Dermal Ornamentation Research Articles

Osteological development of Cope's Gray Treefrog, Hyla chrysoscelis

AbstractHyla chrysoscelis, Cope's Gray Treefrog, is a generalized treefrog found throughout much of east‐central North America. Although it is a model for many behavioural and ecological studies, little is known of its skeletal morphology or development. Herein, we describe the postembryonic skeletal development and adult osteology of H. chrysoscelis. The adult skull is well ossified with slight dermal ornamentation, the postcranial and tadpole skeletons are fairly non‐distinct with no obvious novel morphologies, and the Gosner stage by which bony elements first appear varies. We compare the rank order sequence of ossification to that of its sibling species Hyla versicolor and use examples from this study to demonstrate current complications with conducting ossification sequence meta‐analyses.

A new pteraspidomorph from the Nibil Formation (Katian, Late Ordovician) of the Canning Basin, Western Australia

ABSTRACT Based upon fragmentary remains of dermal armor, a new form of arandaspid fish, Ritchieichthys nibili, gen et. sp. nov., is described from subsurface core material from the Katian (Late Ordovician) Nibil Formation of the Canning Basin, Western Australia. Ritchieichthys nibili represents the first documented record of a fish from the Ordovician of the Canning Basin. Allied to the previous descriptions of arandaspsids from the Amadeus and Warburton basins of the Northern Territory and New South Wales, respectively, this record extends the paleogeographic range of arandaspids across the hypothetical Ordovician Larapintine Seaway and increases the stratigraphic range of the Order Arandaspidiformes into the Katian. The hard tissue histology of Ritchieichthys nibili confirms the presence of a cellular dentine forming the bulk of the dermal armor ornament in arandaspids, a tissue that had not been directly observed previously, and confirms the presence of largely unconnected osteocytes within the dermal bone that forms the majority of the armor.

Testing models of dental development in the earliest bony vertebrates,AndreolepisandLophosteus

Theories on the development and evolution of teeth have long been biased by the fallacy that chondrichthyans reflect the ancestral condition for jawed vertebrates. However, correctly resolving the nature of the primitive vertebrate dentition is challenged by a dearth of evidence on dental development in primitive osteichthyans. Jaw elements from the Silurian-Devonian stem-osteichthyans Lophosteus and Andreolepis have been described to bear a dentition arranged in longitudinal rows and vertical files, reminiscent of a pattern of successional development. We tested this inference, using synchrotron radiation X-ray tomographic microscopy (SRXTM) to reveal the pattern of skeletal development preserved in the sclerochronology of the mineralized tissues. The tooth-like tubercles represent focal elaborations of dentine within otherwise continuous sheets of the dermal skeleton, present in at least three stacked generations. Thus, the tubercles are not discrete modular teeth and their arrangement into rows and files is a feature of the dermal ornamentation that does not reflect a polarity of development or linear succession. These fossil remains have no bearing on the nature of the dentition in osteichthyans and, indeed, our results raise questions concerning the homologies of these bones and the phylogenetic classification of Andreolepis and Lophosteus.

The diverse Cretaceous neobatrachian fauna of South America: Uberabatrachus carvalhoi, a new frog from the Maastrichtian Marília Formation, Minas Gerais, Brazil

Herein we report on a new anuran genus and species, Uberabatrachus carvalhoi, from exposures of the Marília Formation (Maastrichtian) at the well-known fossil site “Ponto 1 do Price” near Peirópolis, Uberaba, Brazil. The holotype consists of an articulated partial skeleton characterized by a broad, well-ossified skull lacking dermal ornamentation, eight procoelous presacral vertebrae, and posteriorly deflected, cylindrical sacral diapophyses; these features, together with the possible presence of a discrete palatine, are consistent with neobatrachian affinities. It clearly differs from the neobatrachian Baurubatrachus, also from the Marília Formation, in having a skull lacking exostosis and with a different temporal architecture, among other features. It also differs from the still unnamed neobatrachian recovered from the Adamatina Formation in the presence of paired frontoparietals with narrow supraorbital flanges in the anterior orbital region and maxilla-squamosal contact. The new frog is not referable to any of the neobatrachian taxa known from the Aptian–Albian Crato Formation owing to different cranial features (e.g., position of the jaw articulation, relative width of the braincase, and morphology of the frontoparietal). With respect to the extant neobatrachians in the context of recent phylogenetic hypotheses, the cylindrical sacral diapophyses of Uberabatrachus contrast with the dilated diapophyses occurring in australobatrachian hyloids and microhyloid ranoids, whereas the procoelous presacral column is unlike the diplasiocoelous condition of most other ranoids. This evidence together with the combination of features of Uberabatrachus suggests a placement among nobleobatrachian hyloids, although this combination is not shared with any extant or extinct species for which skeletal information is known. From the foregoing we conclude that Uberabatrachus represents a new nobleobatrachian taxon, a record that reinforces the hypothesis of a diversified neobatrachian fauna in South America by the Cretaceous and is consistent with the previous proposal that neobatrachian early diversification is related to the breakup of Gondwana.

