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Acanthodes lopatini Rohon, histology of spines and scales: —A. – cross-section of the distal part of the pectoral spine IG KSC-215/10-1, Medvedskoe locality, Lower Os’kin Subformation; —B–D. fragment of the medium part of the left pectoral spine of IG KSC-215/1, Medvedskoe locality, Lower Os’kin Subformation (was glued back to the specimen after observation): —B. cross-section of the distal margin, —C. cross-section of the proximal margin, —D. fragment immersed in anize oil in lateral view; —E. vertical longitudinal section of trunk scale IG KSC-215/1–1, Medvedskoe locality, Lower Os’kin Subformation. Scale bars: A, 0.1 mm; B, C, 0.2 mm; D, 0.5 mm; E, 0.05 mm.
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Investigation of numerous well-preserved specimens of Early Carboniferous acanthodians collected over recent decades from southern central Siberia allowed their redescription as Acanthodes lopatini Rohon, 1889. The morphological characteristics supporting this classification, some peculiarities of ontogeny and a new reconstruction of this species a...
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Context 1
... of A. fritschi and Acanthodes sp. from the Upper Carboniferous of the Czech Republic (Zajic 1985, fig. 5; Zajic 1998, figs 59-77). The central part of the spine is represented by a large 'pulp' cavity (main longitudinal canal). It has a circular to oval cross-section in the proximal part of the spine and is rather polygonal in the distal part (Fig. 6B,C). Proximally the 'pulp' cavity opens into the 'pulp' groove occupying about one-fifth of the spine length. Relatively thin longitudinal canals parallel to the main axis of the spine (zone d sensu Zajic 1985) are located posteriorly from the 'pulp' cavity in the distal half of the spine (Fig. 6A). Their quantity is directly proportional ...
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... spine and is rather polygonal in the distal part (Fig. 6B,C). Proximally the 'pulp' cavity opens into the 'pulp' groove occupying about one-fifth of the spine length. Relatively thin longitudinal canals parallel to the main axis of the spine (zone d sensu Zajic 1985) are located posteriorly from the 'pulp' cavity in the distal half of the spine (Fig. 6A). Their quantity is directly proportional to the size of the specimen and can reach several units. Numerous fine and short canals oblique to the main axis of the spine (zone b sensu Zajic 1985) occupy the anterior part of the spine (Fig. 6D). Unpaired dorsal, anal and ventral spines possess similar morphology. Their histological ...
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... (zone d sensu Zajic 1985) are located posteriorly from the 'pulp' cavity in the distal half of the spine (Fig. 6A). Their quantity is directly proportional to the size of the specimen and can reach several units. Numerous fine and short canals oblique to the main axis of the spine (zone b sensu Zajic 1985) occupy the anterior part of the spine (Fig. 6D). Unpaired dorsal, anal and ventral spines possess similar morphology. Their histological structure has not been ...
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... inner structure of A. lopatini scales is also identical to that in A. bronni (Gross 1947 : fig. 18). The base is composed of compact acellular bone with numerous narrow canals of Williamson. The system of ascending dentine canals is well developed in the neck and lower part of the crown; the crown surface is covered by layers of enameloid (Fig. ...
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Citations
... The unpaired pelvic fin is a diagnostic character for the family Acanthodidae (Zajíc, 1995). Furthermore, in Acanthodidae, paired lateral line canals are located in front of the unpaired pelvic fin, indicating the secondary nature of the unpaired state (Beznosov, 2009). ...
The origin of paired appendages became one of the most important adaptations of vertebrates, allowing them to lead active lifestyles and explore a wide range of ecological niches. The basic form of paired appendages in evolution is the fins of fishes. The problem of paired appendages has attracted the attention of researchers for more than 150 years. During this time, a number of theories have been proposed, mainly based on morphological data, two of which, the Balfour-Thacher-Mivart lateral fold theory and Gegenbaur's gill arch theory, have not lost their relevance. So far, however, none of the proposed ideas has been supported by decisive evidence. The study of the evolutionary history of the appearance and development of paired appendages lies at the intersection of several disciplines and involves the synthesis of paleontological, morphological, embryological, and genetic data. In this review, we attempt to summarize and discuss the results accumulated in these fields and to analyze the theories put forward regarding the prerequisites and mechanisms that gave rise to paired fins and limbs in vertebrates.
... These represent a small-sized (up to 10 cm) fusiform acanthodian, with scales reaching up to, but not covering, the head region of the fish. They likely belong to the genus Acanthodes based on the presence of an unpaired ventral spine and fusiform shape (Beznosov, 2009). From spine length and positioning, along with similarities in the sensory line positioning and orbital bone count, the specimens from the OBF most closely resemble the species Acanthodes lopatini, from the earliest Carboniferous Os'kin Formation in Siberia, Russia (Beznosov, 2009(Beznosov, , 2017. ...
