Cladocyclus geddesi, Berrell & Alvarado-Ortega & Yabumoto & Salisbury, 2014

Berrell, Rodney W., Alvarado-Ortega, Jesús, Yabumoto, Yoshitaka & Salisbury, Steven W., 2014, The first record of the ichthyodectiform fish Cladocyclus from eastern Gondwana: A new species from the Lower Cretaceous of Queensland, Australia, Acta Palaeontologica Polonica 59 (4), pp. 903-920 : 906-910

Cladocyclus geddesi sp. nov.

Etymology: After Kerry Geddes, in recognition of his discovery of the holotype specimen and outstanding preparation work on the Isisford fossil material.

Holotype: QM F44329 , part and counter-part of an articulated cranial and mandibular skeleton, the anterior part of the body, along with associated neural spines and isolated scales ( Fig. 3 ).

Type locality: The specimen comes from a University of Queensland field locality (UQL-Isis-IX) on private land near the town of Isisford, central-western Queensland, Australia (24°15.460’S, 144°26.207’E) .

Type horizon: Winton Formation, Manuka Subgroup, Rolling Downs Group; Lower Cretaceous (upper Albian, 100.5–102.2 Ma; Fig. 2).

Diagnosis.— Cladocyclus geddesi sp. nov. can be diagnosed from other congeners based on the following features: supraoccipital crest that is gently convex posteriorly; horizontal arm of the cleithrum approximately twice as long as the vertical arm.

Description

General features.— Preserved elements of QM F44329 include an almost complete skull, shoulder girdle, pectoral fin, and the first 19 vertebrae. Ribs and some scales are preserved, but the opercular is obscured as a result of the specimen now being embedded in resin. The head is slightly rotated clockwise and the shoulder girdle is disarticulated. The length from the anteriormost end of the head to the posteriormost edge of the 18th vertebrae is 270 mm. The head length (from the tip of the upper jaw to the middle of the corner of lower and upper arms of the cleithrum that represent the posterior border of the opercular is about 121 mm. The depth of the head (from the dorsal end of the supraoccipital crest to the ventral margin of the interopercular is 101 mm. The body depth is almost the same as the depth of the head.

The anterodorsal part of the skull has been flattened mediolaterally slightly, such that the palatine is partly disarticulated from the palatine condyle of the maxilla. For most bones only the lateral surface is visible. Surface colouration on the bones consists of a marbled pattern of yellow, rust orange, chocolate brown and black.

In all the complete ichthyodectiform specimens so far described, the head length (including the opercle) represents 15–25% of the total length (see Bardack 1965; Taverne and Bronzi 1999; Alvarado-Ortega 2004, 2005). The head length of QM F44329 (measured from the tip of the ethmoid area to the anterior border of vertical limb of the cleithrum that represent the posterior border of the opercle, which is missing in the specimen) is about 150 mm. This means that the total length of QM F44329 could be around 600–1050 mm.

The large, oblique, dorsally opening mouth is formed by a well-developed upper and lower jaw bones (assuming that paired bones such as the dentaries are identical) with a single series of conical, slender, pointed teeth. In lateral view, the braincase is triangular in outline, being approximately 1.4 times longer than it is deep. Based on the length of the preserved ribs that enclose the abdominal region, and the height of the preserved neural spines, the anterior part of the trunk of QM F44329 was not deeper than the posterior part of the skull.

Skull and braincase.—The ethmoid region is proportionately very deep dorsoventrally, being almost the same depth as the

A postorbital portion of the braincase. The orbit is also large, with a diameter that is slightly shorter than the length of the posterior part of the cranium. The rostrodermethoid is a complex bone that forms the roof and anterior border of the nasal capsule. It has a posterior projection that is sutured to the frontals. It also shows a rounded anterior tip and a ventral section that is expanded laterally to form two semi-circular, wing-like lateral projections.

B 50 mm premaxilla epineural neural spine maxilla dentary supramaxilla anterior postcleithrum coracoid

The ethmopalatine is a well ossified bone (only the left element is visible) that forms the floor of the nasal capsule. Bony outgrowths attach this bone to the rostrodermethoid anteriorly and to the lateral ethmoid posteriorly.Although the lateral ethmoid is not well preserved, it forms a solid bar that firmly attached to the frontal and the ethmopalatine bone. It additionally separates the nasal capsule from the orbit.

Similar to the condition in most ichthyodectiforms, the frontal is a long smooth bone located above the orbit, being about two thirds of the maximum cranium length. The frontal is attached to the rostrodermethoid anteriorly by a serrated suture, and the mesoparietal, pterotics, and sphenotics posteriorly. Medially, the suture between the two frontals forms a shallow median sulcus, best developed between the orbits.

