Superclass Osteichthyes Huxley,1880
Class Actinopterygii Cope,1887
Subclass Neopterygii Regan,1925
Order Pachycormiformes Berg,1937
Family Pachycormidae Woodward, 1895
Genus Pachycormus Agassiz, 1833
Type species. Pachycormus macropterus (de Blainville 1818), originally diagnosed as Elops macropterus.
Diagnosis (emended from Mainwaring 1978). A member of Pachycormiformes presenting the following combination of morphological traits: fusiform in shape; skull roof raised posteriorly into dome-like structure (cranial boss) anterior to post-parietals; mandible with single row of teeth; teeth stout and needle-shaped; two large suborbitals posterior to a series of nine infraorbital bones; pelvic fins absent; dorsal fin in advance of anal fin; base of anal fin not extended.
Pachycormus macropterus (de Blainville, 1818)
Figures 3, 4, 5, 6, 7, 8, 9, 10, 11
Holotype. MNHN-F-JRE 50 (former catalogue number is 10529-10530), nearly complete specimen (incomplete appendicular skeleton and fins) in counterpart.
Specimens studied. BRLSI M1297, BRLSI M1299, BRLSI M1320, BRLSI M1332, BRLSI M1359, BRLSI M1389, BRLSI M1395 (all Pachycormus macropterus); BRLSI M1341, BRLSI M1351, BRLSI M1366, BRLSI M1393 (all Pachycormus).
Diagnosis (emended from Mainwaring 1978). Pachycormus macropterus is a species of Pachycormus presenting the following features: preorbital region makes up approximately one-third of the skull; opercular bone twice as long as deep, the ventral angle sloping towards the posterior half of the bone.
Remarks. All cranial material studied here belongs to this species.
Skull roof—The parietal is a long and wide paired bone covering two-thirds of the dorsal surface of the skull roof above the orbit but not being part of the dorsal orbital margin (BRLSI M1297, BRLSI M1299). The parietals (Figs. 3, 4, 5) are located posterior to the rostrodermethmoid, and meet medially along an irregular suture. Additionally, the parietals contact the nasal bones at their anterolateral margin. They rise gently backward until they reach a projection, which is named the cranial boss (Figs. 3, 4, 6). This boss is the steepest posterior portion of the parietal bones and is characteristic of members of Pachycormus in general.
In Pachycormus, the post-parietals are small, paired asymmetrical bones which are well separated from each other medially by the parietals and form the posterior portion of the cranial boss (Figs. 3, 4, 5). They are located medial to the dermopterotic (Fig. 3a) and anteromedial to the paired suprascapular bones (Fig. 3b). Post-parietals, nevertheless, are very rarely preserved in the studied specimens, but BRLSI M1395 displays paired post-parietals on the skull roof (Fig. 4).
The dermosphenotics are paired dermal bones with an irregular rectangular outline, which form the dorsal margin of the orbit (BRLSI M1299, BRLSI M1389; Figs. 3, 4, 6). They are located posterior to the nasal bones and anterior to the dermopterotics.
The dermopterotics are paired bones that are located posterior to the dermosphenotic and are usually Y-shaped in Pachycormus when seen in lateral view (Mainwaring 1978) (Fig. 3). This shape can be partially observed in BRLSI M1389 (Figs. 4, 6). However, for most specimens, this shape of the dermopterotic is not preserved because the skulls are substantially crushed in most specimens, where it takes on a more rectangular shape, as can be seen in BRLSI M1332 (Fig. 5). The dermopterotic is situated dorsal to the first suborbital bone (BRLSI M1297, BRLSI M1299).
In Pachycormus, the suprascapulars are usually paired triangular ossifications located posteromedial to the paired post-parietals, with the anterior margins located posterodorsal to the dermopterotics in lateral view (e.g., BRLSI M1332; Figs. 3, 4, 5). Posteriorly, the suprascapular contacts the pre-supracleithrum and supracleithrum.
