Monocerichthys scheuchzeri, Taverne & Capasso, 2013

Taverne, Louis & Capasso, Luigi, 2013, Gladiopycnodontidae, a new family of pycnodontiform fishes from the Late Cretaceous of Lebanon, with the description of three genera, European Journal of Taxonomy 57, pp. 1-30 : 11-19

Monocerichthys scheuchzeri gen. et sp. nov.

Figs 7-14

urn:lsid:zoobank.org:act:74512C09-6F58-4711-A028-93AFA9CB10FF

Diagnosis

Gladiopycnodontid with the prefrontal elongated in a short rostrum, outpacing only slightly the lower jaw level. Prefrontal with a spiny anterior tip. Vomer bearing small rounded molariform teeth irregularly ranged. Frontal short, narrow and with a weakly developed median protuberance. Dermosupraoccipital, parietal and dermopterotic longer than deep and articulated with a large supratemporal. Long nuchal horn with a broad basis, a very spiny posterior border and resting on the dermosupraoccipital, the supratemporal and the posttemporal. Orbitosphenoid articulated with the mesethmoid. Well developed triangular dermosphenotic. Premaxilla long, broad, toothless and sutured by its dorsal margin to the prefrontal. Large, toothless and triangular shaped maxilla. Small lower jaw. Hypertrophied trapezoid preopercle covering a great part of the cheek. Small opercle with a rounded dorsal border and an acuminate ventral tip. Exposed part of the hyomandibula-dermohyomandibula considerably smaller than the preopercle. Very large pectoral girdle closely associated to the skull and forming a sort of cephalothorax. Posttemporal articulated with the supratemporal. Hypertrophied hypercleithrum. Strongly hypertrophied cleithrum with a long and broad ventral branch and a large posterior ventral process. Long pectoral spine articulated on the cleithrum. 17 neural spines, all fused to their neural arches, before the epichordal series. Neural and haemal spines short and broad. Postcoelomic bone obliquely oriented. Short dorsal fin located in the middle of the back, with 13 pterygiophores, 1 small spiny ray and 12 segmented rays. Very short anal fin. Caudal fin with a convex posterior margin and with 17 principal rays, 6 dorsal and 7 ventral procurrent rays. Body entirely covered with very small flake-like scales. No dorsal ridge scutes. One large postcloacal scute below the postcoelomic bone.

Etymology

The species name of this new gladiopycnodontid fossil fish is dedicated to Johann Jacob Scheuchzer (1672-1733), the Swiss physician who was the first to illustrate a Lebanese fossil fish in his book Piscium Queralae et Vindiciae (1708) [cf. Gayet et al. 2012: 10].

Holotype and unique specimen

Sample CLC S-413a, b, the two faces of a complete specimen ( Figs 7-8 ). Total length: 102 mm.

Formation and locality

Marine Late Cenomanian, Haqel, Lebanon.

Holotype morphometric data

The morphometric data are given in % of the holotype standard length (87 mm).

Length of the head (opercle included) ……………………………………… 37.8 % Length of the cephalo-thorax (cleithrum included) ………………………… 66.7 % Depth of the head (with the nuchal horn) …………………………………... 37.3 % Depth of the head (without the nuchal horn) ……………………………...... 64.7 % Maximum depth of the body (just behind the nuchal horn) ………………... 39.4 % Length of the pectoral spine ………………………………………………... 19.9 % Predorsal length …………………………………………………………...... 65.6 % Basal length of the dorsal fin ………………………………………………. 18.2 % Preanal length ………………………………………………………………. 72.6 % Basal length of the anal fin ………………………………………………… 10.0 % Depth of the caudal peduncle ………………………………………………….. 9.5 %

Osteology

1. The skull ( Figs 9-10 )

The dermal bones of the skull are covered with small tubercles, but this ornamentation is not strongly pronounced.

The head is large, longer than high and triangular in shape. The snout is elongated in a short rostrum formed by the long and broad prefrontal and by the long toothless premaxilla. The anterior tip of the prefrontal is spiny. The rostrum outpaces only slightly the lower jaw level. The anterior region of the long mesethmoid and the vomer are hidden by the prefrontal and the premaxilla. However, the most anterior part of the premaxilla is broken on face CLC S-413b and a part of the vomer is visible. The bone bears very small, rounded molariform teeth, irregularly ranged.

The frontal is short, rather narrow and practically not curved. It extends anteriorly beyond the orbit level and exhibits a weakly developed median protuberance located just before the orbit, but there is no frontal horn. The posterior region of the skull roof is formed by the dermosupraoccipital, and the paired parietals and dermopterotics. These bones are rather small and longer than deep. There is no temporal fenestra. A large supratemporal is sutured with the posterior margin of the dermosupraoccipital, the parietal and the dermopterotic. The autosphenotic is located below the dermopterotic. A long and strong pointed nuchal horn with a series of large spines on its posterior border is resting on the dermosupraoccipital, the supratemporal and also the posttemporal. The very broad basis of that horn extends on the beginning of the back.

The parasphenoid is very long and toothless. The orbitosphenoid is pressed against the mesethmoid. The other endochondral bones of the braincase are not accessible.

Small fragments of the metapterygoid and the entopterygoid are visible between the preopercle and the parasphenoid, but a great part of those two bones remains hidden by the preopercle in the same way as the quadrate and the symplectic.

The upper jaw contains a large, more or less triangular and toothless maxilla and a long, broad and toothless premaxilla that is articulated to the prefrontal by its upper margin. The lower jaw is triangular in shape and rather small. The dentary is reduced to its ventral branch. Its ventral margin is spiny. The teeth are not preserved. Other gladiopycnodontid genera have two teeth on the dentary. There is a large angular and a small articular. It is not possible to determine whether the prearticular was toothed or not or whether a marked coronoid process was present. The prearticular bears molariform teeth on its inner face in some other gladiopycnodontid fishes.

