LOPHOSTEUS SUPERBUS PANDER, EIN TELEOSTOME AUS DEM SILUR OESELS

Numerous scales as well as fragments of spines and bones of Lophosteus superbus Pander 1856, a fish which has not been re-studied since 1893, were found in a rock specimen of Ohesaare Beds (marine Downtonian) from the Isle of Oesel (Estonia). The various remains have been studied morphologically and histologically. The structure of the remains demonstrates the close relationship of L. superbus with Andreolepis hedei Gross 1968. Both genera have been placed in the new family Lophosteidae and the new order Lophosteiformes. The position of this order within the Teleostomi will remain unclear until connected remains or intact fish specimens are found. The scales of both genera indicate a possible connection with the Actinopterygii. In addition, the morphology and histology of some bone fragments covered with small teeth and tubercles are described. These fragments may also belong to L. superbus.
Schuppen und Bruchstücke von Stacheln und Knochen des seit 1893 nicht mehr untersuchten Fisches Lophosteus superbus Pander 1856 wurden zahlreich in einem Handstück der Ohesaare Schichten (marines Downton) der Insel Oesel (Estland) gefunden. Die verschiedenen Reste wurden morphologisch und histologisch untersucht. Intakte Schuppen und der histologische Bau der Reste erwiesen die nahe Verwandschaft von L. superbus mit Andreolepis hedei Gross 1968. Beide Gattungen wurden in der neuen Familie der Lophosteidae und der neuen Ordnung Lophosteiformes vereinigt, deren Stellung innerhalb der Teleostomi (Osteichthyes) unklar bleibt, solange keine zusammenhängenden Fischreste oder intakte Fische gefunden werden. Die Schuppen beider Gattungen weisen auf mögliche Beziehungen zu den Actinopterygiern. – Angefügt wird die morphologische und histologische Beschreibung einiger Knochenreste, die mit zahlreichen konischen Zähnchen und Tuberkeln pflasterförmig besetzt sind. Vielleicht gehören auch diese Reste zu L. superbus.     

Histology of “placoderm” dermal skeletons: Implications for the nature of the ancestral gnathostome

The vertebrate dermal skeleton has long been interpreted to have evolved from a primitive condition exemplified by chondrichthyans. However, chondrichthyans and osteichthyans evolved from an ancestral gnathostome stem‐lineage in which the dermal skeleton was more extensively developed. To elucidate the histology and skeletal structure of the gnathostome crown‐ancestor we conducted a histological survey of the diversity of the dermal skeleton among the placoderms, a diverse clade or grade of early jawed vertebrates. The dermal skeleton of all placoderms is composed largely of a cancellar architecture of cellular dermal bone, surmounted by dermal tubercles in the most ancestral clades, including antiarchs. Acanthothoracids retain an ancestral condition for the dermal skeleton, and we record its secondary reduction in antiarchs. We also find that mechanisms for remodeling bone and facilitating different growth rates between adjoining plates are widespread throughout the placoderms. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.     

Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms

The fossil group Placodermi is the most phylogenetically basal of the clade of jawed vertebrates but lacks a marginal dentition comparable to that of the dentate Chondrichthyes, Acanthodii and Osteichthyes (crown-group Gnathostomata). The teeth of crown-group gnathostomes are part of an ordered dentition replaced from, and patterned by, a dental lamina, exemplified by the elasmobranch model. A dentition recognised by these criteria has been previously judged absent in placoderms, based on structural evidence such as absence of tooth whorls and typical vertebrate dentine. However, evidence for regulated tooth addition in a precise spatiotemporal order can be observed in placoderms, but significantly, only within the group Arthrodira. In these fossils, as in other jawed vertebrates with statodont, non-replacing dentitions, new teeth are added at the ends of rows below the bite, but in line with biting edges of the dentition. The pattern is different on each gnathal bone and probably arises from single odontogenic primordia on each, but tooth rows are arranged in a distinctive placoderm pattern. New teeth are made of regular dentine comparable to that of crown-gnathostomes, formed from a pulp cavity. This differs from semidentine previously described for placoderm gnathalia, a type present in the external dermal tubercles. The Arthrodira is a derived taxon within the Placodermi, hence origin of teeth in placoderms occurs late in the phylogeny and teeth are convergently derived, relative to those of other jawed vertebrates. More basal placoderm taxa adopted other strategies for providing biting surfaces and these vary substantially, but include addition of denticles to the growing gnathal plates, at the margins of pre-existing denticle patches. These alternative strategies and apparent absence of regular dentine have led to previous interpretations that teeth were entirely absent from the placoderm dentition. A consensus view emerged that a dentition, as developed within a dental lamina, is a synapomorphy characterising the clade of crown-group gnathostomes. Recent comparisons between sets of denticle whorls in the pharyngeal region of the jawless fish Loganellia scotica (Thelodonti) and those in sharks suggest homology of these denticle sets on gill arches. Although the placoderm pharyngeal region appears to lack denticles (placoderm gill arches are poorly known), the posterior wall of the pharyngeal cavity, formed by a bony flange termed the postbranchial lamina, is covered in rows of patterned denticle arrays. These arrays differ significantly, both in morphology and arrangement, from those of the denticles located externally on the head and trunkshield plates. Denticles in these arrays are homologous to denticles associated with the gill arches in other crown-gnathostomes, with pattern similarities for order and position of pharyngeal denticles. From their location in the pharynx these are inferred to be under the influence of a cell lineage from endoderm, rather than ectoderm. Tooth sets and tooth whorls in crown-group gnathostomes are suggested to derive from the pharyngeal denticle whorls, at least in sharks, with the patterning mechanisms co-opted to the oral cavity. A comparable co-option is suggested for the Placodermi.     

Microsaurs as possible apodan ancestors

The specific ancestry and nature of the relationships of modern amphibians have not yet been established. Detailed comparisons of the anatomy of the skull roof, palate and braincase of living apodans and the Paleozoic microsaur Goniorhynchus demonstrate greater similarities than between apodans and any other group of amphibians, fossil or recent. Unlike any other amphibians, extensive pleurosphenoid ossifications are developed in the area of the Vth nerve, uniting the otic capsule with the sphenethmoid. Other important features that they share (although not uniquely) include the presence of all the primitive dermal elements of the palate, a solidly roofed temporal region, a row of palatal teeth parallel to the marginal dentition and a row of teeth on the medial surface of the lower jaw. The stapes has a similar configuration and position, totally different from that of frogs and salamanders. Such similarities do not necessarily prove close relationship, but indicate the necessity for considering that apodans may have an ancestry distinct from that of frogs and salamanders.     

Fossil fishes from china provide first evidence of dermal pelvic girdles in osteichthyans

Background

The pectoral and pelvic girdles support paired fins and limbs, and have transformed significantly in the diversification of gnathostomes or jawed vertebrates (including osteichthyans, chondrichthyans, acanthodians and placoderms). For instance, changes in the pectoral and pelvic girdles accompanied the transition of fins to limbs as some osteichthyans (a clade that contains the vast majority of vertebrates – bony fishes and tetrapods) ventured from aquatic to terrestrial environments. The fossil record shows that the pectoral girdles of early osteichthyans (e.g., Lophosteus, Andreolepis, Psarolepis and Guiyu) retained part of the primitive gnathostome pectoral girdle condition with spines and/or other dermal components. However, very little is known about the condition of the pelvic girdle in the earliest osteichthyans. Living osteichthyans, like chondrichthyans (cartilaginous fishes), have exclusively endoskeletal pelvic girdles, while dermal pelvic girdle components (plates and/or spines) have so far been found only in some extinct placoderms and acanthodians. Consequently, whether the pectoral and pelvic girdles are primitively similar in osteichthyans cannot be adequately evaluated, and phylogeny-based inferences regarding the primitive pelvic girdle condition in osteichthyans cannot be tested against available fossil evidence.

Methodology/Principal Findings

Here we report the first discovery of spine-bearing dermal pelvic girdles in early osteichthyans, based on a new articulated specimen of Guiyu oneiros from the Late Ludlow (Silurian) Kuanti Formation, Yunnan, as well as a re-examination of the previously described holotype. We also describe disarticulated pelvic girdles of Psarolepis romeri from the Lochkovian (Early Devonian) Xitun Formation, Yunnan, which resemble the previously reported pectoral girdles in having integrated dermal and endoskeletal components with polybasal fin articulation.