Snout—The rostrodermethmoid is the most anterodorsal bone on the skull roof (Figs. 3, 4, 5). It is a compound bone, where the rostral part comprises a large, median shield-like bone that is diagnostic of all pachycormids. The anterior-most ventral portion of the rostrodermethmoid bears teeth. These are the lateral bones that constitute the dermethmoid region of the rostrodermethmoid. Toothed lateral-dermethmoids are similar to the condition of the lateral-dermethmoids seen in Siemensichthys macrocephalus (Patterson 1975; Arratia 2000). Fusion to the rostral bone and lack of contact with the parietal is similar to the conditions seen in the Leptolepidae (Patterson 1975). The rostrodermethmoid medially separates the premaxillae from each other, which is typical of pachycormiforms (Woodward 1895; Berg 1940; Wenz 1967; Mainwaring 1978; Lambers 1988, 1992; Kear 2007; Friedman et al. 2010; Arratia and Schultze 2013).
Nasals are located anterior to the dermosphenotics and parietals (e.g., BRLSI M1297, BRLSI M1299, BRLSI M1395; Figs. 3, 4, 5). They lie posterolateral to the rostrodermethmoid in dorsal view. In lateral view, the nasals are small, sub-triangular bones located anterior to the orbit. The nasal openings are situated on the anteroventral margin of the nasal bones (e.g., BRLSI M1297, BRLSI M1299, BRLSI M1395 and BRLSI M1389) and are semi-ovoid in shape (Fig. 6). The nasal bones are located dorsal to the antorbital.
Orbital bones—The antorbitals are small bones with a roughly rectangular outline and which are longer than high (Figs. 3, 4). They are located ventral to the nasal bones, forming the posteroventral margin of the nasal opening (BRLSI M1389; Fig. 6) and dorsal to the premaxilla (BRLSI M1320). Posterior to the antorbitals is the first infraorbital, which is very elongated, forming the ventral margin of the orbit, and is dorsal to the maxilla (e.g., BRLSI M1389; Figs. 3, 6).
The remaining infraorbital bones 2–10 are rarely preserved in the studied specimens, but when present, they are small, rectangular plate-like ossifications stacked on top of one another, and are located along the posterior edge of the orbit (Figs. 3, 5). The infraorbital plates are fractured, but sutures between bones are still plainly visible.
The two large plate-like suborbital bones lie posterior to the infraorbital series, showing clear sutures between them (Figs. 3, 5). The upper suborbital is trapezoidal in shape, with the ossification becoming broader ventrally. The lower suborbital is triangular in shape and becomes broader posteriorly with a rounded margin. They are located anterior to the opercular series, posterior to the supramaxilla and dorsal to the mandible. A highly ossified sclerotic ring comprising a single plate surrounds the orbit, and it is exceptionally well preserved in some specimens (e.g., BRLSI M1389; Figs. 3, 5). It is located ventral to the dermosphenotic and anterior to the infraorbital series.
Jaw bones—The upper jaw in most specimens is either fragmented or embedded in the rock to such a degree that it prevents identification. Consequently, the premaxilla, which is a fixed bone (Mainwaring 1978; Kear 2007; Arratia and Schultze 2013), is only rarely preserved in the Strawberry Bank specimens, despite the good overall preservation of the skulls. The premaxilla nevertheless is in excellent condition to be identifiable in a few specimens (e.g., BRLSI M1389; Fig. 6). It is a small ovoid dermal ossification, with the antorbital overlying it dorsally. Anteriorly, the premaxilla fits into a small recess in the posteroventral portion of the rostrodermethmoid. This suturing of the premaxilla to the rostrodermethmoid is a diagnostic character of pachycormiforms (Kear 2007; Arratia and Schultze 2013). A concave notch is present on the anterior border of each maxilla, into which the premaxilla fits. The premaxilla carries a series of needle-shaped teeth that are morphologically identical to those of the rostrodermethmoid but which are less robust than the teeth of the lower jaw (Mainwaring 1978).
