Identifying Paleoniscoid Fish from Linton, Ohio

This is my meager attempt to help collectors to identify their Linton paleoniscoids with hopefully, minimal effort.

I have had some experience in identifying Linton fossils and hope to impart some of what I have learned, or should I say, what I have remembered, about the identification of Linton’s paleoniscoid fishes to collectors both old and new to the Linton experience. As my collections from Linton have long been out of my hands (see document “My Linton Collection and Recollections” in this post), I must rely on memory and what literary resources and limited photographic images (only 7) are at my disposal.

Paleoniscoid (Palaeoniscoid) Taxonomy

Murky waters this taxonomy thing, always in constant revision and subjective review leading to much disagreement and confuscation between various workers, authors and ME! This is just one of a variety of ways to express the taxonomic arrangement of the Linton bony fish (exclusive of the other Linton fishes like sharks, lungfish and coelacanths).

Kingdom: Animalia Phylum: Chordata (animals with spinal chords) Subphylum: Vertebrata (animals with backbones) Infraphylum: Gnathostomata (jawed vertebrates) Superclass Osteichthyes (bony fish) Class: Actinopterygii (ray-finned fish) Subclass: Chondrostei Order: Palaeonisciformes Family: Elonychthyidae Elonichthys peltigerus Newberry Family: Haplolepidae Parahaplolepis tuberculata (Newberry) Pyritocephalus lineatus (Newberry) Undescribed Haplolepid Haplolepis corrugata (Newberry) Microhaplolepis ovoidea (Newberry) Microhaplolepis serrata (Newberry)

The scales and dermal bones of the Linton paleoniscoids are easy to distinguish from all other Linton fishes (see the illustration and photos with this post). The polished look of the scales and other body parts stand out above the duller surfaces of other taxa and the cannel. At Linton, paleoniscoids are usually preserved as isolated scales and plates, parts scattered across bedding planes, concentrations of parts in gastric boli (regurgitations of fish, amphibians or reptiles), or as complete or partial individuals. The position in which the fossils have been preserved may hide important characteristics that are needed for identification or important parts or features may be absent. Keep in mind, that the Linton material exposes the surfaces of negative imprints of the original animals, so you are essentially viewing them inside out. Not all specimens will be identifiable to genus and/or species.

Before attempting identifications, be sure your specimens are clean. Sometimes, a wash in water and a gentle brushing is all that will be necessary to expose a feature that otherwise might remain hidden. Other times, a more aggressive preparation may be desired (see acid treatment and scrubbing as mentioned in the “Linton Collection and Recollections” document, accompanying this post). This could result in some minor damage to your specimens, your choice!

Identification

Generalized Paleoniscoid Orientations and Features

Elonychthyidae

Newberry and Worthen (1866) described Elonichthys peltigerus as Palaeoniscus peltigerus from Mazon Creek, Illinois. The skull of the Mazon Creek specimen (see below) has only been briefly described. Newberry (1873) reported it from the Linton cannel coal but the accompanying description and illustration were from a Mazon Creek specimen. I found a large body section (see photo accompanying this post) and a well-preserved complete skull (no photo available) at Linton in the late 1980s; to my knowledge, they are in need of further study.

Palaeoniscus (Elonichthys) peltigerus , Coal Measures, Fulton County, Illinois; Newberry, 1866. Elonichthys peltigerus (see photo accompanying this post) is quite easy to identify, as its scale ornamentation can be readily distinguished from all the other Linton fishes. Isolated scales are found intermittently throughout the cannel but articulated specimens are few. The adult fish are by far the largest of the Linton palaeoniscoids, up to 180 mm. (7 inches) in length. E. peltigerus is rare in the Linton fish assemblage.

Haplolepidae

To identify the various kinds of haplolepids, your specimen should have a skull roof, maxilla, and/or lateral-line scales. The skull roof usually consists of 4 to 6 bones symmetrically arranged on either side of a midline at the center of the fish’s head. The bones of the skull roof are usually found articulated (attached to each other, as in life) even when other parts of the fish are scattered or not present. The maxilla (plural - maxillae) or maxillary can be thought of as the upper jaw of the fish. The flank scales are the scales on the sides of the fish. And, the lateral-line scales are flank scales with sensory organs that can be seen as elongate grooves (lateral line) on a single row of scales along the fish’s length. The lateral lines (only one per side) are sometimes difficult to detect between the flank scales but you can usually locate them by tilting and rotating the fish at varying angles in a bright light source. With a combination, or in some instances only one of these features, identification can be reasonably assured. Usually, more than one feature will be necessary to identify your specimen. Scale counts for identification, like those conducted by Westoll (1944) are quite difficult to perform and I consider them essentially unproductive. Also, immature (juvenile) fish, species variability, and distortion during fossilization can cause problems in identifying your specimen.