The parietals are fused elements, forming the mesoparietal, which in dorsal view has a C-shaped outline, with a posterior section that is concave and a central section joining the supraoccipital, just above the posterior margin of the orbit. The posterior projections of the mesoparietal extend laterally to the base of the supraoccipital, and also contact the epioccipitals. The mesoparietal does not form part of the supraoccipital crest. The supraoccipital bone is proportionately very large and the anterior margin reaches to above the posterior edge of the orbit. The supraoccipital crest is well developed, being gently convex anteriorly with a rounded posterior margin. The epioccipital, which is located along the lateral side of the supraoccipital, is a proportionately large bone with a massive, rounded posterior crest that extends over the middle of the supraoccipital crest. The epioccipital bone is sutured to the mesoparietal anteriorly and sits medial to the middle of the supraoccipital.

The pterotic is the largest bone in the posterior part of the cranium. Posteroventrally, the pterotic contacts the intercalar, while dorsally it contacts the epioccipital and mesoparietal. Ventrally the pterotic forms the middle part of the hyomandibular fossa, while anteriorly it contacts the frontal and the autosphenotic. The pterotic has a long groove that forms a dilator fossa, obscuring its suture with the autosphenotic.

The autosphenotic forms the anterior part of the hyomandibular fossa and has a stout lateral process. The intercalar is a complex bone that forms the posterior part of the hyomandibular fossa. The parasphenoid forms a large, straight, toothless rod along the orbit; although its postocular section is not exposed, the ventral border of the postocular skull section is observable in X-rays. The angle between the orbital and postorbital sections of the parasphenoid is about 130°.

In QM F44329 the hyomandibular fossa is occupied by the articular head of the hyomandibular bone, as when the specimen was alive, such that the longitudinal axes of the hyomandibular head and the hyomandibular fossa have the same inclination. The anterior projection of the hyomandibular fossa crosses the paraesphenoid bone behind the ethmoid area.

Circumorbital and sclerotic bones.—The circumorbital series is not well preserved. Preserved elements include the partial remains of infraorbitals II and IV, which are locat- ed along the ventral border of the orbit. These bones are thin and fragile. The preserved sclerotic bones are flimsy, with semicircular laminar structures that surround the circular basal sclerotic bone with surrounding serrated borders. While the dermosphenotic in most ichthyodectiforms is rarely preserved ( Patterson and Rosen 1977), it is visible on QM F44329 as a small, subrectangular bone that is located dorsal to the preserved sclerotic ring ( Fig. 4 ).

Hyopalatine bones.— The hyomandibula, metapterygoid, quadrate and palatine are partially exposed in lateral view. The exposed dorsal part of the hyomandibula is stout and appears to be elongated and continuous. Just at the base of this opercular condyle, the hyomandibula shows a large deep cavity, regarded by Bardack (1965) as a depression for the adductor hyomandibular muscle in other ichthyodectiforms. The foramen for the hyomandibular branch of cranial nerve VII is visible anterior to the latter depression. The ventral section of the hyomandibula becomes progressively narrower as it curves slightly anteriorly to meet the symplectic. The anterior border of the preopercle matches with a groove that opens dorsoventrally along the ventral section of the hyomandibula.

The quadrate is triangular in outline, with a straight dorsal border. Its articular condyle is directed anteriorly and located below the posterior edge of the orbit. Even though the dorsal tip of the symplectic seems to be broken, this nail-shaped bone is as high as the quadrate, where it is attached in a deep posteri- or notch. The metapterygoid is a smooth bone located between the anterior border of the hyomandibula and the dorsal border of the quadrate. Only the head of the palatine is visible as a malleolus-like element that fits between the palatine condyle of the maxilla and the ethmopalatine bone ( Fig. 4 ). The arrangement and number of branchiostegal rays are uncertain.

Gill arch skeleton.—Elongated gill rakers are preserved between the posterior edge of the preopercle and the anterior end of the anterodorsal border of the cleithrum. The gill rakers comprise hollow tubes and L-shaped projections. No denticles are visible on the lateral surface of each individual gill-raker.

Lower jaw.—In lateral view, the lower jaw is rectangular in outline. The dentary symphysis is almost as deep as the posterior section of the lower jaw along the sinuous alveolar border. The ventral border of the lower jaw is slightly convex, and the coronoid process is reduced to a rounded structure that partially overhangs the dentary teeth. The teeth of the lower jaw are robust, conical, elongate and slightly recurved. The teeth within the dentary are longer than those in the maxilla and premaxilla. The size (crown height) of individual, complete dentary teeth is highly variable along the tooth row, ranging from 2.9–6.4 mm. As in the maxilla and the premaxilla, the dentary teeth form a single line rising from the alveolar border. The left dentary possesses 13–14 alveoli in total, with 4–5 teeth lacking.

