Richard Lund

Hardistiella montanensis n. gen. et sp. (Petromyzontida) from the Lower Carboniferous of Montana, with remarks on the affinities of the lampreys

ABSTRACT Hardistiella montanensis n. gen. et sp. from the Lower Carboniferous (Namurian) Bear Gulch Limestone Member of the Heath Formation, Montana, is the only lamprey known to retain a clearly hypocercal tail and an anal fin provided with rays. These two characters are plesiomorphous and are also met with in “anaspids” and “thelodonts.” An analysis of the relationships of the petromyzontids indicates that they are the sister group of some of the “anaspids.”

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

Chondrichthyan life history styles as revealed by the 320 million years old Mississippian of Montana

SynopsisChondrichthyans comprise 55 of the 94 vertebrate species found to date in the Mississippian marine Bear Gulch Limestone 6 km by 19 km tropical bay. The chondrichthyan-osteichthyan faunal proportions differ strongly from those of modern marine fish faunas. Secondary sexual dimorphism, size partitioning, life period segregation, and sexual segregation, reinforce the considerable morphological specializations among the chondrichthyans that were involved in the maintenance of this diversity. The large number of rare species suggests that many species are represented only by occasional recruitment, reflecting differential life history styles as well as habitat limitations for these species. Repeated fluctuations in environmental parameters through time, producing fine-scale spatial and temporal non-equilibrium, were probably ultimately responsible for the maintenance of this broad-scale high diversity community.

Viviparity and Intrauterine Feeding in a New Holocephalan Fish from the Lower Carboniferous of Montana

A new species of Lower Carboniferous holocephalan chondrichthyan, Delphyodontos dacriformes, is described from two fetal specimens. The well-developed slashing and piercing dentition, enlargement of the abdominal region, and fecal material indicate the probable evolution of intrauterine oophagy and viviparity in Paleozoic Chondrichthyes.

The morphology of Falcatus falcatus (St. John and Worthen), a Mississippian stethacanthid chondrichthyan from the Bear Gulch Limestone of Montana

ABSTRACT The Paleozoic fin spine Physonemus falcatus St. John and Worthen 1883, from the Valmeyeran St. Louis Limestone of St. Louis, Missouri, has been found on sexually mature males of a small, highly sexually dimorphic chondrichthyan from the Chesterian Bear Gulch Limestone of Montana. An amphistylic suspensorium, cladodont teeth and pectoral fin further characterize this species and the group to which it belongs. The first dorsal spine and fin of males are derivable from a form like that of Stethacanthus altonensis. Males also have elongate rostra and dorsal cranial and fin denticles, while females and juveniles lack denticles, first dorsal fin, or spine. The new genus Falcatus (Order Cladodontida, Family Stethacanthidae) is erected for the species and for the related F. hamatus and F. proclivus. Aspects of the sexual dimorphism are discussed; Falcatus falcatus, with many more males than females, fits a behavioral model of male display-courtship and female choice. Possible evidence of lek aggregations...

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.

The rhadinichthyids (paleoniscoid actinopterygians) from the Bear Gulch Limestone of Montana (USA, Lower Carboniferous)

ABSTRACT Three new fusiform Namurian paleoniscoids are described: Wendyichthys dicksoni, gen. et sp. nov., Wendyichthys lautreci, sp. nov. and Cyranorhis bergeraci, gen. et sp. nov. Their most unique features consist of their snout anatomy (with a rostropostrostral, reverse-L shaped antorbitals, a rostral notch, and the lack of premaxillaries) and the presence of extralateral gulars in addition to normal median and lateral gulars. These supplementary gulars and the rostral notch were probably mechanical adaptations for a greater widening of the mouth cavity during abduction of the mandible: as a result these fishes probably fed by suction, either at the bottom of the water column or by intake of floating nutritive particles. Prior to a phylogenetic analysis of the relationships of these and other Bear Gulch actinopterygians, these three new taxa are placed into the Rhadinichthyidae. A revised definition of this family, including new features recognized during this study, is given after a reappraisal of th...

TWO NEW DEEP-BODIED PALAEONISCOID ACTINOPTERYGIANS FROM BEAR GULCH (MONTANA, USA, LOWER CARBONIFEROUS)

Abstract Two new species of deep-bodied Paleozoic actinopterygians, Aesopichthys erinaceus gen. nov., sp. nov. and Proceramala montanensis gen. nov., sp. nov., are described from the Upper Chesterian, Upper Mississippian (Namurian E2b, Lower Carboniferous) Bear Gulch Limestone of Montana, USA, and the new family Aesopichthyidae is erected for them. These two fishes share features usually associated with deep body and deep head specializations, particularly in Palaeoniscimorpha, such as a shortened gape, tall maxilla, vertical suspensorium, deepened flank scales and an elongate dorsal fin. Aesopichthys in particular, for which more complete information is known, also possessed a suite of very specialized adaptations in feeding mechanism, ganoine sculpturing, fin form, and cranial defensive structures that have strong ethological implications. These features include small mobile premaxillae, a spinous and apparently rotatable posteroventral infraorbital, lobed pectoral base, and partially webbed pectoral, dorsal, anal and caudal fins. Within a general scheme of interrelationships of primitive actinopterygians, these two new taxa pertain to Palaeoniscimorpha, i.e., basal Actinopteri. Then, from a cladistic analysis limited, beside these two new taxa, to Wendyichthys and Cyranorhis (two other already described actinopterygians from Bear Gulch), Platysomus, and Cheirolepis, it appears that the Aesopichthyidae display no close relationship to the Platysomidae but should be the sister group of the Rhadinichthyid-group i.e., [Cyranorhis + [Rhadinichthyidae + Wendyichthys]].

On Damocles serratus, nov. gen. et sp. (Elasmobranchii: Cladodontida) from the Upper Mississippian Bear Gulch Limestone of Montana

ABSTRACT An ontogenetic series of spines smaller than but very similar to the form taxon Physonemus attenuatus Davis have been found to be anterior dorsal spines of a cladodont chondrichthyan. The specimens, from the Upper Mississippian (Chesterian) Bear Gulch Limestone of Montana, are closest to the Stethacanthidae although they lack a modified dorsal fin following the spine and have prominent metapterygia in the pelvic fins. The new genus and species Damocles serratus is erected for them and they are provisionally included within the Stethacanthidae together with the referred species Damocles attenuatus (Davis).