A long endoskeletal spiracular canal is classically regarded as a character uniting a clade of post-Devonian ray-finned fishes []. However, the Devonian Cheirolepis [], Mimipiscis, and Moythomasia [] bear a thin bony commissure partially or completely enclosing the spiracle against the lateral wall of the braincase. This bar is anatomically and topologically consistent with the structure in Meemannia and represents the precursor of the elongated canal of younger taxa ( Figure 2 ). Endocranial enclosure of the spiracle is generally absent in sarcopterygians (excluding the highly nested Powichthys [] and Eusthenopteron []), the braincase referred to Ligulalepis [], Acanthodes [], early chondrichthyans [], and ‘placoderms’ []. The stem gnathostome Janusiscus bears symmetrical endocranial fenestrations aligned with its spiracular grooves [], although the absence of similar features in both early sarcopterygians and the stem osteichthyan ‘Ligulalepis’ argues against homology between the bridge in Janusiscus and actinopterygians (see optimizations in the phylogenetic tree available at http://dx.doi.org/10.5061/dryad.t6j72 ).

Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes.

The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia.

The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia.

Zhu et al. [] identified a dorsal embayment in the braincase of Meemannia, Psarolepis, and Ligulalepis as a lateral cranial canal, casting doubt that the feature is restricted to ray-finned fishes. However, these candidates differ from conventional lateral cranial canals. Most significantly, they are dorsally—rather than laterally—oriented and lie anterior to the usual position of the lateral cranial canal. Tomographic study of Meemannia shows that this feature is actually the crus commune, with a genuine lateral cranial canal situated more posteriorly and laterally ( Figure 1 D) but not easily visible in published figures []. Although the presence of a lateral cranial canal in Psarolepis and Ligulalepis cannot be entirely ruled out without further investigation, it seems unlikely.

A lateral cranial canal is known only in actinopterygians []. It is primitively a perichondrally lined diverticulum that exits the lateral wall of the cavum cranii at the level of the hindbrain and passes through the loop of the posterior semicircular canal. It may end blindly (e.g., Mimipiscis [] and Lepisosteus []), pierce the wall of the braincase and exit into the fossa bridgei (e.g., Pteronisculus []), or rejoin the cranial cavity through the loop of the anterior semicircular canal (e.g., Caturus []). Among living taxa, a lateral cranial canal is present in gars, sturgeons, and paddlefishes but is absent in polypterids, Amia, and teleosts. There is abundant fossil evidence that the absence in the latter two groups is secondary []. Absence of a canal in polypterids is conventionally regarded as primary, making the structure a synapomorphy of a subset of crown actinopterygians (Actinopteri) []. However, the posterior semicircular canal is incompletely enclosed within bone and cartilage in polypterids [], questioning whether it is logically possible for a lateral cranial canal of the sort detectable in fossils to be present. Significantly, the nature of absence in Polypterus differs from the condition in actinopterygian outgroups [], where the relevant portions of the endocavity are enclosed in bone but there is no projection through the loop of the posterior semicircular canal.

Endocranial preservation of a Carboniferous actinopterygian from Lancashire, U.K., and the interrelationships of primitive actinopterygians.

The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia.

The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia.

The structure of certain Jurassic holostean fishes with special reference to their neurocrania.

Endocranial preservation of a Carboniferous actinopterygian from Lancashire, U.K., and the interrelationships of primitive actinopterygians.

The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia.

Primitive Dermal Bone Histology in Osteichthyans

10 Schultze H.-P. Scales, enamel, cosmine, ganoine, and early osteichthyans. 36 Qu Q.

Haitina T.

Zhu M.

Ahlberg P.E. New genomic and fossil data illuminate the origin of enamel. Presence of a “cosmine”-like tissue in Cheirolepis suggests that the histological traits argued to link Meemannia with sarcopterygians are general features of bony fishes []. These findings also highlight the ambiguity that often surrounds the use of terms such as “cosmine” and “ganoine” and present an opportunity to review the ways by which these tissues are identified.

10 Schultze H.-P. Scales, enamel, cosmine, ganoine, and early osteichthyans. 37 Richter M.

Smith M.M. A microstructural study of the ganoine tissue of selected lower vertebrates. “Cosmine” is a complex tissue type, the identification of which is contingent on the presence of enamel on dermal bones and scales, enamel being present as a single layer (indicating that resorption is active), dentine underlying the enamel layer, regular pore openings (“pore canals”) on the surface of the enamel, and pore canals extending to pore cavities under the surface of the bone and connected horizontally by horizontal or mesh canals (a “pore canal network”). “Cosmine” is often contrasted with “ganoine,” itself a composite tissue generally considered an actinopterygian character [], identified by the presence of multiple layers of enamel without intervening dentine layers and the absence of pore canals or a pore canal network.

11 Zhu M.

Yu X.

Wang W.

Zhao W.

Jia L. A primitive fish provides key characters bearing on deep osteichthyan phylogeny. 12 Zhu M.

Wang W.

Yu X.-B. Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of China—expanded description and significance. 2 Zhu M.

Yu X.-B.

Janvier P. A primitive fossil fish sheds light on the origin of bony fishes. 36 Qu Q.

Haitina T.

Zhu M.

Ahlberg P.E. New genomic and fossil data illuminate the origin of enamel. 38 Qu Q.

Zhu M.