... They likely belong to the genus Acanthodes based on the presence of an unpaired ventral spine and fusiform shape (Beznosov, 2009). From spine length and positioning, along with similarities in the sensory line positioning and orbital bone count, the specimens from the OBF most closely resemble the species Acanthodes lopatini, from the earliest Carboniferous Os'kin Formation in Siberia, Russia (Beznosov, 2009(Beznosov, , 2017. However, the dorsal spine is missing from the OBF specimens, so it is difficult to determine if they represent a new species or are conspecific with Acanthodes lopatini. ...
The early Tournaisian (Carboniferous) stage represents a key episode in the evolution of vertebrates. It follows the end-Devonian Hangenberg extinction event, which led to a major perturbation to both terrestrial and aquatic vertebrate ecosystems, and resulted in a significant restructuring of assemblages. However, few faunal associations of this age have been described, and our understanding of faunal turnover across the Devonian-Carboniferous boundary remains poor. In this paper, we present an analysis of coprolite material from early Tournaisian lacustrine facies at Celsius Bjerg on Ymer Ø in East Greenland, which overlies the world-famous latest Devonian tetrapod-bearing localities. Fifty-five coprolite specimens (defined as a single coprolite or a piece of shale containing coprolites) were analysed using propagation phase-contrast synchrotron microtomography (PPC-SRμCT). Through a study of external morphology, shape and size combined with information about internal structures, we categorise coprolite morphotypes, and interpret their origin. Notably, we identify a greater number of coprolite morphotypes compared to vertebrate taxa known from skeletal material, indicating the existence of a cryptic ecosystem that has not yet been recovered as body fossils. Vertebrate diversity in the immediate aftermath of the late Devonian extinction is inferred to have been higher than expected, and might have included transient faunal elements within an open system, perhaps involving marine basin connections. Our results show that coprolites offer an alternative fossil data source, revealing diversity that is otherwise not always captured by the skeletal record.
... Chemical composition of the otoconia and otoliths has been uniformly reported to consist of four calcium carbonate polymorphs (calcite, vaterite, aragonite or calcium carbonate monohydrate) in extant gnathostomes [40][41][42][43]. In acanthodians, the otoliths consist of calcite [13,[44][45][46][47]. By comparison, the otoconia and amorphous otoliths in extant jawless agnathans consist of calcium phosphate in the form of apatite [8,24,48]. ...
... Otolith structures are widespread among all major chondrichthyan clades and consist of two pairs in most taxa, not [51], acanthodian distribution [52], basal actinopterygians [53] and sarcopterygians [54] and extant chondrichthyan data [55,56]. Otolith morphology and composition for the other major vertebrate groups was taken from literature: Acanthodii [13,44,45,47], Agnatha [8,27,48], Osteichthyes [8,40,41,43,57,58] exceeding three pairs. Two pairs were found in specimens investigated using CT scans only, while three otoliths were recognized in specimens investigated in situ using dissections. ...
... The difference between two pairs identified in micro CT scans and three pairs from in situ investigations is likely linked to limitations in resolution of these without prior staining of the specific structures, or low mineralization. In addition to our results, two to three singular otoliths have been reported in acanthodians [13,14,[44][45][46][47], which have recently been reinterpreted as stem chondrichthyans [e.g., 1,28,52]. The otoliths of acanthodians (as far as known) consist of calcite rather than the apatite we have identified in the elasmobranchs investigated here. ...
Background: Chondrichthyans represent a monophyletic group of crown group gnathostomes and are central to our understanding of vertebrate evolution. Like all vertebrates, cartilaginous fishes evolved concretions of material within their inner ears to aid with equilibrium and balance detection. Up to now, these materials have been identified as calcium carbonate-bearing otoconia, which are small bio-crystals consisting of an inorganic mineral and a protein, or otoconial masses (aggregations of otoconia bound by an organic matrix), being significantly different in morphology compared to the singular, polycrystalline otolith structures of bony fishes, which are solidified bio-crystals forming stony masses. Reinvestigation of the morphological and chemical properties of these chondrichthyan otoconia revises our understanding of otolith composition and has implications on the evolution of these characters in both the gnathostome crown group, and cartilaginous fishes in particular.
