Ramsay Heatley Traquair, the eminent Victorian Scottish palaeoichthyologist and museum curator, procured an extensive collection of Palaeozoic fishes from across Scotland with the help of local miners and quarrymen. One very productive locality near Edinburgh was Loanhead. Traquair described numerous fossil fish from this Serpukhovian site, including four lungfish taxa: Ctenodus interruptus, Sagenodus quinquecostatus, Uronemus splendens and Ctenodus angustulus. The first three are now quite well known, but the fourth was only briefly described and never figured. It is based entirely on tooth plates, which are unusual both in their very small size and the arrangement of the tooth ridges. They lack the diagnostic characters of Ctenodus tooth plates and are here renamed Clackodus angustulus. A further taxon, Conchopoma sp., has recently been identified. Represented by a spade-shaped parasphenoid and denticulated jaw elements, it is the earliest known occurrence of the genus, extending its range into the Mississippian. A sixth taxon may be represented by an isolated parasphenoid bearing an anterior process, previously only seen in Devonian lungfish. The presence of up to six lungfish taxa at a single locality is unprecedented in the Carboniferous and indicates that the high level of lungfish diversity encountered in the Tournaisian of the Scottish Borders continued throughout the Mississippian, adding to the growing evidence that post-Devonian lungfish evolution was not as limited as previously proposed. This may have been due to changes in tooth plate growth, enabling greater variation in dentition and diet. In most Devonian taxa, tooth plate growth can be explained by comparison with that in extant forms, but analysis of Carboniferous tooth plates suggest growth was different in many taxa, possibly based on more than one pioneer tooth, allowing for novel patterns of tooth ridges and different types of teeth to develop on the same plate.

A revision of the Carboniferous lungfish genus Ctenodus is undertaken. Ctenodus is the longest-surviving genus of lungfish from the Carboniferous, and is the only one to cross the Lower–Upper Carboniferous boundary. Five species span the period, with species correlated to stage. Species are distinguished by differences in tooth plate morphology. Ridge number increases from the Tournaisian to the Moscovian, and the length to width ratio decreases. A new, almost complete articulated individual of C. interruptus allows identification of this species from pterygoid tooth plates and other characteristics. These, and new features of the postcranial skeleton, are described for the first time. Ctenodus retains primitive features of the skull roof, such as a pair of C-bones and retention of a D-bone; its contemporaries Conchopoma, Sagenodus and Straitonia show the derived condition of these features. Ctenodus retains primitive features of the lepidotrichia, which are distally segmented and bifurcated. However, it also shows the derived condition of the tail region in having a combined first and second dorsal fin: whether there was a separate caudal fin remains unknown.

The unique specimen of the Early Carboniferous lungfish Straitonia waterstoni is described in detail. As the only articulated lungfish currently described from the Viséan, it is of particular interest for lungfish evolution. It retains some plesiomorphic features of the skull, and shows some derived features in common with later Carboniferous forms. A thorough description is provided for future phylogenetic analyses. Straitonia has a number of diagnostic features. These include: a relatively narrow B-bone with a broad and curved anterior margin; an unpaired, parallel-sided C-bone whose margin is convex anteriorly; a B-bone approximately the same width as the single C-bone; the I-bones are relatively small compared with the B- and J-bones; the J-bones are tri-lobed; the XY bone is elongate along a posteromedial–anterolateral axis. Elongate fins dorsally and ventrally suggest a condition similar to those of later Carboniferous forms with confluent second dorsal, caudal and anal fins. It may thus be the earliest lungfish to show this condition. The ventral tail fin includes some fused elements, probably haemal and infrahaemal spines. The identity of one of the skull bones is discussed, and concluded to be a C-bone rather than a D-bone as previously described. The D-bone is absent, as in later Carboniferous forms. The history of the D-bone is considered.