The orbit is small. A large triangular dermosphenotic is located below the frontal and before the parietal and the dermopterotic. No other orbital bone is preserved.

The exposed part of the hyomandibula-dermohyomandibula is important; however, it is much smaller than the hypertrophied trapezoid-shaped preopercle, that almost entirely covers the cheek. The opercle is a small bone, deeper than long, dorsally rounded and ventrally acuminate. The subopercle and the interopercle are absent, as in all pycnodontomorph fish.

The hyoid bar, the branchiostegal rays and the branchial skeleton are not visible.

2. The girdles ( Figs 8-9 )

The pectoral girdle is closely associated to the skull, both structures forming a sort of cephalo-thorax. The pectoral bones are ornamented with small tubercles. The posttemporal is a deep bone, narrow in its upper part but ventrally broader, that is sutured anteriorly to the supratemporal and dorsally to the nuchal horn. The hypercleithrum (= supracleithrum) is hypertrophied in a large, more or less ovoid plate. The cleithrum is gigantic, with a well developed dorsal branch, a long and broad ventral branch and a very large posterior ventral process. The pectoral fin is replaced by a long strong spine. There is no soft ray.

No pelvic bones are visible, but small fragments of rays from the ventral fins are preserved just above the posterior point of the pectoral spine on face CLC S-413a.

3. The axial skeleton ( Fig. 8 )

The trunk is fusiform and not deep-bodied as usual in most pycnodontomorph fishes. The abdominal part of the axial skeleton progressively elevates and anteriorly reaches the orbit level. The vertebrae are constituted by the dorsal and ventral arcocentra only. The notochord is not completely surrounded by the arches. There are 17 neural spines before the epichordal series of the caudal skeleton. They are short, but rather broad. All those neural spines are fused to their neural arches. Non of them are autogenous. The first neural spine corresponds to the first ossified neural arch. The fourth neural spine is fused to two neural arches. That is probably an individual variation and not a specific character. A few badly preserved traces of the basiventrals from the abdominal region are visible, but the basiventrals of the caudal region and the haemal spines are lost, except the last ones near the tail. Four long and strong ribs are seen just behind the hypercleithrum on face CLC S-413b. The few preserved haemal spines are short and broad, as the neural spines.

Only fragments of the most ventral part of the postcoelomic bone are preserved. The bone is well developed, obliquely oriented and with an enlarged lower extremity that is directed backwards.

4. The dorsal and anal fins ( Figs 8 , 11-12 )

The dorsal fin is short but rather high and is located in the middle of the back. The fin begins with 1 small spiny ray, followed by 12 long segmented and branched rays. There are 13 pterygiophores, each of them bearing one ray.

The anal fin is partly preserved on face CLC S-413a but is completely missing on face b. The fin is still shorter than the dorsal one and is also located on the middle of the body, far from the tail. Fragments of three pterygiophores and of four rays are visible in the beginning of the fin, just behind the postcoelomic bone. Two very small pterygiophores and three very small rays are visible at the posterior extremity of the fin.

5. The caudal skeleton ( Fig. 13 )

A caudal peduncle is present. The caudal skeleton is not very well preserved, but the two faces CLC S-413a and b complete each other and allow to entirely reconstruct the whole structure. The caudal fin is dorsally supported by 5 epichordals and ventrally by 12 hypochordals. The tenth and eleventh hypochordals are moderately broadened, but there is no real hypertrophy. The hypochordal series begins one vertebral segment before the epichordal series. No urodermal is visible, but this is probably a taphonomic artefact. Indeed, a well developed urodermal does exist in some gladiopycnodontid fishes more specialized than M. scheuchzeri gen. et sp. nov. (pers. obs.).

The caudal fin is not very large and has a convex posterior margin ( Poyato-Ariza & Wenz 2002: fig. 36B). There are 17 principal rays. The most dorsal and ventral principal rays are segmented but unbranched. The other principal rays are segmented and branched. There are 6 dorsal and 7 ventral procurrent rays.

6. Squamation ( Fig. 14 )

The body is entirely covered with very small, flake-like scales that are imbricated one into another. Rather similar scales exist in Gladiopycnodus karami gen. et sp. nov., but they are deeper and much larger. There are no peculiar scales or scutes, either on the dorsal ridge or on the ventral keel.

A large irregular postcloacal scute is present just below the postcoelomic bone.

Figures

Fig. 7. Monocerichthys scheuchzeri gen. et sp. nov. Holotype CLC S-413a (above) and b (below).

Fig. 8. Monocerichthys scheuchzeri gen. et sp. nov. General reconstruction based on the two faces of holotype CLC S-413a, b. Body scales are omitted.

Fig. 9. Monocerichthys scheuchzeri gen. et sp. nov. Reconstruction of the skull and the pectoral girdle based on the two faces of holotype CLC S-413a, b.

Fig. 10. Monocerichthys scheuchzeri gen. et sp. nov. The vomerian region of the skull in holotype CLC S-413b.

Fig. 11. Monocerichthys scheuchzeri gen. et sp. nov. The dorsal fin of holotype CLC S-413a.

Fig. 12. Monocerichthys scheuchzeri gen. et sp. nov. The anal fin of holotype CLC S-413a.

Fig. 13. Monocerichthys scheuchzeri gen. et sp. nov. Reconstruction of the caudal skeleton, based on the two faces of holotype CLC S-413a, b.

Fig. 14. Monocerichthys scheuchzeri gen. et sp. nov. Body scales from holotype CLC S-413a, from the dorsal region near the tail.