Hans-Peter Schultze

Palatoquadrate and its ossifications: Development and homology within osteichthyans

The palatoquadrate and associated dermal bones have significant evolutionary transformations among teleostomes and provide numerous features that characterize teleostomian subgroups. The palatoquadrate forms the upper part of the mandibular arch and is present as a single cartilaginous element in the early ontogeny of teleostomes, except for some advanced teleosts such as siluroids where it is divided into pars autopalatina and pars pterygoquadrata. During ontogeny, the palatoquadrate may ossify as a unit, with a pars autopalatina (absent in Acanthodii), pars quadrata, and pars metapterygoidea in teleostomes (e.g., primitive acanthodians and actinopterygians, onychodonts, and rhipidistians). However, the palatoquadrate may remain cartilaginous (e.g., chondrosteans) or it may ossify as separate elements (e.g., autopalatine, metapterygoid, and quadrate) as occurs in advanced acanthodians, Polypterus and advanced actinopterygians, and advanced actinistians. From the single‐unit pattern, separate autopalatine, metapterygoid, and quadrate evolve in parallel in the three teleostomian subgroups. Therefore, it is necessary to distinguish between actinopterygian and actinistian autopalatines and among acanthodian, actinopterygian, and actinistian metapterygoids and quadrates. A palatoquadrate fused with the neurocranium occurs in parallel in dipnoans.

The urohyal : development and homology within osteichthyans

The formation of the unpaired structure ventral to the basibranchial region, the so‐called urohyal, differs within osteichthyans. A cartilaginous preformed, unpaired “urohyal” is present in sarcopterygians. A three‐tendon ossification is present in Polypterus. An “urohyal” or urohyal is absent in both Amia and Lepisosteus. The urohyal formed as an unpaired ossification of the tendon of the sternohyoideus muscle is a feature of teleosts. A new structure, the parurohyal, arises as a double ossification of the tendon of the sternohyoideus muscle in siluroids; during ontogeny an anterodorsal crest or cup‐like structure derives from the anterior basibranchial region and the tendon bone; therefore, the parurohyal is compound in origin. Judging from their formation and their distribution within osteichthyans the cartilaginous preformed “urohyal” and the teleostean urohyal are nonhomologous, whereas the urohyal and parurohyal are homologous. The urohyal is connected by ligaments with the ventral hypohyals in most teleosts, whereas it articulates with the ventral hypohyals in teleosts such as Anguilla and Chanos. The parurohyal is a synapomorphy of siluroids. The parurohyal in siluroids is articulated with both ventral and dorsal hypohyals, and with the basibranchial region in catfishes such as diplomystids and ictalurids, whereas it articulates only with the ventral hypohyals in other catfishes such as trichomycterines. The passage of the hypobranchial artery through the hypobranchial foramen of the parurohyal is a unique feature of siluroids, like the absence of the basihyal bone.

Ausgangsform und Entwicklung der rhombischen Schuppen der Osteichthyes (Pisces)

Ganoid and cosmoid scales, the two types of rhombic scales within the osteichthyans, can be traced back to a primitive scale similar to the scales ofLophosteus. The primitive rhombic scale did not have a peg-and-socket articulation, it is composed of lamellar bone superposed by many layers of spongy bone + dentine. That kind of superposition of layers of spongy bone + dentine (+ enamel) has been retained in the cosmoid scale; in contrast in the ganoid scale the growth of the dentine has become restricted to the lateral surface, the growth of ganoin to the outer surface and the growth of bone to the inner surface of the scale. The scales ofAndreolepis have a position between the primitive rhombic scale and the ganoid scale. — Scales from the Gedinnian (Lower Devonian) of New Sibirian Islands, USSR, are described asDialipina markae n. sp. The morphological features are very typical for the genus, but the histology is different from the type speciesD. salgueiroensis. Within the two Devonian palaeoniscoid generaDialipina andOrvikuina acellular bone with irregular non-vascular canals of Williamson has developed twice from cellular bone.ZusammenfassungGanoid- und Cosmoidschuppe, die beiden rhombischen Schuppen-formen der Osteichthyes, sind auf eine gemeinsame ursprüngliche Form zurückfiihrbar, der die Schuppen vonLophosteus nahestehen. Die ursprüngliche, rhombische Schuppe besaß keine Dorn-Grube-Artikulation, sie war aufgebaut aus lamellärem Knochen und mehreren Lagen aus spon-giösem Knochen + Dentin darüber. In der Ganoidschuppe ist dieser mehrlagige Bau verschwunden, während er in der Cosmoidschuppe beibehalten wird.Andreolepis steht in der Entwicklungsrichtung auf die Ganoidschuppe hin. — Schuppen aus dem Gedinne, Unter-Devon, der Neusibirischen Inseln, USSR, werden aufgrund der morphologischen Merkmale der GattungDialipina alsD. markae n. sp. zugerechnet, obwohl der histologische Bau sich von dem der Typ-artD. salgueiroensis unterscheidet. Eine Parallelentwicklung von zellulärem zu azellulärem Knochen liegt in den beiden Palaeoniscoidea-GattungenDialipina undOrvikuina vor.

