A large Middle Devonian eubrachythoracid 'placoderm' (Arthrodira) jaw from northern Gondwana

For the understanding of the evolution of jawed vertebrates and jaws and teeth, 'placoderms' are crucial as they exhibit an impressive morphological disparity associated with the early stages of this process. The Devonian of Morocco is famous for its rich occurrences of arthrodire 'placoderms'. While Late Devonian strata are rich in arthrodire remains, they are less common in older strata. Here, we describe a large tooth-bearing jaw element of Leptodontichthys ziregensis gen. et sp. nov., an eubrachythoracid arthrodire from the Middle Devonian of Morocco. This species is based on a large posterior superognathal with a strong dentition. The jawbone displays features considered synapomorphies of Late Devonian eubrachythoracid arthrodires, with one posterior and one lateral row of conical teeth oriented postero-lingually. μCT-images reveal internal structures including pulp cavities and dentinous tissues. The posterior orientation of the teeth and the traces of a putative occlusal contact on the lingual side of the bone imply that these teeth were hardly used for feeding. Similar to Compagopiscis and Plourdosteus, functional teeth were possibly present during an earlier developmental stage and have been worn entirely. The morphological features of the jaw element suggest a close relationship with plourdosteids. Its size implies that the animal was rather large.

An examination of the Devonian fishes of Michigan

We surveyed the taxa, ecosystems, and localities of the Devonian fishes of Michigan to provide a framework for renewed study, to learn about the diversity and number of these fishes, and to investigate their connection to other North American faunas. Nineteen genera of fishes have been found in the Middle and Late Devonian deposits of Michigan, of which thirteen are ‘placoderms’ represented by material ranging from articulated head shields to ichthyoliths. As expected from the marine nature of these deposits, ‘placoderms’ are overwhelmingly arthrodire in nature, but two genera of ptyctodonts have been reported along with less common petalichthyid material. The remaining fish fauna consists of fin-spines attributed to ‘acanthodians’, two genera of potential crown chondrichthyans, an isolated dipnoan, and onychodont teeth/jaw material. There was an apparent drop in fish diversity and fossil abundance between Middle and Late Devonian sediments. This pattern may be attributed to a paucity of Late Devonian sites, along with a relative lack of recent collection efforts at existing outcrops. It may also be due to a shift towards open water pelagic environments at Late Devonian localities, as opposed to the nearshore reef fauna preserved in the more numerous Middle Devonian localities. The Middle Devonian vertebrate fauna in Michigan shows strong connections with same-age assemblages from Ohio and New York. Finally, we document the presence of partially articulated vertebrate remains associated with benthic invertebrates, an uncommon occurrence in Devonian strata outside of North America. We anticipate this new survey will guide future field work efforts in an undersampled yet highly accessible region that preserves an abundance of fishes from a critical interval in marine vertebrate evolution.

Neurocranial anatomy of the petalichthyid placoderm Shearsbyaspis oepiki Young revealed by X-ray computed microtomography

Stem-group gnathostomes reveal the sequence of character acquisition in the origin of modern jawed vertebrates. The petalichthyids are placoderm-grade stem-group gnathostomes known from both isolated skeletal material and rarer articulated specimens of one genus. They are of particular interest because of anatomical resemblances with osteostracans, the jawless sister group of jawed vertebrates. Because of this, they have become central to debates on the relationships of placoderms and the primitive cranial architecture of gnathostomes. However, among petalichthyids, only the braincase of Macropetalichthys has been studied in detail, and the diversity of neurocranial morphology in this group remains poorly documented. Using X-ray computed microtomography, we investigated the endocranial morphology of Shearsbyaspis oepiki Young, a three-dimensionally preserved petalichthyid from the Early Devonian of Taemas-Wee Jasper, Australia. We generated virtual reconstructions of the external endocranial surfaces, orbital walls and cranial endocavity, including canals for major nerves and blood vessels. The neurocranium of Shearsbyaspis resembles that of Macropetalichthys, particularly in the morphology of the brain cavity, nerves and blood vessels. Many characters, including the morphology of the pituitary vein canal and the course of the trigeminal nerve, recall the morphology of osteostracans. Additionally, the presence of a parasphenoid in Shearsbyaspis (previously not known with confidence outside of arthrodires and osteichthyans) raises some questions about current proposals of placoderm paraphyly. Our detailed description of this specimen adds to the known morphological diversity of petalichthyids, and invites critical reappraisal of the phylogenetic relationships of placoderms.

New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes

Arthodire placoderms have been proposed as the sister group of Chinese ‘maxillate’ placoderms plus all the more crownward gnathostomes. These basal groups provide key information for understanding the early evolution of jaws. Here, we test previous assumptions about placoderm jaw structure and function by using high-resolution computed tomography, digital dissection, and enlarged 3D printouts on a unique articulated 400 million-year-old buchanosteid arthrodire. The upper jaw has a double ethmoid and a palatobasal connection, but no otic connection; the dermal bone attachment for the quadrate is different to other placoderms. A separately ossified cartilage behind the mandibular joint is comparable to the interhyal of osteichthyans. Two articular facets on the braincase associated with the hyomandibular nerve foramen supported a possible epihyal element and a separate opercular cartilage. Reassembling and manipulating 3D printouts demonstrates the limits of jaw kenetics. The new evidence indicates unrecognized similarities in jaw structure between arthrodires and osteichthyans, and will help to clarify the sequence of character acquisition in the evolution of basal gnathostome groups. New details on the hyoid arch will help to reformulate characters that are key in the heated debate of placoderm monophyly or paraphyly.