Daniel Goujet

A primitive placoderm sheds light on the origin of the jawed vertebrate face

Extant vertebrates form two clades, the jawless Cyclostomata (lampreys and hagfishes) and the jawed Gnathostomata (all other vertebrates), with contrasting facial architectures. These arise during development from just a few key differences in the growth patterns of the cranial primordia: notably, the nasal sacs and hypophysis originate from a single placode in cyclostomes but from separate placodes in gnathostomes, and infraoptic ectomesenchyme migrates forward either side of the single placode in cyclostomes but between the placodes in gnathostomes. Fossil stem gnathostomes preserve cranial anatomies rich in landmarks that provide proxies for developmental processes and allow the transition from jawless to jawed vertebrates to be broken down into evolutionary steps. Here we use propagation phase contrast synchrotron microtomography to image the cranial anatomy of the primitive placoderm (jawed stem gnathostome) Romundina, and show that it combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of cyclostomes and the galeaspid (jawless stem gnathostome) Shuyu. This combination seems to be primitive for jawed vertebrates, and suggests a decoupling between ectomesenchymal growth trajectory, ectomesenchymal proliferation, and cerebral shape change during the origin of gnathostomes.

Interrelationships ofplacoderms revisited

Abstract From a set of new characters, the phylogeny of placoderms taken as a whole has been investigated using the computer programs PAUP and Mac Clade. The result corroborate the hypothesis of an inclusion of the Phylolepida within the Arthrodira and the basal position of the Acanthothoraci among Placodermata.

PRIMITIVE JAW STRUCTURE IN AN ARTICULATED BRACHYTHORACID ARTHRODIRE (PLACODERM FISH; EARLY DEVONIAN) FROM SOUTHEASTERN AUSTRALIA

Abstract A unique ‘buchanosteid’ arthrodire specimen from Emsian limestones at Burrinjuck represents the complete articulated remains of the head and cheek, and most of the trunk armor bones from a single fish, together with well-ossified braincase and jaw cartilages. The structure of the toothplates and jaws are described, and compared with primitive brachythoracid material from Saudi Arabia of similar age. Both the palatoquadrate and meckelian cartilages are well preserved, and perichondrally ossified as single elements. As previously described in some phlyctaeniids, the dermal gnathal elements carry crowded denticles similar to the normal tubercular ornament, enlarged along anterior and lateral margins. Six characters concerning gnathal plates of brachythoracids, which have been used in phylogenetic analysis, are discussed and reformulated.

A new genus of placoderm (Arthrodira: ‘Actinolepida’) from the Lower Devonian of Podolia (Ukraine)

Abstract A new genus of ‘actinolepid’ arthrodire placoderm, Erikaspis, is named to contain “Kujdanowiaspis” zychi Stensiö, 1945, from the Lochkovian–Pragian deposits (Lower Devonian) of Podolia (Ukraine). This new genus occurs together with the genus Kujdanowiaspis Stensiö, 1942, to which its type species was previously assigned. Distinctive ornamentation and pattern of dermal plates justify this new assignment. A phylogenetic analysis places Erikaspis, gen. nov., as a close relative of Kujdanowiaspis and of the contemporaneous species Sigaspis lepidophora Goujet, 1973, from Spitsbergen, all of them belonging to the family Kujdanowiaspididae Berg, 1955.

Thelodont Scales from the Lower Devonian Red Bay Group, Spitsbergen

Scales of nine thelodont taxa are described from the Red Bay Group of Spitsbergen, also characterized by its assemblage of other typical Early Devonian vertebrates. The four thelodont–bearing horizons of the lower Fraenkelryggen Formation yield typical Lochkovian thelodont assemblages, containing Boreania minima, Canonia grossi, Nikolivia depressa sp. nov., Nikolivia elongata, Nikolivia gutta and Turinia pagei. One horizon from the younger Ben Nevis Formation has a similar thelodont assemblage, except for two new taxa; Apalolepis angelica sp. nov. and Turinia barentsia sp. nov. The appearance of Turinia polita and the disappearance of Boreania minima is also characteristic for this assemblage. Comparison with similar faunas from the Northern Hemisphere supports this faunal change and allows further elaboration of Early Devonian thelodont biostratigraphy.