Results: Dissections of Amblyraja radiata, Potamotrygon leopoldi, and Scyliorhinus canicula revealed three pairs of singular polycrystalline otolith structures with a well-defined morphology within their inner ears, as observed in bony fishes. IR spectroscopy identified the material to be composed of carbonate/collagen-bearing apatite in all taxa. These findings contradict previous hypotheses suggesting these otoconial structures were composed of calcium carbonate in chondrichthyans. A phylogenetic mapping using 37 chondrichthyan taxa further showed that
the acquisition of phosphatic otolith structures might be widespread within cartilaginous fishes.
Conclusions: Differences in the size and shape of otoliths between taxa indicate a taxonomic signal within elasmobranchs. Otoliths made of carbonate/collagen-bearing apatite are reported for the first time in chondrichthyans. The intrinsic pathways to form singular, polycrystalline otoliths may represent the plesiomorphic condition for vertebrates but needs further testing. Likewise, the phosphatic composition of otoliths in early vertebrates such as cyclostomes and elasmobranchs is probably closely related to the lack of bony tissue in these groups, supporting a
close relationship between skeletal tissue mineralization patterns and chemical otolith composition, underlined by physiological constraints.
... The absence of an anterior dorsal fin is considered as a derived condition in acanthodiforms (Burrow, 2004;Denison, 1979;Hanke, 2002). Acanthodians generally have pelvic fins, although members of the Acanthodidae lack paired pelvic fin spines (Beznosov, 2009;Burrow & Young, 2005;Zajíc, 1995). ...
... Instead, Acanthodes species have a single ventral median spine inserted close behind the pectoral fins, often bearing a long and shallow fin-web (Beznosov, 2009;Heidtke, 1990;Zajíc, 1995Zajíc, , 1998. ...
Fishes are both extremely diverse and morphologically disparate. Part of this disparity can be observed in the numerous possible fin configurations that may differ in terms of the number of fins as well as fin shapes, sizes and relative positions on the body. Here, we thoroughly review the major patterns of disparity in fin configurations for each major group of fishes and discuss how median and paired fin homologies have been interpreted over time. When taking into account the entire span of fish diversity, including both extant and fossil taxa, the disparity in fin morphologies greatly complicates inferring homologies for individual fins. Given the phylogenetic scope of this review, structural and topological criteria appear to be the most useful indicators of fin identity. We further suggest that it may be advantageous to consider some of these fin homologies as nested within the larger framework of homologous fin‐forming morphogenetic fields. We also discuss scenarios of appendage evolution and suggest that modularity may have played a key role in appendage disparification. Fin modules re‐expressed within the boundaries of fin‐forming fields could explain how some fins may have evolved numerous times independently in separate lineages (e.g., adipose fin), or how new fins may have evolved over time (e.g., anterior and posterior dorsal fins, pectoral and pelvic fins). We favour an evolutionary scenario whereby median appendages appeared from a unique field of competence first positioned throughout the dorsal and ventral midlines, which was then redeployed laterally leading to paired appendages.
... For more than 30 years, ''acanthodian'' growth series have been recognised but frequently based on limited size series including already large individuals ( Chevrinais, Sire & Cloutier, 2017, see review). Nevertheless, more than 15 ontogenies have been documented: one possible Ischnacanthiformes (Nerepisacanthus denisoni ( Burrow & Rudkin, 2014)), two Diplacanthiformes (Diplacanthus horridus ( Cloutier et al., 2009), Uraniacanthus curtus ( Newman et al., 2012)), one Climatiiformes (Tetanopsyrus breviacanthias (Hanke, Davis & Wilson, 2001)), two species of uncertain order (Machaeracanthus goujeti ( Botella, Martinez-Perez & Soler-Gijon, 2012), Lupopsyrus pygmaeus ( Hanke & Davis, 2012)), and nine Acanthodiformes (Lodeacanthus gaujicus ( Upeniece, 1996;Upeniece, 2001;Upeniece & Beznosov, 2002), Triazeugacanthus affinis ( Chevrinais, Cloutier & Sire, 2015;Chevrinais, Balan & Cloutier, 2015;Chevrinais, Sire & Cloutier, 2017), Homalacanthus concinnus ( Cloutier et al., 2009), Acanthodes bridgei ( Zidek, 1985), A. bronni ( Heidtke, 1990), A. gracilis ( Zajic, 2005), A. lopatini ( Beznosov, 2009), A. ovensi ( Forey & Young, 1985), and an acanthodiform indet. ( Coates, 1993)). ...
... The record of growth lines in the otoliths in Triazeugacanthus ( Chevrinais, Cloutier & Sire, 2015, Annexe II) is congruent with observations already made by Gagnier (1996), who reported the presence of concentric growth zones enclosing minor secondary order zones. Three pairs of otoliths are recorded early in the ontogeny of Acanthodes lopatini ( Beznosov, 2009) and A. bronni ( Heidtke, 1990) (in which statoconia are followed by three otoliths in ontogeny) and osteichthyans, whereas they are absent in chondrichthyans ( Schultze, 1990). Schultze (1990) considered the presence of three pairs of otoliths as a synapomorphy shared by ''acanthodians'' and osteichthyans. ...