Newberry (1856) was the first to describe briefly the Linton Haplolepid fish as species of the genus Mecolepis . Westoll (1944) placed Newberry’s Mecolepis species into his newly established family the Haplolepidae and assigned them with some detail to the genera Parahaplolepis , Haplolepis and Pyritocephalus . Lowney (1980) made revisions to Westoll’s Haplolepis ovoidea reassigning it to Microhaplolepis ovoidea and described a new species Microhaplolepis serrata . The following is a “down and dirty” breakdown of these earlier works into a description of the Linton Haplolepidae that will hopefully be useful to those interested. I’ve assembled an illustration in which I have arranged all the currently known Linton haplolepid species to help compare their characteristic features (refer to illustration “Linton Haplolepids”, accompanying this post).

Parahaplolepis tuberculata (see photo accompanying this post) is recognizable by its large size (45-75 mm), its large and uniquely shaped skull roof, distinctive maxillae, unique lateral-line scales and large shiny, deep flank (side) scales. Skull Roof : The skull roof ornamentation, sometimes pronounced, usually looks like groups of small “isolated depressions (or islands)”. The posterior margin of the skull roof is somewhat angular and indented anteriorly. Overall, it is of similar shape to the skull roof of Microhaplolepis ovoidea except, differing somewhat in ornamentation and usually much larger. Maxilla : The maxilla of P. tuberculata is very narrow anteriorly and is quite high and nearly vertical along the posterior margin. Scales : The lateral-line scales are elongated vertically and have a very narrow “V” shaped indentation in the upper 1/3 of the scale along the posterior margin. Lateral Line : The lateral-line groove is easily located along the “V”-shaped indentation on the lateral-line scale’s posterior margin. It is not in the center of the scale. Other : A single lateral-line scale with groove, a skull roof, or a maxilla from this species is all that is usually necessary for identification. Occurrence : P. tuberculata is relatively common in the Linton fish assemblage.

Pyritocephalus lineatus (see photo accompanying this post) is a medium to large sized haplolepid of 50-67 mm. in length. Skull Roof : Looking directly down on the skull roof, an outstanding feature is the presence of two circular membranes (called fossa (-ae) or fenestra (-ae)) that are sometimes interpreted as a position for eye openings by the uninitiated. The function of these membranes is not fully understood. The eyes are actually located farther down the side of the head and are usually observed only when the skull is splayed from above or compressed from the side. The posterior margin of the skull roof is very rounded and indented anteriorly. Maxilla : The maxilla of P. lineatus narrows anteriorly. Posteriorly, it resembles a trapezoid with a wide base and a narrower, relatively flattened top. Scales : The flank scales are typically shiny and smooth along the edges and the lateral-line scales are not noticeably elongated vertically. Lateral Line : The lateral line, fairly easily seen, appears as a groove near the center of the lateral-line scales. Other : The fossa of the skull roof, the shape of the maxillae, and the centralized lateral-line groove are diagnostic. Occurrence : This fish is uncommon in the Linton fish assemblage.

Undescribed Haplolepid (photo not available) is a medium to large sized haplolepid of unmeasured length but agreeing closely in size and body plan to Pyritocephalus lineatus . Skull Roof : The skull roof lacks the circular membranes (fenestra, fossa) of P. lineatus and its posterior margin appears very angular and deeply indented anteriorly. Maxilla : The maxilla of the undescribed haplolepid roughly agrees with that of P. lineatus but posteriorly appears rotated slightly in a counterclockwise direction. Scales : The flank scales are typically shiny, smooth along the edges, and the lateral-line scales are not noticeably elongated vertically. Lateral Line : The lateral line, fairly easily seen, appears as a groove near the center of the lateral-line scales. Other : Resembles Pyritocephalus lineatus in the body, fin, and tail regions and also in some portions of the head (i.e., maxillae). Could this be an example of sexual dimorphism (physical differentiation between males and females of the same species)? More studies are needed to understand this fish. Occurrence : This fish is rare (only 4 specimens known) in the Linton fish assemblage.