The internal cranial anatomy of Romundina stellina Ørvig, 1975 (Vertebrata, Placodermi, Acanthothoraci) and the origin of jawed vertebrates—Anatomical atlas of a primitive gnathostome

Placoderms are considered as the first jawed vertebrates and constitute a paraphyletic group in the stem-gnathostome grade. The acanthothoracid placoderms are among the phylogenetically most basal and morphologically primitive gnathostomes, but their neurocranial anatomy is poorly understood. Here we present a near-complete three-dimensional skull of Romundina stellina, a small Early Devonian acanthothoracid from the Canadian Arctic Archipelago, scanned with propagation phase contrast microtomography at a 7.46 μm isotropic voxel size at the European Synchrotron Radiation Facility, Grenoble, France. This is the first model of an early gnathostome skull produced using this technique, and as such represents a major advance in objectivity compared to past descriptions of placoderm neurocrania on the basis of grinding series. Despite some loss of material along an oblique crack, most of the internal structures are remarkably preserved, and most of the missing structures can be reconstructed by symmetry. This virtual approach offers the possibility to connect with certainty all the external foramina to the blood and nerve canals and the central structures, and thus identify accurate homologies without destroying the specimen. The high level of detail enables description of the main arterial, venous and nerve canals of the skull, and other perichondrally ossified endocranial structures such as the palatoquadrate articulations, the endocranial cavity and the inner ear cavities. The braincase morphology appears less extreme than that of Brindabellaspis, and is in some respects more reminiscent of a basal arthrodire such as Kujdanowiaspis.

A new placoderm fish (Acanthothoraci) from the Early Devonian Jauf Formation (Saudi Arabia)

ABSTRACT An exceptionally well-preserved new acanthothoracid (placoderm fish) presenting complete endocranium and skull roof, from Saudi Arabia (Qasr Limestone Member, Jauf Formation), Early Devonian in age, is described as Arabosteus variabilis n. gen., n. sp. Compared to other Acanthothoraci, the new taxon exhibits a skull roof variable in its dermal plate distribution (e.g., a supernumerary plate: the posterior central plate) and sensory line pattern. Even the different specimens of this taxon show such variability between them. Arabosteus variabilis n. gen., n. sp. enlarges the scope of our knowledge of Acanthothoraci and Placodermi in general. Traditionally, the Arthrodira, one of the historically earliest described groups, provide the standard reference model for the placoderm skull roof pattern. The interpretation of the acanthothoracid skull roof pattern of the Saudi taxon from an arthrodiran model and the establishment of homologies is impossible to resolve unless based on prior assumptions (e.g., hypothesis of fragmentation or fusion for the paranuchal plate complex). The lateral line system of A. variabilis n. gen., n. sp. is unstable (e.g., presence or absence of the postmarginal sensory line canal) making it ambiguous to define bone homology and terminology. Consequently spatial distribution of skull roof plates has been retained for such a task with a minimal reference to the instable features of the sensory line groove distribution.

New observations on the squamation patterns of articulated specimens of Loganellia scotica (Traquair, 1898) (Vertebrata: Thelodonti) from the Lower Silurian of Scotland

ABSTRACT Articulated squamations of Loganellia scotica (Traquair, 1898) from the Lower Silurian Lesmahagow inliers of southern Scotland have been studied. They include seven articulated specimens, three of them representing complete thelodont fossils, and four partially preserved postpectoral, precaudal and caudal parts of the exoskeleton. All the five main types of thelodont scales that is rostral, cephalo-pectoral, postpectoral, precaudal and pinnal sacles, as well as respective squamation patterns have been observed on articulated specimens. The specific orbital, branchial, and bucco-pharyngeal scales, characteristic of L. scotica, have not been found due to the poor or non-preservation of these particular areas within the specimens studied. Tail morphology and squamation pattern of the caudal fin have been studied with particular attention, and the constitution of the caudal fin rays has been analyzed. The rostral squamation pattern is argued to be characteristic to L. scotica.

Thelodont scales from the Lower and Middle Devonian Andree Land Group, Spitsbergen

Scales of six thelodont taxa are described from the Devonian of Spitsbergen. Numerous samples from localities widely dispersed on Spitsbergen yield several assemblages considered to represent different depositional phases of the late Lower lower Middle Devonian of the Andrée Land Group, but also support the view that certain lithostratigraphic units of the Andrée Land Group should be regarded as contemporaneous lithofacies subjected to different sedimentary environments, rather than as separate stratigraphic members. The description of Woodfjordia collisa gen. et sp. nov., Talivalia svalbardia sp. nov., Canonia cf. C. grossi, Amaltheolepis montiwatsonia sp. nov., Amaltheolepis winsnesi and Amaltheolepis austfjordia sp. nov. also allows for a comparison with similar faunas from other regions of the Northern Hemisphere and motivates further elaboration of Early-Middle Devonian thelodont biostratigraphy.

Multicuspidate shark teeth associated with chondrichthyan and acanthodian scales from the Emsian (Devonian) of southern Algeria

ABSTRACT Previously undescribed Emsian vertebrates from southern Algeria (southern Ahaggar) include a new chondrichthyan Tassiliodus lessardi n. gen., n. sp. with multicuspidate teeth and scales with a distinctive histology. This is the first Emsian chondrichthyan taxon reported from the north-western margin of Gondwana. Rare acanthodian scales are assigned to Milesacanthus cf. ancestralis Burrow, Lelièvre & Janjou, 2006, an Emsian species also known from Saudi Arabia, reinforcing the Gondwanan faunal affinity.