Wednesday, June 12, 2019

[PaleoIchthyology • 2019] Flagellipinna rhomboides • A New Genus and Species of Pycnodontid Fish (Neopterygii, Pycnodontiformes), from the Upper Cretaceous (Cenomanian) of Lebanon, with Notes on Juvenile Form and Ecology


Flagellipinna rhomboides
Cawley & Kriwet, 2019


ABSTRACT
The Upper Cretaceous (Cenomanian) limestone quarry of Haqel, Lebanon, is home to one of the largest diversities of fossil actinopterygians in the Mesozoic, particularly of pycnodontiform fishes. Here, we describe a pycnodontiform fish, Flagellipinna rhomboides, gen. et sp. nov., from this locality based on four specimens. It is considered a member of the derived family Pycnodontidae due to the presence of a postparietal process. This taxon is distinct from other pycnodontids due to its diamond-shaped body, whip-like dorsal fin, postcloacal scales with forward-pointing spines, and acute anterior profile with a concave slope, giving it a ‘hunchback’ appearance. The prognathous snout armed with molariform teeth suggests that this pycnodont preyed on a variety of shelled animals from crevices. The smallest specimen is distinct in that it has a larger orbit size, no spines on the contour scales, poorly ossified skull roof bones, a notochord partially covered by arcocentra, and lacks whip-like filament on the dorsal fin, which suggest that it is a juvenile/subadult. The differences between the juvenile/subadult and other larger specimens suggest a change in ecological niche occupation during ontogeny, going from a generalized forager that lived in complex, reef habitats to moving into deeper waters to feed from crevices on the reef edge. These findings provide a more complete picture of the possible life history strategies that pycnodontiforms may have used in order to exploit different resources throughout their lives.

the holotype of Flagellipinna rhomboides, gen. et sp. nov. [MNHN.F.HAK2003]


SYSTEMATIC PALEONTOLOGY

Class OSTEICHTHYES Huxley, 1880 
Subclass ACTINOPTERYGII Cope, 1887 
Series NEOPTERYGII Regan, 1923 

Order PYCNODONTIFORMES Berg, 1937 
Family PYCNODONTIDAE sensu Nursall, 1996 

FLAGELLIPINNA, gen. nov.


Age— Early late Cenomanian, Late Cretaceous.

Type Species— Flagellipinna rhomboides, sp. nov.

Etymology— The name is a combination of the Latin words ‘flagellum’ (whip) and ‘pinna’ (fin), which refers to the whip-like dorsal fin present in this genus.

FIGURE 1. Specimens of Flagellipinna rhomboides, gen. et sp. nov. A, MNHN.F.HAK2003, holotype. B, MNHN.F.HAK2001, paratype, possible late juvenile/subadult stage. C, MNHN.F.HAK1972a, paratype, part. D, MNHN.F.HAK1972b, paratype, counterpart. Scale bars equal 1 cm.



FLAGELLIPINNA RHOMBOIDES, sp. nov.

Diagnosis— Small pycnodontid fish with a deep, rhomboid body shape. Anterior profile of the fish is extremely steep, being sloped at a mean 57.3° angle in relation to the vertebral column. Dorsal fin has a whip-like filament. Dorsal and anal pterygiophores inserted very deeply into the body. Dorsal and anal fins falcate anteriorly and become progressively strap-like posteriorly. Ventral apex present anterior to the insertion of anal fin. Two dentalosplenial (dentary) teeth are broad and incisiform, and the anterior (mesial) premaxillary tooth is bicuspid. Skull dorsoventrally flattened and obliquely oriented, with an elongate snout. Paired preparietal (prefrontal) bone present. Dermocranial fenestra absent in skull roof. Cleithrum narrow and elongate with just two limbs, the dorsal limb being far narrower than the ventral limb. Large, semicircular preoperculum with small exposed dermohyomandibular. Comparatively large operculum broad and club-shaped. Notochord completely surrounded by arcocentra. Ten to 15 dorsal ridge scales with three to six backward-pointing spines. Ventral ridge scales consist of 11 precloacal scales, with two backward curved spines and a single postcloacal scale. Cloaca is roofed by two comma-shaped scales. Four spines present on the postcloacal ventral ridge scale, with the first three being strongly curved and forward pointing and the most posterior spine straight and backward pointing in a posteroventral angle. Complete scales restricted to abdominal region, scale bars on rest of body.