A rhinesuchid-like temnospondyl from the Permo-Triassic of Uruguay

A new temnospondyl species, Arachana nigra, from the Permo-Triassic Buena Vista Formation of Uruguay is described. The holotypic and only known specimen consists of an almost complete skull lacking most of the snout, the tabular horns and the posterolateral corners of the skull table. As with other specimens from the same unit, Arachana shows a transitional morphology. It shares several characters with rhinesuchids, such as the large size and the general shape of the skull, orbits positioned slightly posterior to the skull roof mid-length, a coarsely pitted dermal ornamentation lacking pustules, and a smoothly convex cheek contour. The palatal ramus of the pterygoid suturing with the vomer, and excluding the palatine and ectopterygoid from the margin of the interpterygoid vacuity, and the presence of a small basioccipital, visible in both occipital and ventral view, also resemble the conditions found in rhinesuchids and other basal temnospondyls. Other characters present in Arachana, however, are commonly found in lydekkerinids, but are absent in almost all rhinesuchids: supratemporal excluded from otic notch; supraorbital and infraorbital sensory sulci encroaching the lacrimal, although lacking a step-like lacrimal flexure; otic notch not deeply incised; post-temporal fenestra large and rounded; occipital condyles well-separated from each other; palatine tooth row behind the palatine tusk reduced; pterygoid corpus slightly ornamented; and presence of an interorbital depression. This combination of primitive and derived characters is consistently present in most components of the Buena Vista fauna, which could thus be transitional between typical Permian and Triassic tetrapod communities found elsewhere. The location of the PTB in the Uruguayan sequence is controversial, mainly due to the lack of clear faunal correlations with other well-known sequences, such as those of southern Africa and Russia. Moreover, the mosaic-like character combinations in most of the recorded tetrapods ally them to both Triassic and Paleozoic groups, and this has complicated even more the possibility of age assignment. Transitional faunas associated with the PTB, such as the Russian Uppermost Permian faunas, could be equivalent to the unique Colonia Orozco fauna. If true, this scenario will substantially change estimates about survivorship rates, suggesting a speciation rate increase in temnospondyls after the Permian-Triassic event. Based on taxonomic, phylogenetic, and geochronologic data, the Buena Vista fauna allows us to quantify faunal turnover across the PTB and in the aftermath of the end Permian extinction event.

A new species of Enchodus (Aulopiformes: Enchodontidae) from the Cretaceous (Albian to Cenomanian) of Zimapán, Hidalgo, México

ABSTRACT A new species of Enchodus (Aulopiformes: Enchodontidae) is described from a quarry exposure of the La Negra facies of the El Doctor Formation (Cretaceous: Albian to Cenomanian) in the state of Hidalgo, central México. It is an elongate fish with many generic and familial characters, including dermal ornamentation of tubercles atop rows of ridges; a palatine with a single, large tooth; and a triangular preopercle. It has a number of unique characters that include the presence of an orbitosphenoid, absence of dorsal scutes, and dorsal fin rays that share common pterygiophores. A phylogenetic analysis places the new species as the sister to Enchodus gladiolus. The new species of Enchodus is the most abundant fish species at the locality. Using mandible length as an indicator of overall size, the range in lengths suggests that this species lived at or near the locality. This is further supported by other species that have individuals of large and small sizes.