The maxilla is an elongate bone which extends beyond the orbit and as far back as the second suborbital (Figs. 3, 5). Anteriorly, it meets the posterior edge of the premaxilla. As it meets the premaxilla, it arches forward and becomes thinner. The posterodorsal portion of the maxilla is deeply excavated to accommodate the supramaxilla. The posterior margin of the premaxilla is convex.
The supramaxilla is an oval-shaped ossification that covers the posterior border of the maxilla (e.g., BRLSI M1299, BRLSI M1395; Figs. 3, 5, 6).
The most anterior bones in the mandible are the dentaries, which cover approximately three-quarters of its total length. The dentaries meet each other at the anterior-most tip from either side, forming a shallow symphysis.
Posterior to the dentaries are the angular and supra-angular bones that meet in an uneven zigzag suture. The angular and supra-angular are difficult to distinguish on the majority of specimens, as the surface has flaked away. The angular forms the posteroventral corner of the mandible and articulates with the quadrate (BRLSI M1297; Figs. 3, 5, 7).
The supra-angular is only exposed when the upper jaw is displaced. It is found to be dorsal in relation to the angular and is a small bone bearing a long anterior process with a concave anteroventral notch and a straight ventral margin (BRLSI M1320; Fig. 7). This seems to be a fragmentary supra-angular; however, as it has a wider ventral margin in BRLSI 1332 (Fig. 5), it correlates better with the typical triangular ossification shown in Pachycormus (Fig. 1b; Mainwaring 1978).
The dentition consists of needle-like teeth. The teeth of the dentary are larger, more robust and less numerous than the teeth of the upper jaw. There is almost no variation in the size of the teeth along the margins of either jaw (Figs. 5, 7).
Gular plate—The gular plate is situated anteroventrally in relation to the branchiostegal rays and between the left and right dentary bones (BRLSI M1299, BRLSI M1320; Fig. 7). Radiating ridges are found all around the margin of the gular plate. The gular plate is triangular in form with rounded corners, broadening as it extends posteriorly to the branchiostegal rays.
The branchiostegal rays are a series of elongate, curved bones that support the branchiostegal membrane, completing the opercular series (Figs. 3, 5, 7). They lie ventromedial to the interoperculum and posterior to the gular plate (BRLSI M1299, BRLSI M1320). A total of 66–74 branchiostegal rays are usually present.
Opercular series—The preoperculum is semi-lunate and has a broad ventral limb (Figs. 3, 5). The preoperculum tapers dorsally to a narrow splint. It is located posterior to the suborbital bones.
The operculum is posterodorsal to the preoperculum, contacting the posterodorsal margin of the preoperculum. It is a large dermal ossification in the shape of a triangle and is dorsal to the suboperculum.
The suboperculum is trapezoidal in shape but becomes deeper posteriorly, with a posteroventral rounded margin (Figs. 3, 5). The dorsal margin in contact with the operculum is gently sloping. The anterior and dorsal margins are heavily ossified relative to the rest of the suboperculum.
The interoperculum is the smallest element of the opercular series and is rectangular in outline. It is located along the anterior margin of the suboperculum and lies ventral to the preoperculum (Figs. 3, 5). It is positioned dorsally in relation to the branchiostegal rays.
Pectoral girdle—The cleithrum is a sigmoid-shaped bone located posteriorly in relation to the opercular series (Figs. 3, 5). Ventrally, it slopes gently posterior to the branchiostegal rays. The ventral margin is concave and it is overlapped by the posterior margin of the suboperculum (e.g., BRLSI M1395; Fig. 8).
A pre-supracleithrum is preserved ventral to the suprascapular and anterior to the supracleithrum in BRLSI M1332 (Figs. 3, 5). It is a roughly falcate to irregularly shaped ossification with a convex anterior margin.