To analyze myelin structure and the composition of myelinated tissue in the African lungfish (Protopterus dolloi), we used a combination of ultrastructural and biochemical techniques. Electron microscopy showed typical multilamellar myelin: CNS sheaths abutted one another, and PNS sheaths were separated by endoneurial collagen. The radial component, prominent in CNS myelin of higher vertebrates, was suggested by the pattern of staining but was poorly organized. The lipid and myelin protein compositions of lungfish tissues more closely resembled those of teleost than those of higher vertebrates (frog, mouse). Of particular note, for example, lungfish glycolipids lacked hydroxy fatty acids. Native myelin periodicities from unfixed nerves were in the range of those for higher vertebrates rather than for teleost fish. Lungfish PNS myelin had wider inter-membrane spaces compared with other vertebrates, and lungfish CNS myelin had spaces that were closer in value to those in mammalian than to amphibian or teleost myelins. The membrane lipid bilayer was narrower in lungfish PNS myelin compared to other vertebrates, whereas in the CNS myelin the bilayer was in the typical range. Lungfish PNS myelin showed typical compaction and swelling responses to incubation in acidic or alkaline hypotonic saline. The CNS myelin, by contrast, did not compact in acidic saline but did swell in the alkaline solution. This lability was more similar to that for the higher vertebrates than for teleost.

New cranial material of the ‘rhynchodipterid’ lungfish Soederberghia groenlandica from Upper Devonian (Famennian) strata in East Greenland is described. Previously unknown structures identified here include components of the hyoid arch (ceratohyal, hypohyal) and the lower jaw (prearticular, dentary). Earlier interpretations of the cheek and mandible of Soederberghia are reconsidered in the light of new fossil specimens. Some of the difficulties in assessing the homologies between cheek bones in Soederberghia and those of other lungfishes stem from confusion over the arrangement in Rhynchodipterus, and a revised interpretation of this genus is proposed. The single infraorbital bar found in Soederberghia probably originates, in part, from an expanded bone 10 (quadratojugal) of the kind found in Griphognathus. Hypotheses that posit ‘rhynchodipterid’ polyphyly seem unlikely in light of a set of derived cranial characters that define a coherent radiation of long-snouted, denticle-bearing lungfishes known from the Late Devonian. The hypothesis presented here places ‘rhynchodipterids’ as a paraphyletic grade with respect to fleurantiids. Rhynchodipterus, Soederberghia, and fleurantiids form a clade to the exclusion of the species of Griphognathus. G. minutidens is the sister taxon to this apical group, while G. sculpta and G. whitei are more remote from it.

Lungfish (order Dipnoi) evolved during the Devonian period and are believed to be the closest living relatives to the land vertebrates. Here we describe the previously unknown morphology of the lungfish eye in order to examine ocular adaptations present in early sarcopterygian fish. Unlike many teleosts, the Australian lungfish Neoceratodus forsteri possesses a mobile pupil with a slow pupillary response similar to amphibians. The structure of the eye changes from juvenile to adult, with both eye and lens becoming more elliptical in shape with growth. This change in structure results in a decrease in focal ratio (the distance from lens center to the retina divided by the lens radius) and increased retinal illumination in adult fish. Despite a degree of lenticular correction for spherical aberration, there is considerable variation across the lens. A re-calculation of spatial resolving power using measured focal ratios from cryosectioning reveals a low ability to discriminate fine detail. The dipnoan eye shares more features with amphibian eyes than with most teleost eyes, which may echo the visual needs of this living fossil.

Australian lungfish Neoceratodus forsteri may be the closest living relative to the first tetrapods and yet little is known about their retinal ganglion cells. This study reveals that lungfish possess a heterogeneous population of ganglion cells distributed in a horizontal streak across the retinal meridian, which is formed early in development and maintained through to adult stages. The number and complement of both ganglion cells and a population of putative amacrine cells within the ganglion cell layer are examined using retrograde labelling from the optic nerve and transmission electron-microscopic analysis of axons within the optic nerve. At least four types of retinal ganglion cells are present and lie predominantly within a thin ganglion cell layer, although two subpopulations are identified, one within the inner plexiform and the other within the inner nuclear layer. A subpopulation of retinal ganglion cells comprising up to 7% of the total population are significantly larger (>400 μm2) and are characterized as giant or alpha-like cells. Up to 44% of cells within the retinal ganglion cell layer represent a population of presumed amacrine cells. The optic nerve is heavily fasciculated and the proportion of myelinated axons increases with body length from 17% in subadults to 74% in adults. Spatial resolving power, based on ganglion cell spacing, is low (1.6–1.9 cycles deg−1, n = 2) and does not significantly increase with growth. This represents the first detailed study of retinal ganglion cells in sarcopterygian fish, and reveals that, despite variation amongst animal groups, trends in ganglion cell density distribution and characteristics of cell types were defined early in vertebrate evolution.