Two new species of Dunkleosteus Lehman, 1956, from the Ohio Shale Formation (USA, Famennian) and the Kettle Point Formation (Canada, Upper Devonian), and a cladistic analysis of the Eubrachythoraci (Placodermi, Arthrodira)

Two pachyosteomorph arthrodires are described from the Upper Devonian of North America, Dunkleosteus raveri sp. nov. and Dunkleosteus amblyodoratus sp. nov. (from the Appalachian and the Michigan basins, respectively). Dunkleosteus raveri sp. nov. is found stratigraphically below the only other species within the Ohio Shale Formation (Famennian), and is characterized by the retention of a primitive dermal ornamentation, and shares with Dunkleosteus terrelli (Newberry, 1873) the possession of articular facets on the parasphenoid. Dunkleosteus amblyodoratus sp. nov. (Kettle Point Formation, Upper Devonian), known only from incomplete nuchal, paranuchal, and parasphenoid plates, also retains dermal tuberculation, and is characterized by tapering articular facets on the parasphenoid. The current study reviews three North American species of Dunkleosteus (Du. terrelli, Du. raveri sp. nov., and Du. amblyodoratus sp. nov.). Formerly, two subgroups were recognized within the Dinichthyidae, the terrelli and herzeri groups, which are shown here to be members of distinct clades. Dinichthys herzeriNewberry, 1868 (the namesake member of the herzeri group) is now the sole member of the family Dinichthyidae comb. nov., and is assigned to the Aspinothoracidi sensuMiles & Dennis, 1979. HeintzichthysWhitley, 1933 and GorgonichthysClaypole, 1892 (additional members of the herzeri group) were previously placed among the aspinothoracid arthrodires. Likewise, Holdenius holdeniDunkle & Bungart, 1942 (the final member of the herzeri group) and Hadrosteus rapaxGross, 1932 are now placed within the Aspinothoracidi. The remaining taxa, formerly assigned to the Dinichthyidae (i.e. DunkleosteusLehman, 1956; EastmanosteusObruchev, 1964; GolshanichthysLelievre, Janvier, & Goujet, 1981; and WestralichthysLong, 1987), are united within the Dunkleosteidae Stensio, 1963. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 159, 195–222.

A NEW TRISTICHOPTERID (SARCOPTERYGII, TETRAPODOMORPHA) FROM THE UPPER FAMENNIAN EVIEUX FORMATION (UPPER DEVONIAN) OF BELGIUM

Abstract: Additional material of a large specimen of tristichopterid fish from the Upper Famennian Evieux Formation of Belgium is described. This large fish was previously assigned to Tristichopteridae gen. et sp. indet. due to the lack of diagnostic anatomical data. New available material consists of the internal surface of the parietal shield, vomers and anterior part of the parasphenoid, subopercular and submandibulo‐branchiostegal bones, and an internal view of the anterior part of the mandible. A possible autapomorphy of the new form from Belgium, Langlieria socqueti gen. nov. et sp. nov., is the absence of marginal teeth on the vomer except on its most lateral part. Apart from these features, it only differs from the genus Mandageria from Australia in the absence of marginal teeth between the dentary fang and the mandibular symphysis, in the presence of a raised marginal crest lateral to the anterior coronoid fang, and in the presence of numerous small marginal teeth on the premaxilla. It differs from the cosmopolitan genus Eusthenodon in a number of respects: the supratemporal, tabular, and postparietal bones are superficially fused, as are the intertemporal and parietal bones, the dermal ornament is proportionally very fine, and the denticulated field of the parasphenoid stands proud rather than being recessed into the body of the bone.

New arthrodires (Family Williamsaspididae) from Wee Jasper, New South Wales (Early Devonian), with comments on placoderm morphology and palaeoecology

AbstractTwo new arthrodire species represented by articulated trunk armours from the Early Devonian (Emsian) limestones of the Burrinjuck area are placed in a new genus Elvaspis (E. tuberculata, E. whitei), assigned to the Family Williamsaspididae. On new evidence of the dermal neck‐joint and shape of the median dorsal plate this family is reassigned from the Phlyctaeniina to the Brachythoraci. The strongly ornamented post‐branchial lamina of the trunk armour relates to a recent hypothesis that special post‐branchial denticles in placoderms are homologous to pharyngeal denticles of crown‐gnathostomes rather than modified external tubercles. Variable development of the post‐branchial lamina and its ornament in different placoderm subgroups, with specific resemblance to the external ornament characteristic of that subgroup, indicates that modification of normal external dermal ornament is the most parsimonious interpretation. A comparison of fish diversity in modern and ancient tropical reef environments is consistent with an equilibrium model for species diversification through time.

