Eileen D. Grogan

Relationships of the Chimaeriformes and the basal radiation of the Chondrichthyes

The origin and early evolution of the cartilaginous fishes (Chondrichthyes) has been the subject of considerably more debate than of data. The two modern groups, Chimaeriformes and Elasmobranchii, differ so radically in morphology that in the past they have often been considered unrelated -- descended from some remote and unknown common ancestor. The current consensus promotes the Chimaeriformes and Elasmobranchii as sister taxa of the Class Chondrichthyes which are linked by an assemblage of Palaeozoic fossil taxa, but no taxonomic or phylogenetic scheme has been accepted for the Class. Of the two groups, the Chimaeriformes is the less understood. The few species of Chimaeriformes existing today are enigmatic, principally deeper-water fish that are not readily accessible for study. In the past the fossil record of both groups has been relatively scanty, primarily due to the poor potential for skeletal fossilization, and so has provided little useful input into fundamental discussions of vertebrate diversification. However, these situations are changing. Chimaerids are increasingly becoming the subject of renewed biological and limited fisheries interests. Regarding extinct chondrichthyans, the last 30 or so years have entailed discoveries of new fossils that illuminate our view of Palaeozoic life and are eliciting dramatic changes in our understanding of these early fishes, their relations, and the origins of jawed conditions.Morphological examination of fossil chondrichthyans indicates that the plesiomorphous state of the gnathostome suspensorium is autodiastylic and that complex labial cartilages are primitive and likely to have been critical to the mechanical architecture of the first jaws. Analysis of cranial morphology, cranial proportions, the phyletic and developmental history of calcified tissues, and postcranial data including the evolution of the prepelvic tenaculum are now feasible. Cumulatively, when the results of these analyses are subject to cladistical evaluation, the result is one predominant cladogram supporting two monophyletic subclasses: the Elasmobranchii and the Euchondrocephali. The latter subclass contains a monophyletic group of holocephalans including the Cochliodontomorpha, and within this taxon, the restricted Chimaeriformes. Alternative cladograms of the non-holocephalan Euchondrocephali are dependent upon whether whole-body or cranial characters alone are employed in the analysis, or the additive or non-additive treatment of characters. Otherwise, only the discovery and description of additional members of this diverse assemblage are expected to alter these patterns of associations

Debeerius ellefseni (Fam. Nov., Gen. Nov., Spec. Nov.), an autodiastylic chondrichthyan from the Mississippian Bear Gulch Limestone of Montana (USA), the relationships of the chondrichthyes, and comments on gnathostome evolution

ABSTRACT

Description of the chimaerid jaw and its phylogenetic origins

Anatomical delineation of the holocephalan palatoquadrate has proven to be difficult and, so, has been an extensively debated topic as it relates to the evolutionary derivation of jaws, modes of jaw suspension, and the interrelationships of the hondrichthyes (Elasmobranchii and Holocephali). Embryological analyses of the chimaerid jaw and cranium are presented to provide an anatomical description of the palatoquadrate in modern chimaerids. The palatoquadrate fuses, anteriorly, to the nasal capsule early in development. This marks the first point of contact between the mandibular arch and cranium. Orbitonasal canal foramina delineate the dorsal palatoquadrate margin. The posteriormost margin is marked by fusion of the upper jaw with trabecular and parachordal cartilages in the region of the efferent eudobranchial artery foramen and by a suborbitally positioned basitrabecular cartilage. This basitrabecula generates a subocular shelf as it fuses medially to the parachordal cartilage and posteriorly to the postorbital wall and cranial otic process.