The supracleithrum is best preserved on the right side in BRLSI M1395 (Fig. 8). It is an elongated thin and blade-like bone with a posterior rounded margin that increases in depth posteriorly. It is located dorsal to the opercular series, with its posterior margin in contact with the dorsal margin of the cleithrum.
Two bones found posterior to the cleithrum visible in BRLSI MI395 are the post-cleithra (Figs. 3, 5, 8). The dorsal post-cleithrum is an elongated plate that is overlain anteriorly by the cleithrum, and the ventral post-cleithrum is a large triangular bone with a convex posteroventral margin.
Appendicular skeleton and unpaired fins—A complete pectoral fin is preserved in specimens BRLSI M1332 and BRLSI M1395 (Fig. 5). It has a characteristic scythe-like shape. Each fin ray shows an anterior dichotomous branching pattern, but there does not appear to be extensive division of the posterior half of each fin ray bifurcation, so the distal edge does not have “a particularly thick fringe”, as described by Mainwaring (1978: 83). This could be because the preserved pectoral fins belong to small individuals, so the pectoral fin has not developed to the extent where extensive bifurcation occurs as seen in specimens such as MNHN 10530 (Wenz 1967: fig. 60). As a result, this gives the distal margin of the pectoral fins of BRLSI M1395 a particularly frayed border. Fringing fulcra lie on the surface of the simple principal leading fin ray. These series of fringing fulcra are small spine-like elements, which Arratia (2009) referred to as “Pattern C” fringing fulcra. Anterior to the principal leading fin ray is a series of what can be interpreted as either three elongated basal fulcra or procurrent rays. Arratia (2008a) comments that determining the identity of these bones is problematic because of the need for a better understanding of pachycormiform pectoral fin structures. Basal fulcra are usually associated with the caudal fin rather than with paired fins in actinopterygians, and are usually low-lying structures that are lanceolate, leaf-like or arrow-like in shape (Arratia 2009). This description does not match with the structures anterior to the principal leading fin ray, so we have interpreted the structures in BRLSI M1395 as procurrent rays. These procurrent rays have a broad proximal base that narrows as it reaches the distal end. Each procurrent ray increases in length in an anteroposterior direction from the proximal radials.
Since the pectoral girdle of Pachycormus is for the most part endoskeletal, and CT scanning of specimens is outside the scope of this study, we will just discuss the only exposed part of the pectoral girdle present in any of our specimens (BRLSI M1395; Fig. 5), the scapular region. No proximal radials are visible in any of our specimens. The condition of the propterygium, with respect to its fusion to the principal fin ray, also cannot be deduced. This scapular is moderately fragmented, but the general elongated, deep shape of the ossification can be easily deduced. The anterior portion of the scapular has a rounded margin and is overlapped by the ventral post-cleithrum in non-disarticulated specimens (e.g., Mainwaring 1978). The posterior end of the scapular region gently slopes from the midpoint of the ventral post-cleithrum. The posteroventral portion of the bone extends slightly beyond the fin rays. The openings of a sensory canal are identifiable as dotted lines throughout the scapular region, which bifurcates towards the posterior part of the bone, with one line heading in a dorsal direction and the other towards the posteroventral part of the scapular. The number of sensory pores decreases in density posteriorly. Mainwaring (1978: 82) had only this comment regarding this sensory canal: “There is no large scapular foramen beneath the mesocoracoid arch, though a narrow canal does open onto the medial surface of the scapular region.”
The dorsal and anal fins are rarely preserved, and they are only visible in specimens BRLSI M1366 (Fig. 9) and BRLSI M1341, respectively. The dorsal fin is small and falcate, containing 23 rays. It is located on the posterior half of the trunk. The anal fin is located close to the caudal fin. It is severely fragmented but seems to have a falcate shape with the distal edge fanning out. No interhaemal spines are present between the haemal spines and the anal fin rays. Thirteen fin rays are preserved in the anal fin.