Conclusions/Significance

The new findings reveal hitherto unknown similarity in pectoral and pelvic girdles among early osteichthyans, and provide critical information for studying the evolution of pelvic girdles in osteichthyans and other gnathostomes.     

The relationships of Uronemus: a carboniferous dipnoan with highly modified tooth plates

Previous accounts of the dentition of the Carboniferous dipnoan Uronemus have stressed the significance of the scattered small denticles. These, together with the marginal teeth and ridges, have been interpreted as primitive characters of the dipnoan dentition shared with three other genera: the Devonian Uranlophus and Griphognathus and the Carboniferous to Permian Conchopoma. Genera with tooth plates have been considered to be a monophyletic group in which tooth plates are a derived character; Uronemus has been excluded from this group in all previous investigations dealing with the significance of the dentition for determining relationships among dipnoans. The macromorphology of the dentition of Uronemus has been re-examined and correlated with the histology of all the dental tissues. Optical study of thin sections and scanning electron microscope study of the adjacent cut surfaces has shown that the hard, wear-resistant dentine of the teeth and ridges is petrodentine. The arrangement, growth, wear and histology of the dental tissues have been compared with those of denticulated and tooth-plated genera. The arrangement of new teeth relative to the tooth ridge, the pattern of wear along the ridge, and the type of dentine and its growth indicate that the dentition of Uronemus is best interpreted as a tooth plate with one long lingual tooth ridge and reduced lateral tooth rows. Therefore the marginal tooth ridges are not considered to be homologous with those of denticulate dipnoans such as Uranolophus. The presence of petrodentine, a tissue type only found in forms with tooth plates, is consistent with the view that the dentition is derived by modification of a radiate tooth plate. The denticles covering restricted regions of the palate and lower jaw are considered to have been a secondary acquisition. The suggestion that Conchopoma is a close relative of Uronemus is not accepted, and possible new relationships have been proposed. New data on Scaumenacia and Phaneropleuron, two other genera previously compared with Uronemus, are presented. Rhinodipterus, a form with elongate lingual ridges, is also discussed. Phaneropleuron is shown to have radiate tooth plates and not a marginal row of conical teeth as previously described. It is proposed that the tooth plate of Uronemus is derived from a dipterid type of plate. A discussion of some of the other factors involved in determining the relationships of the genus is given.(ABSTRACT TRUNCATED AT 400 WORDS)     

The tessellated pattern of dermal armour in the Heterostraci

The earliest and most primitive heterostraeans possessed a tessellated carapace. Isolated dentine tubercles scattered in the skin formed primordia around which concentric rings of further tubercles were laid down. This pattern of growth produced a characteristic terrazzo. From this stage the gradual elimination of tesserae can be traced in several groups. Beginning with areally growing or cyclomorial tesserae, the individual units appear simultaneously or synchronomorially, thereafter they become fused into a system of large discrete plates. Finally these synehroriomorial units appear earlier and earlier in ontogeny with progressively wider zones of cyclomorial growth being added on to them. Thus a pattern of cyclomorial plates is eventually produced.
In the psammosteids there was a redevelopment of tesserae so that the latest stages were comparable to the very early stages, although the tesserae were developed on an already existing pattern of large plates.
The possible origin of the tessellated pat tern of dermal armour is discussed. The apparent macular nature of dermal structures is not considered to be an inherent property of skin but instead due to simple physico-chemical factors.     