A new stereospondyl from the German Middle Triassic, and the origin of the Metoposauridae

Recent finds of well-preserved temnospondyl skeletons from the Lower Keuper (Ladinian, Middle Triassic) in southern Germany are assigned to a new genus and species, Callistomordax kugleri. This taxon is characterized by the following autapomorphies: (1) wide unpaired frontal; (2) vomerine fangs greatly enlarged to occupy entire width of element; (3) intercentra elongated and massive, anterior face being convex; (4) humerus semilunar with enlarged deltopectoral crest; (5) cleithrum strongly curved and bow-shaped; (6) trunk extremely elongated to reach three times the length of the skull. Callistomordax shares with the Metoposauridae the pattern of dermal ornamentation, the proportion of both posterior skull table and snout, the position of the lacrimal, the morphology of the basicranial region, and the structure of the clavicle and interclavicle. Phylogenetic analysis suggests Callistomordax to be the sister taxon of the Metoposauridae, nested within a grade formed by various trematosaurian taxa. In this assemblage, Lyrocephaliscus and a clade formed by Almasaurus, Rileymillerus, Callistomordax, and the Metoposauridae are sister taxa. In all variants of the cladistic analysis, Callistomordax and the Metoposauridae form immediate sister groups. According to the present findings, neither plagiosaurids nor brachyopoids and rhytidosteids are closely related to this ‘trematosaurian’ monophylum, although these taxa share a range of homoplasies. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 152, 79–113.

A new Middle Devonian arthrodire (placoderm fish) from the Broken River area, Queensland

Associated bones of the trunk-armour belonging to a large brachythoracid arthrodire are described as Confractamnis johnjelli n.gen. and n.sp. The specimen comes from strata of probable Eifelian age in the Broken River Group of Queensland. It shows a range of derived brachythoracid features including reduction of dermal ornament, and strong development of the dermal neck joint articulation. The posterior dorsolateral and posterior lateral plates of the trunk armour were high and narrow, and the anterior lateral plate had a distinctive narrow bilobed dorsal angle as indicated by the shape of its overlap area on the anterior dorsolateral plate. A provisional reconstruction suggests that the bones came from a large fish over 2 m in length. A cross section of the trunk armour is compared with the large arthrodire Taemasosteus from the Early Devonian of Burrinjuck, New South Wales. A possible close relative of this new taxon has been illustrated from the Early Devonian of Morocco, supporting other indications of resemblance in placoderm fish assemblages between these regions of east and north Gondwana during the Early-Middle Devonian.

Pederpes finneyae, an articulated tetrapod from the tournaisian of Western Scotland

Synopsis The Tournaisian tetrapod Pederpes finneyae is described in detail, from the holotype and only specimen. The specimen derives from the Ballagan Formation, Tournaisian CM palynozone and is the earliest post‐Devonian tetrapod known from articulated remains. It is preserved almost complete except for the tail, some skull regions and a few limb extremities. The animal resembles Whatcheeria deltae from the Viséan of Iowa, especially in the skull morphology, although the two differ in characters such as the lateral line expression, dermal ornament expression, proportions of the sub‐orbital portion of the jugal and several other dermal skull characters. The postcranial skeleton of Pederpes shows several unique features, including the structure of the leading edges of the cleithrum and clavicle, the form of the rib flanges and a possible supernumerary digit on the manus. The humerus is also unusual in possessing a spike‐like latissimus dorsi process, most like that of Baphetes. An investigation of the phylogenetic position of the whatcheeriids is carried out by comparing the results from two recently published databases. The clade lies very close to the base of the tetrapod stem group, although according to the dataset employed, the whatcheeriids compete with Crassigyrinus for the next most basal node after the Devonian forms. Possible whatcheeriids also occur in the Viséan of Australia and the Tournaisian of Ireland and less certainly elsewhere, making the whatcheeriids a long‐lasting clade, widely distributed in time and space.

