensory canal passes along the midpart of the bone (e.g., AMNH 20059). The posttemporal also has a long anterior process although it is often broken; it can be seen in AMNH 20050, 20053, 20059, 20060, and 20064. According to Weitzman (1962) an identical arrangement is present in Brycon meeki.

The supracleithrum is an elongate, slightly curved, thin and narrow bone covering the upper part of the cleithrum but inserting under the posttemporal. The laterosensory canal enters the supracleithrum and extends through the upper length of this bone at the level of the upper tip of the cleithrum (e.g., AMNH 20060, 20068). The cleithrum is a large bone lying just posterior and medial to the opercle. Its upper arm is slender and very pointed, but its lower portion is expanded into a large posterior wing (e.g., AMNH 20050, 20052). The anteroventral part of the bone is also narrow.

The scapula and the coracoid can be seen in a few specimens (e.g., AMNH 20052, 20060, 20071), although few details can be discerned. The scapula is well developed, jutting out of the cleithrum ventrally. The coracoid is visible at the base of the fin rays, but its shape is uncertain. An extrascapula has not been positively identified. The bone is usually extremely small and thin in teleosts and in addition is often fused with the parietal. In AMNH 20050, a small bone which may contain a canal lies adjacent to the left posterior part of the braincase (fig. 2). This may represent a fused parietal–extrascapula but it is too poorly preserved for accurate identification.

Sixteen fin rays are present in AMNH 19439. No pectoral splint was observed.

Weberian Apparatus:

In some specimens of Santanichthys there is evidence of a well-preserved Weberian apparatus. Most of the characteristic elements have been identified, although not all are observable in any one specimen. The general arrangement of this Weberian apparatus nevertheless seems to differ from that of all extant otophysans, but shares some similarities with the (presumably primitive) Weberian apparatus of Chanoides illustrated by Patterson (1984).

The first four vertebrae form elements of the Weberian apparatus, and are shorter and more elaborate than the following ones. An element interpreted as the scaphium is visible in AMNH 20064 and 20068. This bone has a ventral process that fits within a pit in the anterior border of the first centrum. It also has a shell-like concave anterior edge extending anteriorly beyond the end of the basioccipital, and an ascending process projecting posteriorly and dorsally (fig. 6). In extant otophysans, the concave portion of the bone (concha strapedis) is applied to the sinus impar. There is a foramen for the root of the spinal nerve passing through the bone above the articulating process. Just above this foramen is a knob, which is interpreted as the attachment of the ligament from the intercalarium.

A claustrum has not been identified in any of the specimens we examined, and an intercalarium has been identified in only two specimens (AMNH 20059 and 20064). Its shape is similar to that of Chanoides and unlike that of extant otophysans. The intercalarium of Santanichthys has a pedicel ventrally that articulates with the second vertebra. The bone is similar to the scaphium in shape, but its anterior edge is pointed (fig. 6). No foramen has been observed in the intercalarium, as in extant otophysans but not in Chanoides, where a foramen is present.

A manubrium has not yet been identified in Santanichthys, and the third vertebra is not reduced (unlike Chanoides). Nevertheless this vertebra bears a large transformed neural arch (e.g., AMNH 20052, 20068), resembling that of modern cypriniforms. The side of the third vertebral centrum also has a strong oblique crest, which we regard as evidence for the attachment of a tripus (figs. 5, 7). In support of this interpretation, AMNH 20052 has an element ventral to this vertebra that may represent a displaced tripus. This element is stretched and triangular with a convex posterior margin and is separated into two pieces. This configuration conforms to current hypotheses on the origin of the tripus as being formed from two different bones (parapophysis and rib). Rosen and Greenwood (1970) described a tripus in two pieces in a juvenile Brycon, suggesting that this is the primitive condition for the bone.