Haplolepis corrugata (photo not available) is a small to large sized haplolepid of 31.5 to 73 mm. in length. Skull Roof : The skull roof is typically strongly ornamented with high ridges and its posterior margin is only slightly rounded and indented anteriorly. Maxilla : The maxillae are slightly thickened anteriorly and posteriorly almost form an equilaterally triangular shape. Scales : In the mid section of the body the flank scales are quite smooth and somewhat elongate vertically. The flank scales overlap each other in a way to exhibit a very flattened appearance, unlike the other Linton haplolepids. Where the upper rows of flank scales juncture (above the lateral-line scales), they form rows of quite visible “V” shaped notches that are directed anteriorly. Lateral Line : The lateral-line groove is very easily recognized and is centrally located in the upper 1/2 of the lateral-line scales. Other : The upper flank scale notches, flattened flank scales, easily recognized lateral-line groove, shape of the maxillae, and coarse ornamentation of the skull roof are diagnostic of H. corrugata. Occurrence : This fish is quite rare in the Linton fish assemblage.

Microhaplolepis ovoidea (see photo accompanying this post) is a small to medium sized haplolepid of 27 to 45 mm. in length. Skull Roof : Skull roof ornamentation usually consists of coarse furrows roughly paralleling each other. The posterior margin is somewhat angularly rounded and indented anteriorly. It is similar in shape to the skull roof of Parahaplolepis tuberculata except, the ornamentation is not in the form of “isolated depressions (or islands)” and is usually much smaller. Maxilla : The maxilla is of equal width along its length and accompanied by thick furrows. The maxilla conforms to a roughly banana-shaped outline with rounded ends. Scales : The flank scales in the mid-body region are elongate vertically and markedly toothed. Lateral Line : The lateral-line groove, which is moderately easy to locate, is centrally located as a dash in the upper 1/4 of each lateral line-scale Other : The small size, the skull roof, and especially the shape of the maxilla are diagnostic Occurrence : M. ovoidea is common in the Linton fish assemblage.

Microhaplolepis serrata (see photo accompanying this post) is a small sized haplolepid of 20-30 mm. in length. It is the smallest and most abundant of the Linton haplolepids. It was overlooked as M. ovoidea in all the Linton fish material until Lowney (1980) recognized it as a different and new species. Skull Roof : The skull roof is easily recognized. It is quite small with the ornamentation, in most instances, difficult to see. The anterior margin of the skull roof is very rounded and the posterior is straight. Maxilla : The maxilla is very small with a slight curvature and coarse furrows. It has a noticeable anterior narrowing and is very rounded posteriorly. Scales : The markedly toothed flank scales in the mid-body region are narrow and much elongated vertically. They have a very minimal thickness, usually appearing with a film-like expression and many times exposing individual vertebrae to the surface. Lateral Line : The lateral-line groove, which is difficult to locate is centrally positioned as a very vertically narrow dash (shortened horizontally) in the upper 1/4 of each lateral line-scale. Other : Often, this species can be found in abundance as thin, film-like specimens on the conchostracan layers in the cannel. The very small size, the shape of the maxilla, an inability to locate a lateral line, and the shape of the skull roof are diagnostic. Occurrence : M. serrata is abundant in the Linton fish assemblage.

REFERENCES:

LOWNEY, K.A. 1980. A revision of the family Haplolepidae (Actinopterygii, Paleonisciformes) from Linton, Ohio (Westphalian D, Pennsylvanian) Journal of Paleontology, September 1980, v. 54, p. 942-953

NEWBERRY, J. S. and WORTHEN, A. H. 1866. Descriptions of new species of vertebrates, mainly from the Sub- Carboniferous limestone and Coal Measures of Illinois. Geological Survey of Illinois 2, 9-134

NEWBERRY, J.S. 1873. The classification and geological distribution of our fossil fishes. Report of the Geological Survey of Ohio, 1 (2): 245-355

WESTOLL, T. STANLEY. 1944. The Haplolepidae, a new family of late Carboniferous bony fishes: a study in taxonomy and evolution. Bulletin of the AMNH; v. 83, article 1