The caudal fin is homocercal (e.g., BRLSI M1393; Fig. 10) and carries 70 fin rays that articulate with the hypural plate. The fin rays of the dorsal and ventral lobes of the caudal fin are segmented and bifurcate dichotomously at their distal tips. In BRLSI M1393, the ventral lobe has the fin rays preserved in what must have been their original condition while still alive. Dorsal lobe fin rays are fragmented and overlap one another. The true distal tips of the fin rays of the dorsal and ventral lobes are absent in BRLSI M1393 because they protrude beyond the posterior edge of the enclosing nodule. The middle portion of the fin also shows a change in pattern in the fin rays. While they retain their segmentation, the rays become thinner and branch out continuously, which results in brush-like tips at the distal end of the rays. The principal caudal fin ray cannot be identified, as the caudal fin rays are not fully preserved, being broken off by the posterior margin of the nodule.
The hypural plate is a large, almost fan-shaped compound bone made up of an undetermined number of hypurals (Wenz 1967; Arratia and Lambers 1996) that broadens posteriorly with rounded margins and is overlain by hypaxial fin rays in its ventral portion (Fig. 10), whereas in a less taphonomically distorted specimen this would have been overlain by both epaxial and hypaxial fin rays (Arratia and Lambers 1996). It is located posterior to the preural vertebrae. In other specimens (Patterson 1973; Mainwaring 1978), the hypural plate is ventral to the “epurals”, but in BRLSI M1393 the hypural plate is directly posterior to the “epurals”. This position is a result of the postmortem burial process, which has fragmented and transported an assortment of bones in this specimen. This same process appears to account for what seems at first to be the fusion of two hypural bones at the dorsal margin of the hypural plate. However, no sutures are present to suggest fusion, indicating that the impression of a fusion event is created by two splits along the dorsal margin of the hypural plate (Fig. 10b). Along the base of the hypural plate is what appears to be an medial bone with a club-shaped head and a smaller ventral extension with rounded margins. A suture between this bone and the hypural plate indicates what we interpret to be a possible parhypural.
The series of preural vertebrae is anterior to the hypural plate (Fig. 10). In other specimens, these bones are overlapped by the hypaxial principal fin rays (Wenz 1967; Mainwaring 1978; Arratia and Lambers 1996), but here displacement seems to have raised the preural vertebrae above the fin rays. The morphology of these bones and their position relative to the hypural plate is an indication that they are preural vertebrae despite their lack of overlap by fin rays. The preural vertebra anterior to the hypural plate is curving gently backward. It is a large, narrow bone with a greatly expanded ventral arcocentrum with a rounded tip. On first inspection, it looks as though the ventral portion of the dorsal expansion spreads outward anteriorly. Upon closer observation, the anterior half of the dorsal expansion is crushed in, forming a groove that gives the impression of a projection.
The rest of the preural vertebrae share the large, rounded dorsal head of the anterior-most preserved bone, but here the anterior projection is very evident. The preceding preural vertebrae curves slightly more than the first bone, whereas each successive haemal spine curves backward more strongly as they face anteriorly toward the trunk vertebrae. Only the arcocentra of the first four preserved preural vertebrae are present in BRLSI M1393, as the last two are cut off at the edge of the nodule. As the spines face anteriorly towards the trunk vertebrae, they are raised substantially higher than the rest of the caudal endoskeleton as a result of the burial process.
A large scute is displaced anteriorly to the “epurals” (Fig. 10). It is heavily fragmented, but appears to broaden posteriorly and possesses a convex ventral margin, which gives it a rhomboid shape similar to that seen in a Lower Toarcian Pachycormus specimen from Holzmaden, Germany, SMNH SW P.6152 (Arratia and Lambers 1996). The scute is usually found anterior to the “epurals” but has moved inward to cover the “epurals” following distortion.