A new buchanosteid arthrodire (placoderm fish) from the Early Devonian of the Ural Mountains

ABSTRACT A new primitive brachythoracid arthrodire, Uralosteus bashkiricus gen. et sp. nov., is described from two Emsian (late Early Devonian) localities in the Ural Mountains of the Autonomous Republic of Bashkortostan, Russia. The holotype includes bones of the skull and trunk armor associated with numerous scales from one individual, which facilitates the study of isolated microremains. The dermal ornament of distinctive tuberculate ridges suggests a relationship to Errolosteus Young, 1981 from the Emsian of southeastern Australia, but neither form is well enough known for this to be strongly supported in a character analysis. Some isolated placoderm bones with ridged ornament from southeastern Australia are figured, and interpreted to belong to a new form of williamsaspid rather than to Errolosteus. The new genus Uralosteus is referred to the family Buchanosteidae, defined by a unique overlap arrangement of the posterior lateral onto the anterior dorsolateral plate of the trunk armor. Buchanosteids, as a basal brachythoracid group, are a key to understanding the phylogeny of that clade. The type genus Buchanosteus comes from southeastern Australia, but it is now evident that the family was widely distributed in shallow marine environments during the Emsian.

A revised reconstruction of the dermal skull roof of Acanthostega gunnari, an early tetrapod from the Late Devonian

ABSTRACTA revised reconstruction of the dermal skull roof of Acanthostega gunnari from the Upper Devonian of East Greenland is provided, incorporating modifications consequent upon further study of the specimens and in the light of new information from the Upper Devonian genus Ventastega from Latvia. The new reconstruction features a midline gap between the frontal and nasal bones and median rostrals. A revised profile for the skull and a rendition of the dermal ornament is also provided.

A primitive actinopterygian neurocranium from the Early Devonian of southeastern Australia

ABSTRACT A well-preserved neurocranium with attached partial skull roof is described from the Early Devonian Taemas Formation at Wee Jasper, southern New South Wales, Australia. The dermal ornament, skull roof pattern, and general proportion and structure of the endocranium are typically actinopterygian and the specimen is tentatively assigned to the actinopterygian genus Ligulalepsis. Other features more closely resemble some other groups, such as sarcopterygians (widely-spaced orbital walls, and short, broad telencephalon cavity), acanthodians (position of hyo-mandibular facet), and placoderms (many similarities including position of foramina for oculomotor, profundus, and trigeminal nerves, pituitary vein, and ophthalmic and orbital arteries in and around the orbit). This specimen is the first early osteichthyan to demonstrate the presence of an eyestalk, previously known only in placoderms and chondrichthyans. The unusual mix of characters and presence of an eyestalk provide new insights into primitive osteichthyan anatomy.

A second incisoscutid arthrodire (Pisces, Placodermi) from the Late Devonian Gogo Formation, Western Australia

Long, J. A., 1994:03:28. A second incisoscutid arthrodire (Pisces, Placodermi) from the Late Devonian Gogo Formation. Western Australia. Alcheringa 18. 59–69. ISSN 0311-5518. A new incisoscutid arthrodire is described from the Upper Devonian Gogo Formation of Western Australia as Gogosteus sarahae gen. et sp. nov. It is characterised by its narrow headshield with cheek unit firmly attached to skull roof, crushing gnathal plates, deep postnasal plates, elongate anterior lateral plate with deeply embayed postbranchial notch and squarely cut posterior margin, and fine dermal ornamentation. The family Incisoscutidae Denison 1984 is redefined to include Incisoscutum ritchiei Dennis & Miles 1981 and Gogosteus gen. nov., and the superfamily Incisoscutoidea nov. defined to include Incisoscutidae and Camuropiscidae.

Crossopterygian fishes from the Devonian of Antarctica: systematics, relationships and biogeographic significance