Development and Evolution of Dentition Pattern and Tooth Order in the Skates And Rays (Batoidea; Chondrichthyes)

Shark and ray (elasmobranch) dentitions are well known for their multiple generations of teeth, with isolated teeth being common in the fossil record. However, how the diverse dentitions characteristic of elasmobranchs form is still poorly understood. Data on the development and maintenance of the dental patterning in this major vertebrate group will allow comparisons to other morphologically diverse taxa, including the bony fishes, in order to identify shared pattern characters for the vertebrate dentition as a whole. Data is especially lacking from the Batoidea (skates and rays), hence our objective is to compile data on embryonic and adult batoid tooth development contributing to ordering of the dentition, from cleared and stained specimens and micro-CT scans, with 3D rendered models. We selected species (adult and embryonic) spanning phylogenetically significant batoid clades, such that our observations may raise questions about relationships within the batoids, particularly with respect to current molecular-based analyses. We include developmental data from embryos of recent model organisms Leucoraja erinacea and Raja clavata to evaluate the earliest establishment of the dentition. Characters of the batoid dentition investigated include alternate addition of teeth as offset successional tooth rows (versus single separate files), presence of a symphyseal initiator region (symphyseal tooth present, or absent, but with two parasymphyseal teeth) and a restriction to tooth addition along each jaw reducing the number of tooth families, relative to addition of successor teeth within each family. Our ultimate aim is to understand the shared characters of the batoids, and whether or not these dental characters are shared more broadly within elasmobranchs, by comparing these to dentitions in shark outgroups. These developmental morphological analyses will provide a solid basis to better understand dental evolution in these important vertebrate groups as well as the general plesiomorphic vertebrate dental condition.     

Scales and Tooth Whorls of Ancient Fishes Challenge Distinction between External and Oral ‘Teeth’

The debate about the origin of the vertebrate dentition has been given fresh fuel by new fossil discoveries and developmental studies of extant animals. Odontodes (teeth or tooth-like structures) can be found in two distinct regions, the ‘internal’ oropharyngeal cavity and the ‘external’ skin. A recent hypothesis argues that regularly patterned odontodes is a specific oropharyngeal feature, whereas odontodes in the external skeleton lack this organization. However, this argument relies on the skeletal system of modern chondrichthyans (sharks and their relatives), which differ from other gnathostome (jawed vertebrate) groups in not having dermal bones associated with the odontodes. Their external skeleton is also composed of monoodontode ''placoid scales'', whereas the scales of most early fossil gnathostomes are polyodontode, i.e. constructed from several odontodes on a shared bony base. Propagation phase contrast X-ray Synchrotron microtomography (PPC-SRµCT) is used to study the polyodontode scales of the early bony fish Andreolepis hedei. The odontodes constructing a single scale are reconstructed in 3D, and a linear and regular growth mechanism similar to that in a gnathostome dentition is confirmed, together with a second, gap-filling growth mechanism. Acanthodian tooth whorls are described, which show that ossification of the whorl base preceded and probably patterned the development of the dental lamina, in contrast to the condition in sharks where the dental lamina develops early and patterns the dentition.The new findings reveal, for the first time, how polyodontode scales grow in 3D in an extinct bony fish. They show that dentition-like odontode patterning occurs on scales and that the primary patterning unit of a tooth whorl may be the bony base rather than the odontodes it carries. These results contradict the hypothesis that oropharyngeal and external odontode skeletons are fundamentally separate and suggest that the importance of dermal bone interactions to odontode patterning has been underestimated.     

Conserved deployment of genes during odontogenesis across osteichthyans

Odontogenesis has only been closely scrutinized at the molecular level in the mouse, an animal with an extremely restricted dentition of only two types and one set. However, within osteichthyans many species display complex and extensive dentitions, which questions the extent to which information from the mouse is applicable to all osteichthyans. We present novel comparative molecular and morphological data in the rainbow trout (Oncorhynchus mykiss) that show that three genes, essential for murine odontogenesis, follow identical spatial-temporal expression. Thus, at all tooth bud sites, epithelial genes Pitx-2 and Shh initiate the odontogenic cascade, resulting in dental mesenchymal Bmp-4 expression, importantly, including the previously unknown formation of replacement teeth. Significantly, this spatial-temporal sequence is the same for marginal and lingual dentitions, but we find notable differences regarding the deployment of Pitx-2 in the developing pharyngeal dentition. This difference may be highly significant in relation to the theory that dentitions may have evolved from pharyngeal tooth sets in jawless fishes. We have provided the first data on operational genes in tooth development to show that the same signalling genes choreograph this evolutionary stable event in fishes since the osteichthyan divergence 420 Myr ago, with the identical spatial-temporal expression as in mammals.     