Wang W. Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues. Meemannia has been described as displaying the first step toward the evolution of “cosmine” proper [], possessing only some of the tissue’s identifying features: pore openings on the dermal surface, a pore-canal network, dentine, and superimposed layers of enamel, suggesting that resorption was absent. New histological data for Cheirolepis indicate that a precursor of both “cosmine” and “ganoine” was present in the last common ancestor of actinopterygians and sarcopterygians. This hypothesis gains support if interpretations of Psarolepis as a stem osteichthyan rather than stem sarcopterygian are corroborated by subsequent analyses [].

23 Gardiner B.G. The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia. 19 Giles S.

Darras L.

Clément G.

Blieck A.

Friedman M. An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes. 15 Grogan E.D.

Lund R. Two new Actinopterygii (Vertebrata, Osteichthyes) with cosmine from the Bear Gulch Limestone (Heath Fm., Serpukhovian, Mississippian) of Montana USA. 39 Dunkle D.H.

Schaeffer B. Tegeolepis clarki (Newberry), a palaeonisciform from the Upper Devonian Ohio shale. 27 Long J.A. New palaeoniscoid fishes from the Late Devonian and Early Carboniferous of Victoria. 23 Gardiner B.G. The relationships of the palaeoniscid fishes, a review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia. 40 Arratia G.

Cloutier R. A new cheirolepidid fish from the Middle-Upper Devonian of Red Hill, Nevada, USA. 12 Zhu M.

Wang W.

Yu X.-B. Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of China—expanded description and significance. 38 Qu Q.

Zhu M.

Wang W. Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues. Actinopterygians above Cheirolepis show modifications to this ancestral tissue type: loss of pore canals and underlying network and retention of multiple layers of enamel, but with loss of intervening dentine. Large, irregular pores are visible on the lower jaw, gulars, and scales of certain Devonian (e.g., Moythomasia [] and Raynerius []) and Carboniferous (e.g., Paphosiscus []) actinopterygians but appear to be absent in taxa such as Tegeolepis [], Howqualepis [], and Mimipiscis []. It is not possible to say whether they are homologous with the pore canal network of sarcopterygians and the earliest actinopterygians without more complete histological investigation. Younger species of Cheirolepis appear to lack porous ornament, possessing the ridges more typically associated with actinopterygians [], although dermal bone histology is unknown. These species nest within Cheirolepis, suggesting that a primitively present pore canal system may have been secondarily lost. The stepwise evolution of “cosmine” proper is well documented in sarcopterygians [], and the combination of characters that diagnoses the tissue is restricted to a subset of sarcopterygians: rhipidistians (lungfishes plus tetrapods).

5 Schultze H.-P.

Cumbaa S.L. Dialipina and the characters of basal actinopterygians. 10 Schultze H.-P. Scales, enamel, cosmine, ganoine, and early osteichthyans. 41 Friedman M. The early evolution of ray-finned fishes. 8 Friedman M.

Brazeau M.D. A reappraisal of the origin and basal radiation of the Osteichthyes. 10 Schultze H.-P. Scales, enamel, cosmine, ganoine, and early osteichthyans. 41 Friedman M. The early evolution of ray-finned fishes. 41 Friedman M. The early evolution of ray-finned fishes. 8 Friedman M.

Brazeau M.D. A reappraisal of the origin and basal radiation of the Osteichthyes. 42 Sallan L.C.

Coates M.I. End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates. 43 Anderson P.S.L.

Friedman M.

Brazeau M.D.

Rayfield E.J. Initial radiation of jaws demonstrated stability despite faunal and environmental change. 44 Chang, M.-M. (1995). Diabolepis and its bearing on the relationships between porolepiforms and dipnoans. Bulletin du Muséum National d’Histoire Naturelle, Paris, 4e Série 17, 235–268. 16 Zhu M.

Yu X. A primitive fish close to the common ancestor of tetrapods and lungfish. 43 Anderson P.S.L.

Friedman M.

Brazeau M.D.

Rayfield E.J. Initial radiation of jaws demonstrated stability despite faunal and environmental change. Recognition of Meemannia as an actinopterygian punctuates a puzzling stratigraphic gap for half of the bony fish tree of life. Despite numerous reports of candidate ray-finned fishes from the Early Devonian and late Silurian [], few of these remains can be placed with confidence []. Characters recently proposed as indicating actinopterygian affinity for this material []—an anterodorsal process on the scale, peg and socket articulation, and multilayered enamel—only diagnose membership of total-group Osteicththyes. While considerable sarcopterygian diversity is known from the earliest Devonian, and increasingly the late Silurian, the oldest unequivocal actinopterygian material is some 30 million years younger []. By this point, even deep-diverging ray-fin lineages like Cheirolepis had acquired considerable specializations, pointing to an extensive—but as yet unsampled—evolutionary history []. It has been hypothesized that this gap stems from low actinopterygian richness and abundance [], a pattern apparent throughout the Devonian record []. Meemannia reinforces this pattern; in contrast to hundreds of sarcopterygian and placoderm fossils known from the shallow marine Xitun Formation, it is represented by only five specimens. The mandibles of co-occurring sarcopterygians point to considerable early trophic specialization in that group, with examples like toothplates and a palatal bite in Diabolepis [] and the combination of a long adductor fossa, short dentary, and double jaw joint in Styloichthys []. By contrast, the lower jaws attributed to Meemannia lack such extreme modifications and, like those of other early ray-finned fishes, fall near the center of a function space for early gnathostome mandibles [].