Four new genera of osteolepiforms and a new rhizodontiform genus are described from the Middle-Late Devonian Aztec Siltstone (Beacon Supergroup) of Antarctica. Other indeterminate osteolepid, eusthenopterid and rhizodontid remains, and a single indeterminate porolepiform scale, are described from the same formation. Koharalepis jarviki n.gen., n.sp. is a large cosmine-covered osteolepiform with a broad flat head, two pairs of dermosphenotics, large extratemporals, a large postorbital in the cheek which does not reach the orbit, an elongate jugal, lachrymal separating the maxilla from the premaxilla at the jaw margin, and large anteriorly pointed median and triangular lateral extrascapulars. It is placed with Canowindra Thomson and Beelarongia Long in the new family Canowindridae. Platyethmoidia antarctica n.gen., n.sp. is a poorly known form with a very broad, flat fronto-ethmoidal shield with dorsomesially oriented slit-like nares, and a lower jaw with a deep articular region. It may be closely related to Gyroptychius? australis Young & Gorter. Mahalalepis resima n.gen., n.sp. is interpreted on the morphology of the fronto-ethmoidal shield as an early megalichthyid, and a new definition of this family is presented. Vorobjevaia dolonodon n.gen., n.sp. is a poorly known osteolepid with a distinctive jaw morphology. The rhizodontiform Notorhizodon mackelveyi n.gen., n.sp. was the largest fish in the Aztec fauna, attaining a length of over 3 m. It is characterised by elongate frontals and small dermosphenotics in the skull, Rhizodus-type tusks, and strong tooth flanges on the coronoid and dermopalatine series bones in the jaws. The braincase in a rhizodontiform is described for the first time, and in general morphology closely resembles that of Osteolepiformes. Notorhizodon resembles Screbinodus Andrews in dermal ornament and Barameda Long in skull pattern. All previously described crossopterygian material from the Aztec Siltstone is revised, and Gyroptychius antarcticus (Smith Woodward) is regarded as a nomen nudum. The stratigraphic distribution of rhipidistians in the Aztec Siltstone is summarised, and the biogeography and phylogenetic relationships of the new taxa discussed. The canowindrids were an endemic group of East Gondwana. The megalichthyids and rhizodontiforms may have originated in the East Gondwana Province during the Middle Devonian, but did not reach Euramerica until the uppermost Late Devonian. They flourished in the Carboniferous Period following the decline of the eusthenopterids and the initial radiation of tetrapods.

A new bothriolepid antiarch (Pisces, Placodermi) from the Devonian of Dô Son peninsula, northern Vietnam

A new bothriolepid antiarch, Vietnamaspis trii gen. et sp. nov., is described from the Middle-Upper Devonian sandstones of the Dô Son peninsula, near Haiphong, Vietnam. The site is north of the Song Ma suture zone on the South China Terrane, and contains a fauna including Bothriolepis and groenlandaspids. Vietnamaspis gen. nov. is characterized by having a long, parallel-sided posterior median dorsal plate, and an anterior median dorsal plate with a narrow anterior margin. Vietnamaspis gen. nov. is regarded as a sister taxon to Bothriolepis and Grossilepis because it has a relatively large axillary foramen on the anterior ventrolateral plate. The Australian bothriolepid Bothriolepis warreni Long 1983 is referred to a new genus, Briagalepis gen. nov., because it has very short overlap surfaces for the anterior dorsolateral plate on the anterior median dorsal plate, and lacks a reticulate dermal ornament.

A Pennsylvanian aeduelliform (Osteichthyes, Actinopterygii) from North America with comments on aeduelliform interrelationships

A specimen assignable to the lower actinopterygian order Aeduelliformes (Family Aeduellidae) is reported from the Upper Pennsylvanian (Virgilian) Wild Cow Formation of central New Mexico, USA. This is the first record outside of Europe. Based on the reduction of the branchiostegal series, lateral position of the supraorbital sensory canal, and distinctive pattern of dermal ornamentation, the Aeduelliformes is monophyletic. The group is indeterminate in position between palaeonisciform actinopterygians and ginglymodians.

A new late Devonian bothriolepid (placodermi, antiarcha) from Victoria, with descriptions of other species from the state

A new species of bothriolepid antiarch, Bothriolepis tatongensis sp. nov., from the Late Devonian basal conglomerate of the Hollands Creek Rhyodacite, near Tatong, northern Victoria, is described. B. tatongensis sp. nov. can be distinguished from all other species possessing a trifid preorbital recess by the short postpineal plate with paired pits opening anteriorly into the orbital fenestra, and by coarsely reticulate dermal ornament. B. gippslandiensis Hills, B. cullodenensis Long, B. fergusoni Long, B. bindareei Long and B. warreni Long are described and illustrated. Metric analysis of headshield and premedian plate measurements of B. gippslandiensis, B. cullodenensis and B. fergusoni are compared with ontogenetic variation in B. canadensis Whiteaves. The phylogenetic position of B. fergusoni and B. tatongensis sp. nov. is discussed and the significance of the preorbital recess in bothriolepid evolution re-evaluated. A trifid preorbital recess is suggested to be apomorphic within the Bothriolepidoidei ...