Variability in the number of tooth rows in the pharyngeal dentition of Barbus intermedius (Teleostei; Cyprinidae): genetic,hormonal and environmental factors

The variability of pharyngeal dentition in a natural population of B. intermedius and effects of genetic, hormonal and environmental factors on the number of tooth rows in the pharyngeal dentition in offspring from wild‐caught parents have been investigated. It was revealed that: (i) about 10% of fish from natural population have four‐rowed dentition instead of three‐rowed dentition characteristic for this species; (ii) the presence of the additional tooth row is not an abnormality of tooth replacement since it occurs symmetrically on both sides; (iii) occurrence of the fourth row of teeth is heritable since laboratory‐reared offspring from parents with four‐rowed dentition have the same dentition. Even if one of the parents had four‐rowed dentition the percentage of four‐rowed individuals in the progeny was significantly higher than in progeny from parents with normal (three rowed) dentition; (iv) the number of tooth rows appears to be hormonally controlled: high levels of thyroid hormone result in a decrease in the number of tooth rows to two. In contrast, deficiency of this hormone results in an increase to four rows; (v) no differences in the number of tooth rows were found in fish reared under 17°C, 24°C, 30°C and room temperature (20–26°C).     

Developmental mechanisms underlying tooth patterning in continuously replacing osteichthyan dentitions

The dentition of osteichthyans presents an astonishing diversity with regard to the distribution of teeth in the oral cavity, tooth numbers, arrangements, shapes, and sizes. Taking examples from three unrelated teleosts--the most speciose group of osteichthyans--and from the literature, this study explores how the initial tooth pattern is set up, and how this relates to the establishment and maintenance (or modification) of the tooth replacement pattern. In teleosts, first-generation teeth (the very first teeth in ontogeny to develop at a particular locus) are commonly initiated in adjacent or in alternate (odd and even) positions. The mechanisms responsible for these divergent developmental patterns remain to be elucidated, in particular, whether they reflect a field or local type of control. However, patterns of adjacent or alternate tooth initiation, set up by the first-generation teeth, can easily turn into replacement patterns where new teeth are initiated simultaneously every second, or even every third position, by synchronizing the formation of new first-generation teeth to the formation of replacement teeth at older loci. Our observations suggest that, once established, the replacement pattern appears to be maintained, as a kind of "default" state. Variations and modifications in this pattern are nevertheless common and suggest that tooth replacement is under local control, exerted at the level of the initiation of replacement teeth. Further studies are needed to test the hypothesis that regular replacement patterns are more frequent in association with the plesiomorphic condition of extramedullary replacement (replacement on the surface of the dentigerous bone) and more rare in the derived condition of intramedullary replacement (replacement within the medullary cavity of the dentigerous bone).     

LOPHOSTEUS SUPERBUS PANDER: ZÄHNE, ZAHNKNOCHEN UND BESONDERE SCHUPPENFORMEN

Morphological and histological studies of numerous remains of Lophosteus superbus Pander from erratic boulders of Beyrichia Limestone prove that the peculiar teeth recently brought to notice by Gross (1969) belong to Lophosteus and are not of acanthodian origin. The structure of these teeth as well as that of some tooth-bearing bones and scales is described. The investigation was extended to cover the teeth of smaller Devonian actinopterygians and cross-opterygians to establish whether Lophosteus is an actinopterygian, a crossopterygian, or even a member of a hitherto unknown group of Osteichthyes. Results unfortunately proved negative, as the very small teeth of various species offer too few distinct and symptomatic histological characteristics. A solution of this question must await the discovery of determinable skull and lower jaw bones of the genus Lophosteus.
Die morphologische und histologische LTntersuchung zahlreicher in nordwest-deutschen Geschieben des Beyrichienkalkes gefundener Reste von Lophosteus superbus Pander beweist, daß die kürzlich von Gross (1969) erwähnten, eigenartigen und nicht von Acanthodiern stammenden Zähne zu Lophosteus gehören. Der Bau der Zähne und einiger Zahnknochen und Schuppen wird beschrieben. Um die Fragen zu klaren, ob Lophosteus zu den Actinopterygiern oder zu den Crossopterygiern gehört oder gar zu einer bisher unbekannten Gruppe der Osteichthyes, wurde die Untersuchung auch auf die Zähne kleiner devonischer Actinopterygier und Crossopterygier ausgedehnt. Das Ergebnis war leider negativ: die sehr kleinen Zähne der verschiedenen untersuchten Arten bieten zu wenig kennzeichnende und unterscheidende histologische Merkmale. Nur von der Entdeckung bestimmbarer Schädel- und Unterkiefer-Knochen der Gattung Lophosteus kann die Entscheidung dieser Frage erhofft werden.     