Three new palaeoniscoid fishes from the Bear Gulch Limestone (Serpukhovian, Mississippian) of Montana (USA) and the relationships of lower actinopterygians

ABSTRACT Three new palaeoniscoid fishes (Osteichthyes, Actinopterygii), representing two new genera, Lineagruan judithi n. gen., n. sp., L. snowyi n. gen., n. sp., Beagiascus pulcherrimus n. gen., n. sp., are described from the Bear Gulch Limestone Member of the Heath Formation (Serpukhovian) of Montana, a 318 million year old lagerstätte. Morphological, morphometric, and meristic data were analyzed and compared to data for other Paleozoic actinopterygians. Differences among the species were noted in character complexes that may have played a role in feeding or propulsive regimes and fine-scale niche partitioning. A matrix of 111 characters and 40 taxa was constructed using relatively complete taxa ranging from the Devonian to the Recent. Cladistic analysis using Hennig86 and Winclada resulted in two trees. Branch-and-bound treatment generated one tree, in which the palaeoniscoids were paraphyletic. In all instances, the relationships of the three new species were consistent. It is noted that a number of the palaeoniscoid clades proposed by the current cladistic analysis have also been generated in earlier analyses of Bear Gulch fishes and other independent analyses. This recurring pattern implies some robustness to these associations in spite of variations between the matrices generating them.

A Basal Elasmobranch, Thrinacoselache gracia n. gen & sp., (Thrinacodontidae, New Family) from the Bear Gulch Limestone, Serpukhovian of Montana, USA

Abstract A new elasmobranch, Thrinacoselache gracia, is described from the Bear Gulch Limestone lens (Heath Formation; Sepukhovian) of Montana, with an elongate neurocranial region, occipital region that incorporates several separate vertebral zygal elements, extremely elongate, low body, and anatomically heterocercal, but horizontally oriented tail. Teeth are borne upon the rostrum, palatoquadrate, and mandible. The palatoquadrates articulate with both palatobasal and postorbital processes, as well as with the ethmoid; the hyomandibula articulates with the posterior upper corner of the neurocranium remote from the postorbital process. The mandible is longer than the neurocranium and meets its contralateral element in an extended, but mobile symphysis. Squamation is placoid, except for a dorsal pre-otic shield of elongate denticles. Digestive cavity contents identifiable as elasmobranch and holocephalan indicate that Thrinacoselache was predominantly a piscivore. The teeth are close to isolated teeth described as Thrinacodus, and particularly to the twisted, asymmetrical teeth of T. ferox Turner rather than the symmetrical teeth of the genotype, T. nanus. Tooth asymmetry and elongated preotic scale morphology in Thrinacoselache gracia are attributed to the strong positive allometry of the rostral region relative to the remainder of the neurocranium. The implications of distinctly different growth patterns between Thrinacodus and Thrinacoselache support the removal of T. ferox and its assignment to the new genus. The Thrinacodus-complex is removed from the Phoebodontidae and assigned to the new family Thrinacodontidae. Phylogenetic analysis indicates that Thrinacoselache and Diplodoselache occupy a basal position relative to other elasmobranchs for which holomorphological information is available.

Soft tissue pigments of the Upper Mississippian chondrenchelyid, Harpagofututor volsellorhinus (Chondrichthyes, Holocephali) from the Bear Gulch Limestone, Montana, USA

Reexamination of the chondrenchelyid Harpagofututor volsellorhinus from the Bear Gulch Limestone (Health Formation, Upper Chesterian, Mississippian), has revealed that the preserved fossil pigments are those originally associated with particular, well-vascularized abdominal organs (liver, spleen, and gonads) and major venous sinuses (orbital, gonadal, pelvic). The pattern of pigment localization reflects circulatory pathways of fossilized vessels. This determination was confirmed by comparison of fossil patterns to the visceral and circulatory anatomy of extant chondrichthyans. The arrangement of these pigments conveys strong, and otherwise unavailable, evidence for the internal reproductive features of these sexually mature, Paleozoic chondrichthyans. Under the appropriate preservational conditions the pigments also reveal asphyxia as the cause of death. Thus, the value of these pigments cannot be underestimated. Unfortunately, they are prone to spontaneous and progressive degradation that starts immediately upon excavation. Consequently, it is imperative to record data accurately and in a timely fashion. This report thus introduces the use of a color flatbed scanner as a particularly effective laboratory research tool for the collection and archiving of ephemeral fossil data.