The bones classified as “epurals” in the caudal fin of Pachycormus may not be epurals due to the revised definition of these bones given by Arratia and Schultze (1992), which they used to refer to detached neural spines of the preural and ural vertebrae. Since the development and homology of these structures is still unknown, Arratia and Schultze (1992) suggest they be referred to as “epurals” until such homology has been demonstrated. We will be following the same naming convention when referring to these bones. Six “epurals” are present in BRLSI M1393 (Fig. 10). Previous descriptions of the caudal endoskeleton of Pachycormus revealed seven “epurals” (Patterson 1973; Arratia and Lambers 1996). It is possible that more “epurals” are present, but that they are overlain by the scute or have been lost during burial. Five “epurals” are located anterodorsal to the hypural plate. The “epurals” have an expanded base that narrows in a ventrodorsal direction and begins to expand laterally as it reaches the dorsal margin of the bone. The dorsal margin has a flat surface with rounded edges, and the dorsal margins become broader in a posterior direction towards the hypural plate. A roughly ovoid fragment of bone, most likely part of the scute, overlies “epurals” 2 and 3. A displaced “epural” is situated ventral to the “epurals”, with its proximal surface facing dorsal side up. The expanded base of this displaced “epural” has a projecting anterior process.
In other specimens, all “epurals” except “epurals” 2 and 3 articulate with ural neural arches (Arratia and Lambers 1996), but no ural neural arches appear to be preserved in any of the caudal fin specimens, and so articulation cannot be shown.
At least 12 hypaxial fringing fulcra are located along the ventral margin of the caudal fin. The five anterior-most fringing fulcra are found singly along the anteroventral margin of the leading edge of each procurrent caudal fin ray. The fringing fulcra have an expanded base that narrows towards the distal end in a rounded tip. The posterior-most fringing fulcra cluster together in groups of three along the posteroventral margin of the procurrent ray, with the exception of one fulcrum found on the posterior margin of the nodule. These posterior-most fringing fulcra have a pointed instead of a rounded tip (Fig. 10c). Epaxial fringing fulcra are not preserved on any of the caudal fins.
In BRLSI M1341, the lateral line is shown terminating dorsally into the axial lobe before the caudal fin.
Axial skeleton—Most specimens have scales covering nearly all of the trunk, so that the vertebral column is difficult to examine. One specimen, BRLSI M1389, however, has a part of its flank covering the vertebral column removed (Fig. 11). A few dorsal and ventral arcocentra are exposed. The ventral arcocentra are flattened and become broader posteriorly, but their elongate shape is evident, along with their ventral arches being curved in a posterior direction. There are 25 haemal spines visible in this specimen, which are strongly curved in a posterior direction. The base of some of the haemal spines widens to a fan shape, which we interpret to be disarticulated haemal arches. The high number of haemal spines exposed, along with the close proximity to the pectoral girdle, suggests that these spines are paired ribs. The specimen shows only four dorsal arcocentra, which appear to be less distorted by taphonomic processes and display a similar fan-shaped neural arch. From what little is shown of the neural spines, their morphology is similar to the haemal spines. Eight-nine neural spines are preserved. The space between the dorsal and ventral arcocentra is the unossified notochord.
Squamation—Body scales of Pachycormus are of amioid-type sensu Schultze (1966, 1996, 2008) (specimens BRLSI M1366 and BRLSI M1395). The scales of Pachycormus, as is seen in all pachycormids with scales, are small and rhombic-shaped. Each scale is a thin bony basal plate with striations. The anterior portion of the scale structure is ornamented by extremely thin radiating ridges with closely packed growth lines appearing on the posterior region, giving it a superficial resemblance to the concentric ridges of cycloid scales (Schultze 1966; Wenz 1967: fig. 64). The ridges are extremely thin, unlike those seen in amioid scales of fishes from the family Amiidae (Schultze 1996; Meunier and Brito 2004), but their anterior position on the scale is similar to that seen in other amioid-type scales. The reduced size of the scales is a common characteristic of pachycormids (Lambers 1992). Rounded edges found in well-preserved scales show a superficial resemblance to cycloid scales. Ganoin is absent from the scales, another characteristic of most pachycormids (Schultze 1966, 2016; Lambers 1992).