Tubular phosphatic microproblematica from the Early Ordovician of China

Fenhsiangia zhangwentangi gen. et sp. nov., is named for an animal represented by a phosphatic tube-like exoskeleton with the internal walls ornamented by stellate-based rounded tubercles. As vertebrates are the only animals known to possess stellate tubercles of phosphatic material in the dermal skeleton, yet our remains do not show any morphological or histological similarity to primitive fish bone, we suggest that Fenhsiangia was an ancestral protovertebrate, possibly an ascidian or related form. A second form, here referred to as Fenhsiangia sp. differs in that the tubercles are flat-topped or concave, and the external walls are deeply pitted with lines of small pustules bordering the depressions. The Upper Cambrian and Ordovician could have been a time of great diversity and radiation for protovertebrates. with the evolution of the first true vertebrates resulting from this radiation. □ Ordovician, China, phosphatic microproblematica, protovertebrate.     

Evolution of developmental pattern for vertebrate dentitions: an oro-pharyngeal specific mechanism

Classically the oral dentition with teeth regulated into a successional iterative order was thought to have evolved from the superficial skin denticles migrating into the mouth at the stage when jaws evolved. The canonical view is that the initiation of a pattern order for teeth at the mouth margin required development of a sub-epithelial, permanent dental lamina. This provided regulated tooth production in advance of functional need, as exemplified by the Chondrichthyes. It had been assumed that teeth in the Osteichthyes form in this way as in tetrapods. However, this has been shown not to be true for many osteichthyan fish where a dental lamina of this kind does not form, but teeth are regularly patterned and replaced. We question the evolutionary origin of pattern information for the dentition driven by new morphological data on spatial initiation of skin denticles in the catshark. We review recent gene expression data for spatio-temporal order of tooth initiation for Scyliorhinus canicula, selected teleosts in both oral and pharyngeal dentitions, and Neoceratodus forsteri. Although denticles in the chondrichthyan skin appear not to follow a strict pattern order in space and time, tooth replacement in a functional system occurs with precise timing and spatial order. We suggest that the patterning mechanism observed for the oral and pharyngeal dentition is unique to the vertebrate oro-pharynx and independent of the skin system. Therefore, co-option of a successional iterative pattern occurred in evolution not from the skin but from mechanisms existing in the oro-pharynx of now extinct agnathans.     

Zebrafish dentition in comparative context

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.     

Formation of dermal skeletal and dental tissues in fish: a comparative and evolutionary approach

Osteichthyan and chondrichthyan fish present an astonishing diversity of skeletal and dental tissues that are often difficult to classify into the standard textbook categories of bone, cartilage, dentine and enamel. To address the question of how the tissues of the dermal skeleton evolved from the ancestral situation and gave rise to the diversity actually encountered, we review previous data on the development of a number of dermal skeletal elements (odontodes, teeth and dermal denticles, cranial dermal bones, postcranial dermal plates and scutes, elasmoid and ganoid scales, and fin rays). A comparison of developmental stages at the tissue level usually allows us to identify skeletogenic cell populations as either odontogenic or osteogenic on the basis of the place of formation of their dermal papillae and of the way of deposition of their tissues. Our studies support the evolutionary affinities (1) between odontodes, teeth and denticles, (2) between the ganoid scales of polypterids and the elasmoid scales of teleosts, and (3) to a lesser degree between the different bony elements. There is now ample evidence to ascertain that the tissues of the elasmoid scale are derived from dental and not from bony tissues. This review demonstrates the advantage that can be taken from developmental studies, at the tissue level, to infer evolutionary relationships within the dermal skeleton in chondrichthyans and osteichthyans.