In lateral view, the angular bone occupies the posterior third of the lower jaw. It possesses a short, rounded postarticular process. The retroarticular bone is a proportionately small bone, located at the posterior end of the angular, between the posterior border of the dentary and the postarticular process of the angular. The articular, retroarticular and angular bones participate in the articular facet for the quadrate ( Fig. 5 ).

Upper jaw.—The maxilla is a long rectangular-shaped bone, about seven times longer than it is deep. The maxillary teeth are similar in shape to those of the lower jaw, but are smaller and more regular in size, ranging from 2.7–3.9 mm. As in the lower jaw, these teeth are singularly implanted within the jaw, such that they are almost equally spaced.

The premaxilla is ovoid in lateral view, and carries the first four teeth of the upper jaw. These teeth increase in size posteriorly from 3.6–1.3 mm. The premaxilla and maxilla attach to each other along their entire height. The left premaxilla bears four alveoli, all with teeth, while the left maxilla has 22 alveoli in total, with five teeth lacking. Patterson and Rosen (1977) state that all ichthyodectiforms possess two supramaxilla bones except for the saurodontids. In QM F44329 both supramaxillary bones are preserved. The anteri- or supramaxilla is an irregularly shaped, elongated bone with a posterior keel. The posterior supramaxilla has a triangular outline with a notch in its anterior part where it intersects with the bony outgrowth of the anterior supramaxilla ( Fig. 3 ).

Opercular series.—The opercle is not visible on the exposed section of the specimen. Although the posterior edge of the preopercle is not well preserved, it is a flat boomerang-shaped bone with wide limbs, the vertical limb being twice as long as the horizontal one. The dorsal posterior edge of the preopercle shows fine grooves giving the appearance of a wide-toothed comb. The preopercle has five openings with short grooves of the preopercular sensory canal along its vertical limb, and about 14 on its horizontal limb. The interopercle is a thin, laminar, and gently curved bone placed below the ventral limb of preopercles. Behind the interopercle, the anterior part of the subopercle is preserved showing the same laminar aspect.

Pectoral girdle and fin.—In lateral view, the cleithrum is boomerang-shaped, with the long axes of the two arms oriented at approximately 90° to each other. The length of the horizontal arm of the cleithrum is about twice the length of the vertical arm. The coracoid is as long as the horizontal arm of the cleithrum, and has a rounded ventral border. Only a small surface of the scapula is visible, just in the posterior angle formed by both arms of the cleithrum, at the point where these bones meet the upper posterior part of the coracoid. Two poorly preserved postcleithra sit behind the vertical arm of the cleithrum. These elements are only partially exposed, but X-rays reveal their presence and full extent. The dorsalmost part of the postcleithrum has a rectangular shape with an undulating posterior margin. The ventralmost part of the postcleithrum has a hammer-like outline anteriorly, and a tear-drop-like outline posteriorly. The supracleithrum is not preserved. The supratemporal is a large triangular plate.At least seven rays of the pectoral fin are present in this specimen. The two anterior rays of each fin are saber-shaped, being wider and longer, while the posterior ones tend to be shorter and narrower.

Vertebral column.—Only the anterior 19 abdominal centra are preserved. These are proportionately massive, well ossified, and slightly longer in an anteroposterior direction than they are wide.All centra have long, longitudinally ovoid fossae on their ventral and lateral surfaces, except for the two anterior-most ones, which have essentially smooth surfaces. All the neural arches are autogenous and fused to neural spines. There is no evidence of supraneurals, but the remains of several large, fine epineurals are preserved. The ribs enclose the abdominal area.

Scales.—Although very fragmentary, the preserved scales are all oval-shaped, with a dorsoventrally-aligned long axis. The external surface of each scale displays numerous concentric circuli.

Figures

Fig. 3. Ichthyodectiform fish Cladocyclus geddesi sp. nov. (QM F44329) from near Isisford, central-western Queensland, Australia; Lower Cretaceous (upper Albian) strata of the Winton Formation, in lateral view. Photograph (A) and interpretive drawing (B).

Fig. 4. Ichthyodectiform fish Cladocyclus geddesi sp. nov. (QM F44329) from near Isisford, central-western Queensland, Australia; Lower Cretaceous upper Albian) strata of the Winton Formation, braincase in dorsolateral view. Photograph (A) and interpretive drawing (B).

Fig. 5. Ichthyodectiform fish Cladocyclus geddesi sp. nov. (QM F44329) from near Isisford, central-western Queensland, Australia; Lower Cretaceous (upper Albian) strata of the Winton Formation, area of articulation for the lower jaw. Photograph (A) and interpretive drawing (B). Hatched area indicates broken bone.