Type Locality and Horizon—Haqel, northern Lebanon; Sannine Formation, early late Cenomanian, Upper Cretaceous.

Etymology— Latin for ‘rhomboid’ in allusion to the diamond-like body shape of the new species.

FIGURE 2. Flagellipinna rhomboides, gen. et sp. nov., reconstruction of the skeleton based on MNHN.F.HAK1972a, MNHN.F.HAK1972b, MNHN.F.HAK2001, and MNHN.F.HAK2003.
Anatomical characters based on the holotype and the paratypes; skull roof restored from the holotype, whereas the lower jaw is based on both the holotype and MNHN.F.HAK2001; pectoral girdle is intermediate between the conditions seen in the holotype and MNHN.F.HAK2001; dorsal ridge scale series based mainly on MNHN.F.HAK1972b; caudal endoskeleton based on a combination of MNHN.F.HAK1972b and specimen figured on page 93 of Gayet et al. (2012 Gayet, M., P. Abi Saad, and O. Gaudant. 2012. Les fossiles du Liban: Memoire du Temps. Éditions Desiris, Meolan-Revel, France, 184 pp. [Google Scholar] ); arcocenta based on paratype MNHN.F.HAK1972b. Reconstruction of paired fins and the caudal fin is hypothetical because they are poorly preserved in all specimens. Caudal endoskeleton that is not preserved in any specimen is represented by hypothethical bones shown here in gray. Scale bar equals 1 cm.


CONCLUSIONS: 
Flagellipinna rhomboides, gen. et sp. nov., is a recent addition to the high biodiversity of the exceptional pycnodont fauna of the Cenomanian Lebanese fossil beds (Appendix 1). Anatomical characters such as the presence of a postparietal process indicate that this taxon is a pycnodontid, which are well represented in the Cenomanian of Lebanon (Appendix 1).

The characteristics distinguishing Flagellipinna, gen. nov., from other pycnodonts is its acute anterior profile with a diamond-shaped body; a reduced operculum, which is nevertheless broader than typical pycnodont opercula and lies posterior to the dermohyomandibular and the preoperculum; a narrow and elongate cleithrum with two limbs; a posteroventral spine of the postcloacal scale, which is straight and backward pointing alongside three forward-pointing spines; a dorsoventrally flattened skull with a prognathous snout; a deep insertion of the dorsal and anal pterygiophores; and whip-like extended dorsal fin rays reminiscent of modern coral reef fishes such as Zanclus cornutus.

A combination of inferences from the specimens, the paleoenvironment of Haqel, and the invertebrate fauna that were found alongside Flagellipinna, gen. nov., suggests that it possibly inhabited reef slopes and fed from crevices on elusive armored prey such as bivalves, echinoderms, and gastropods. The interpretation of the smallest specimen of Flagellipinna, gen. nov., being a juvenile raises interesting questions regarding ontogeny and change in ecology during growth of this fish. The juvenile has an even deeper body shape than the adult along with a shift in skull position and shape, indicating that it was a more typical durophagous pycnodont that lived in shallower reef habitats and may have moved into deeper waters to feed from the crevices of the reef edge as it became older. Of course, more specimens of Flagellipinna, gen. nov., are necessary in order to confirm the validity of these morphological changes throughout its ontogeny, but this interpretation is supported by similar ontogenetic changes observed in modern taxa.

The Lebanese Cenomanian pycnodont fauna contains some of the most diversified assemblages of non-teleost actinopterygians in the Cretaceous. Such a diverse range of forms must have had a wide range of ecological requirements and life histories. Flagellipinna rhomboides, gen. et sp. nov., is not only another new species from this extraordinary assemblage, but the specimens described here also hint at how these fishes could have filled a particular niche in the Haqel ichthyofauna and how that niche may have changed during its life span.


John Joseph Cawley and Jürgen Kriwet. 2019. A New Genus and Species of Pycnodontid Fish Flagellipinna rhomboides, gen. et sp. nov. (Neopterygii, Pycnodontiformes), from the Upper Cretaceous (Cenomanian) of Lebanon, with Notes on Juvenile Form and Ecology. Journal of Vertebrate Paleontology.  DOI: 10.1080/02724634.2019.1614012   

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