A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans

The Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and thus our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons. Here, we report a previously undescribed symmoriiform shark, Ferromirum oukherbouchi, from the Late Devonian of the Anti-Atlas. Computed tomography scanning reveals the undeformed shape of the jaws and hyoid arch, which are of a kind often used to represent primitive conditions for jawed vertebrates. Of critical importance, these closely fitting cartilages preclude the repeatedly hypothesized presence of a complete gill between mandibular and hyoid arches. We show that the jaw articulation is specialized and drives mandibular rotation outward when the mouth opens, and inward upon closure. The resultant eversion and inversion of the lower dentition presents a greater number of teeth to prey through the bite-cycle. This suggests an increased functional and ecomorphological disparity among chondrichthyans preceding and surviving the end-Devonian extinctions.

Osteocranium of Tor tambroides (Cypriniformes: Cyprinidae) from Tangse River, Aceh, Indonesia

Akmal Y, Zulfahmi I, Dhamayanti Y, Paujiah E. 2020. Osteocranium of Tor tambroides (Cypriniformes: Cyprinidae) from Tangse River, Aceh, Indonesia. Biodiversitas 21: 442-450. We report the first detailed descriptive osteocranium of keureling, Tor tambroides (Cypriniformes: Cyprinidae) collected from Tangse River, Indonesia. This study aimed to describe the cranium morphology of keureling (Tor tambroides). Keureling fish used in this study were obtained from the catch of fishers in the Tangse River, District of Pidie, Aceh Province, Indonesia. Stages of research include sample preparation, bone preparations, and identification of cranium nomenclature. For osteological examination, the cranium of the keureling prepared physically and chemically. Based on the results, the cranium of the keureling is divided into two major parts, namely the neurocranium and the facial bone,s including the branchial apparatus. The neurocranium is divided into (anterior to posterior) olfactory (ethmoid), orbital, otic, and occipital regions while the rest divided into oromandibular, opercular series, branchial arch, mandibular arch, and hyoid arch (suspensorium) regions. The neurocranium includes bones in the dermal series of the skull and endochondral bones protecting the brain. At the same time, branchiocranium are those located in the oromandibular part, opercular series, and branchial arch.

Bichir external gills arise via heterochronic shift that accelerates hyoid arch development

In most vertebrates, pharyngeal arches form in a stereotypic anterior-to-posterior progression. To gain insight into the mechanisms underlying evolutionary changes in pharyngeal arch development, here we investigate embryos and larvae of bichirs. Bichirs represent the earliest diverged living group of ray-finned fishes, and possess intriguing traits otherwise typical for lobe-finned fishes such as ventral paired lungs and larval external gills. In bichir embryos, we find that the anteroposterior way of formation of cranial segments is modified by the unique acceleration of the entire hyoid arch segment, with earlier and orchestrated development of the endodermal, mesodermal, and neural crest tissues. This major heterochronic shift in the anteroposterior developmental sequence enables early appearance of the external gills that represent key breathing organs of bichir free-living embryos and early larvae. Bichirs thus stay as unique models for understanding developmental mechanisms facilitating increased breathing capacity.

The hyoid arch and braincase anatomy of Acanthodes support chondrichthyan affinity of ‘acanthodians’

Solving the evolutionary relationships of the acanthodians is one of the key problems in reconstructing ancestral anatomical conditions for the jawed vertebrates (gnathostomes). Current debate concerns whether acanthodians are an assemblage of stem chondrichthyans, or a more generalized grade encompassing some early stem osteichthyans. The skull anatomy of Acanthodes bronni has been pivotal in these debates, owing to tension between chondrichthyan- and osteichthyan-like models of reconstruction. We use computed tomography scanning and traditional palaeontological techniques to resolve the long-standing debate about the anatomy of the jaw suspension. We establish the correct length of the hyomandibula and show that it attaches to a process on the ventrolateral angle of the braincase below the jugular vein groove. This condition corresponds precisely to that in chondrichthyans. This character represents an unambiguously optimized synapomorphy with chondrichthyans given current gnathostome phylogenies, corroborating the growing consensus of the chondrichthyan affinity of acanthodians.

The visceral skeleton and jaw suspension in the durophagous hybodontid shark Tribodus limae from the Lower Cretaceous of Brazil

The visceral skeleton (including complete mandibular, hyoid, and branchial arches) and teeth of the Lower Cretaceous hybodontid shark Tribodus limae are described based on well preserved fossil material. Jaw suspension and musculature are reconstructed, representing the first reconstruction of jaw musculature in a hybodont. The jaw suspension of Tribodus is similar to batoids and advanced galeomorphs in lacking direct cranio–palatine articulations and having skeletal jaw support by the hyoid arch alone (unlike most other hybodonts), but differs from batoids in that an intact hyoid arch is present. As in Asteracanthus and Lonchidion, the jaws do not extend to the snout, and were connected symphysially but not fused. CT scanning reveals the presence of supportive ‘trabecular cartilage’ struts in force-bearing regions of the jaws, representing the first report of these structures in an extinct chondrichthyan. Five branchial arches are present, of which pharyngobranchial, epibranchial, and ceratobranchial elements are observed although hypobranchials and basibranchials were presumably also present. A pharyngobranchial blade is present, as in some other hybodonts (e.g., Lissodus) and extant galeomorphs (e.g., Heterodontus), and the posteriormost pharyngobranchials are unfused. Tribodus is considered durophagous, based on presence of ‘trabecular cartilage’ struts and a weakly heterodont monognathic pavement dentition of flattened hexagonal teeth, as in extant myliobatoid rays. SEM examination shows that teeth of T. limae are anaulacorhize with a double layer of single crystallite enameloid (SCE), and confirms the presence of columnar osteodentine, as in other Acrodontidae.