Reevaluation of the caudal skeleton of certain actinopterygian fishes: III. Salmonidae. Homologization of caudal skeletal structures

The ontogenetic development of caudal vertebrae and associated skeletal elements of salmonids provides information about sequence of ossification and origin of bones that can be considered as a model for other teleosts. The ossification of elements forming the caudal skeleton follows the same sequence, independent of size and age at first appearance. Dermal bones like principal caudal rays ossify earlier than chondral bones; among dermal bones, the middle principal caudal rays ossify before the ventral and dorsal ones. Among chondral bones, the ventral hypural 1 and parhypural ossify first, followed by hypural 2 and by the ventral spine of preural centrum 2. The ossification of the dorsal chondral elements starts later than that of ventral ones. Three elements participate in the formation of a caudal vertebra: paired basidorsal and basiventral arcocentra, chordacentrum, and autocentrum; appearance of cartilaginous arcocentra precedes that of the mineralized basiventral chordacentrum, and that of the perichordal ossification of the autocentrum. Each ural centrum is mainly formed by arcocentral and chordacentrum. The autocentrum is irregularly present or absent. Some ural centra are formed only by a chordacentrum. This pattern of vertebral formation characterizes basal teleosts and primitive extant teleosts such as elopomorphs, osteoglossomorphs, and salmonids.

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Reevaluation of the caudal skeleton of some actinopterygian fishes: II. Hiodon, Elops, and Albula

The vertebral centra of Hiodon, Elops, and Albula are direct perichordal ossifications (autocentra) which enclose the arcocentra as in Amia. An inner ring of ovoid cells forms in late ontogeny from the intervertebral space inside the autocentrum. The chordacentrum is reduced or completely absent in centra of adult Elops, whereas it forms an important portion of the centra in adult Hiodon. The posterior portion of the compound ural centrum 3+4+5 is partially (Hiodon) or fully formed by the chordacentrum (Elops, Albula). The haemal arches and hypurals are fused medially by cartilage or bone trabecles of the arcocentrum with the centra, even though they appear autogenous in lateral view in Elops and Albula. The composition of the caudal skeleton of fossil teleosts and the ontogeny of that of Hiodon, Elops, and Albula corroborate a one‐to‐one relationship of ural centra with these dorsal and ventral elements. The first epural (epural 1) of Elops relates to ural centrum 1, whereas the first epural (epural 2) of Hiodon and Albula relates to ural centrum 2. In Albula, the first ural centrum is formed by ural centrum 2 only. With 4 uroneurals Hiodon has the highest number within recent teleosts. Juvenile specimens of Hiodon have eight, the highest number of hypurals within recent teleosts; this is the primitive condition by comparison with other teleosts and pholidophorids. Reduction of elements in the caudal skeleton is an advanced feature as seen within elopomorphs from Elops to Albula. Such reductions and fusions occur in osteoglossomorphs also, but the lack of epurals and uroneurals separates most osteoglossomorphs (except Hiodon) from all other teleosts.