Further consideration of the earliest known lamprey, Hardistiella montanensis Janvier and Lund, 1983, from the Carboniferous of Bear Gulch, Montana, U.S.A.

PHILIPPE JANVIER, RICHARD LUND and EILEEN D. GROGAN, UMR 8569 of CNRS, Muséum National d’Histoire Naturelle, 8 rue Buffon, 75005 Paris, France, and Honorary Research Fellow, Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom; Department of Vertebrate Paleontology, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, U.S.A.; Biology Department, St. Joseph’s University, 5600 City Avenue, Philadelphia, Pennsylvania 19131, U.S.A.

Pigment Patterns, Soft Anatomy and Relationships of Bear Gulch Chondrichthyes (Namurian E2b, Lower Carboniferous, Montana, USA)

Abstract Examination of pigment patterns and of skin remains of chondrichthyans from Bear Gulch; and comparisons with extant chimaerids permit assessment of their soft anatomy; physiologic states and relationships: such as presence of a soft operculum upon a hyoid arch of chimaeroid design (Heteropetalus elegantulus); jaw suspension; mode of death; anatomy of reproductive organs; sex and sexual maturity (Harpagofututor volsellorhinus). The evolutionary conservatism of the preserved vascular systems of these fossils add new information to systematic relationships. Cladistic analysis indicates that the position of H. elegantulus is pivotal whithin the basal complex of chondrichthyans and supports the clade and taxon Paraselachii.

Similarity of morphological composition and developmental patterning in paired fins of the elephant shark

Jawed vertebrates, or gnathostomes, have two sets of paired appendages, pectoral and pelvic fins in fishes and fore- and hindlimbs in tetrapods. As for paired limbs, paired fins are purported serial homologues, and the advent of pelvic fins has been hypothesized to have resulted from a duplication of the developmental mechanisms present in the pectoral fins, but re-iterated at a posterior location. Developmental similarity of gene expression between pectoral and pelvic fins has been documented in chondrichthyans, but a detailed morphological description of the progression of paired fin development for this group is still lacking. We studied paired fin development in an ontogenetic series of a phylogenetically basal chondrichthyan, the elephant shark Callorhinchus milii. A strong similarity in the morphology and progression of chondrification between the pectoral and pelvic fins was found, which could be interpretated as further evidence of serial homology in paired fins, that could have arisen by duplication. Furthermore, this high degree of morphological and developmental similarity suggests the presence of morphological and developmental modules within paired fins, as observed in paired limbs. This is the first time morphological and developmental modules are described for the paired fins of chimaeras.

Reactivity of Human White Blood Cells to Factors of Elasmobranch Origin

(leukocytes, WBC) obtained from different groups within the class Chondrichthyes and those derived from the structurally distinct immune system of the class Mammalia. The culture medium components of elasmobranch origin that are responsible for the conditioned nature of media exhibit similar effects upon peripheral blood leukocytes irrespective of vertebrate classification. This cross reactivity is considered to be discriminatory; reactivity is restricted to lymphocyte and monocyte-like cells. The cellular composition of the individual cultures was consistently similar among Carcharhinus, Rhizoprionodon, Dasyatis, and Homo specimens. Despite the phylogenetic distance between Chondrichthyes and Mammalia, similar patterns of peripheral white blood cell reactivity to molecules of elasmobranch origin suggests phyletic conservation of cell mediator production and regulatory activity. These observations also suggest that elasmobranchs, cartilaginous animals with an efficient immunosurveillance system, may be used to study the evolutionary and developmental potentials of the vertebrate immune system from a perspective not afforded by work on mammals or teleosts. Further investigation of the responsible culture system components should facilitate a cladistic analysis of immune systems and of the vertebrates.