The fourth vertebra bears unfused and flattened neural arches, pointed and backwardly directed. In AMNH 20052 there is a large bone anterodorsal to the third vertebra that is regarded as a broken piece of a well-developed third supraneural.

Pelvic Girdle and Dorsal Fin:

The pelvic bones are triangular with a thick posterior portion, although their exact shape is unclear. These bones are located under the vertebrae 15–8. The pelvic fin is composed of nine rays, whose length is equal to that of the pelvic bones. No pelvic splint has been seen.

The base of the dorsal fin is situated under vertebrae 16–19, but the first pterygiophore reaches the neural spine of the 12th vertebra. This pterygiophore has two branches (AMNH 20050) and bears at least two short rays (AMNH 20074) as well as the first long ray. At least nine pterygiophores can be counted (AMNH 20071), but the original number of fin-rays is unknown, although the dorsal fin was probably short anteroposteriorly. It is unknown whether an adipose fin was present.

Axial Skeleton:

There are between 38 and 40 vertebrae, including a compound ural centrum, the independent second ural centrum, and the anterior vertebrae bearing the Weberian apparatus (fig. 8). Most of the vertebrae are long, especially those from the midpart of the body, and all are moderately constricted. Their ornamentation is restricted to two crests on each side, and all the vertebrae generally have a smooth appearance. Unbranched epineurals are visible as far back as the 18th vertebra, attached at the base of the neural spine (AMNH 20050). No epicentral or epipleural elements were identified. Ribs are present up to the 25th vertebra, but on vertebrae 23–25 the hemapophyses are extended to form a small hemal spine supporting the rib. Hemal spines of the 26th and subsequent vertebrae are completely formed. Throughout the column, the neural and hemal spines are fused to their respective centrum in a very anterior position.

Caudal Skeleton:

In Santanichthys the first ural and preural centra are fused to form a compound centrum, although the second ural centrum remains independent, is elongate in shape, and extends far posteriorly (fig. 9). The parhypural and first two hypurals are usually all fused to the compound centrum, as well as the first uroneural, but in AMNH 12826 the first two hypurals, parhypura,l and first two hemal spines appear to be unfused (fig. 10). However, this specimen is unusual in having been found in the stomach content of another fish (Rhacolepis) and is partially digested, so the surface of the bone and the crests of the vertebrae have been removed. Digestion may have secondarily separated caudal elements from the centra, and the specimen is of considerable interest because it suggests that these elements were fused only superficially with their centrum but remained unfused internally.

The hemal spines of preural centra 2 and 3 are expanded. The spine of preural centrum 2 has a large but thin anterodorsal wing and a flattened extremity. The neural spine of preural centrum 3 is not modified, whereas that of preural centrum 2 is short and slightly flattened. The neural spine of the compound centrum (first preural spine?) is even shorter and leaflike. Uroneural 1 is fused with the compound centrum only and forms a ridge on its surface. There is a space for a second uroneural, but this element has not been observed. Hypural 1 is enlarged and lies between the parhypural and hypural 2, which are similarly shaped. Hypural 3 is approximately as large as the first (e.g., AMNH 20055, 20058, 20059, 20062, 20068) and is unfused (it is missing in some specimens). It is clearly associated with the small and elongate independent second ural centrum in AMNH 20053, 20059, 20068, and 20072, which is similar to the arrangement in Chanoides (Patterson, 1984). There is no diastema between hypurals 3 and 4. The fourth hypural lies just above the third and is considerably smaller. There may have a fifth and sixth hypural but these have not been identified. An elongate and slightly sigmoid dorsal element (possibly an epural) is present in AMNH 20068. The exact number of caudal finrays is uncertain. The bases of the finrays are not expanded and ray segmentation is almost straight.

Squamation:

The scales are cycloid and arranged in oblique rows, although the arrangement has only been seen in AMNH 19439. In the abdominal region, each transverse row contains approximatively 15 scales. The total number of scale rows is unknown, as AMNH 19439 lacks the tail.