False teeth: conodont-vertebrate phylogenetic relationships revisited

ABSTRACT An evidence-based reassessment of the phylogenetic relationships of conodonts shows that they are not “stem” gnathostomes, nor vertebrates, and not even craniates. A significant group of conodont workers have proposed or accepted a craniate designation for the conodont animal, an interpretation that is increasingly becoming established as accepted “fact”. Against this prevailing trend, our conclusion is based on a revised analysis of traditional morphological features of both discrete conodont elements and apparatuses, histological investigation and a revised cladistic analysis modifying that used in the keystone publication promoted as proof of the hypothesis that conodonts are vertebrates. Our study suggests that conodonts possibly were not even chordates but demonstration of this is beyond the scope of this paper. To summarize, in conodonts there is low cephalization; presence of simple V-shaped trunk musculature and unique large-crystal albid material in the elements; lack of a dermal skeleton including characteristic vertebrate hard tissues of bone, dentine and enamel; lack of odontodes with bone of attachment and a unique pulp system; lack of segmentally-arranged paraxial elements and dermal elements in median fins, all of which supports neither a vertebrate nor a craniate relationship for conodonts.

Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (osteolepiform rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada

ABSTRACT A size series of thirty-five specimens of Eusthenopteron foordi Whiteaves, 1881, shows isometric and allometric changes. As in Recent fishes, the main difference between small (juvenile) and large (adult) specimens is the relative size of the orbit and of the head. The large relative decrease of the orbit diameter from 31% to 12–7% of head length (b = 0.632) explains the positive allometric growth of bones surrounding the orbit, i.e., the change from a palaeoniscoid-like juvenile to a typical rhipidistian with an elongated postorbital region. With the exception of the caudal prolongation, all fin positions remain isometric to standard length.

Reevaluation of the caudal skeleton of actinopterygian fishes: I. Lepisosteus and Amia

The centra of Lepisosteus are perichondral ossifications of arcualia (i.e., arcocentra), whereas those of Amia are direct perichordal ossifications (i.e., autocentra) that enclose the arcualia. The preural centra of Lepisosteus are monospondylous, whereas the ural centra are formations of inter‐ and basidorsal arcualia. In contrast, the preural centra of Amia are diplospondylous, whereas preural centrum 1 (and sometimes preural centrum 2) and ural centra are monospondylous. The ural centra of Lepisosteus are expansions of dorsal arcualia, but those of Amia are expansions of the basiventral autocentrum. This explains the fusion of the neural arches with the ural centra and the presence of autogenous hypurals in Lepisosteus, in contrast to the situation in Amia in which the compound ural neural arch (the fused ural neural arches) is free, and the hypurals are fused to the ural centra. Lepisosteus possesses true epurals, which are modified neural spines, whereas in Amia the “epurals” are positioned between the neural spines like radials. Lepisosteus and Amia possess a polyural caudal skeleton with a one‐to‐one relationship between ural centra and hypurals; the number of hypurals may be reduced in adult Lepisosteus.

Relative importance of molecular, neontological, and paleontological data in understanding the biology of the vertebrate invasion of land.

SummaryMeyer and Wilsons (1990) 12S rRNA phylogeny unites lungfish and tetrapods to the exclusion of the coelacanth. These workers also provide a list of morphological features shared in common between modern lungfish and tetrapods, and they conclude that these traits were probably present in their last common ancestor. However, the exquisite fossil records of the abundant extinct lungfishes and rhipidistians show that at least 13 out of Meyer and Wilsons 14 supposed ancestral traits were not present in the last common ancestor of lungfishes and tetrapods. Using extant taxa to infer ancestral morphologies is fraught with difficulties; just like molecular sequences, ancestral character states of morphological traits may be severely overprinted by subsequent modifications. Modern lungfish are air-breathing nonmarine forms, yet their Devonian forebears were marine fish that did not breathe air. Fossils dating from the time of origin of tetrapods in the Devonian offer the only hope of understanding the morphological innovations that led to tetrapods; morphological analysis of the “living fossils,” the coelacanth and lungfish, only lends confusion.