... The paired fin spines of Triazeugacanthus grow by distal accretion of odontodes. Small odontodes have also been observed in other acanthodiforms: pectoral, intermediate and pelvic fin spines in juvenile Lodeacanthus ( Upeniece, 1996;Upeniece, 2011), and paired fin spines of the juvenile A. lopatini ( Beznosov, 2009). In the so-called 'juveniles' of Tetanopsyrus breviacanthias (Lower Devonian), paired fin spines are completely formed by small odontodes (Hanke, Davis & Wilson, 2001). ...
The study of vertebrate ontogenies has the potential to inform us of shared developmental patterns and processes among organisms. However, fossilised ontogenies of early vertebrates are extremely rare during the Palaeozoic Era. A growth series of the Late Devonian ''acanthodian'' Triazeugacanthus affinis, from the Miguasha Fossil-Fish Lagerstätte, is identified as one of the best known early vertebrate fossilised ontogenies given the exceptional preservation, the large size range, and the abundance of specimens. Morphological, morphometric, histological and chemical data are gathered on a growth series of Triazeugacanthus ranging from 4 to 52 mm in total length. The developmental trajectory of this Devonian ''acanthodian'' is characteristic of fishes showing a direct development with alternating steps and thresholds. Larvae show no squamation but a progressive appearance of cartilaginous neurocranial and vertebral elements, and appendicular elements, whereas juveniles progress in terms of ossification and squamation. The presence of cartilaginous and bony tissues, discriminated on histological and chemical signatures, shows a progressive mineralisation of neurocranial and vertebral elements. Comparison among different body proportions for larvae, juveniles and adults suggest allometric growth in juveniles. Because of the phylogenetic position of ''acanthodians'', Triazeugacanthus ontogeny informs us about deep time developmental conditions in gnathostomes.
... Acanthodian species known from complete specimens are relatively rare compared to the number of taxa known solely from isolated scales [5][6][7]. Furthermore, only a few acanthodian ontogenies based on complete specimens have been discovered [1,8]: one possible ischnacanthiform [Nerepisacanthus denisoni [3]], two diplacanthiforms [Diplacanthus horridus [9] and Uraniacanthus curtus [10]], one "climatiiform" [Tetanopsyrus breviacanthias [11]], one species of uncertain order Lupopsyrus pygmaeus [12]], and nine acanthodiforms [Triazeugacanthus affinis [13,14], Lodeacanthus gaujicus [15][16][17][18], Homalacanthus concinnus [9], Acanthodes bridgei [19], A. bronni [20], A. gracilis [21], A. lopatini [22], A. ovensi [23]], and an acanthodiform indet. [24]. ...
Growth series of Palaeozoic fishes are rare because of the fragility of larval and juvenile specimens owing to their weak mineralisation and the scarcity of articulated specimens. This rarity makes it difficult to describe early vertebrate growth patterns and processes in extinct taxa. Indeed, only a few growth series of complete Palaeozoic fishes are available; however, they allow the growth of isolated elements to be described and individual growth from these isolated elements to be inferred. In addition, isolated and in situ scales are generally abundant and well-preserved, and bring information on (1) their morphology and structure relevant to phylogenetic relationships and (2) individual growth patterns and processes relative to species ontogeny. The Late Devonian acanthodian Triazeugacanthus affinis from the Miguasha Fossil-Lagerstä tte preserves one of the best known fossilised ontogenies of early vertebrates because of the exceptional preservation, the large size range, and the abundance of complete specimens. Here, we present morphological, histological, and chemical data on scales from juvenile and adult specimens (scales not being formed in lar-vae). Histologically, Triazeugacanthus scales are composed of a basal layer of acellular bone housing Sharpey's fibers, a mid-layer of mesodentine, and a superficial layer of ganoine. Developmentally, scales grow first through concentric addition of mesodentine and bone around a central primordium and then through superposition of ganoine layers. Onto-genetically, scales form first in the region below the dorsal fin spine, then squamation spreads anteriorly and posteriorly, and on fin webs. Phylogenetically, Triazeugacanthus scales show similarities with acanthodians (e.g. " box-in-box " growth), chondrichthyans (e.g. squamation pattern), and actinopterygians (e.g. ganoine). Scale histology and growth are interpreted in the light of a new phylogenetic analysis of gnathostomes supporting acantho-dians as stem chondrichthyans.
... Machaeracanthus goujeti n. sp. and M. peracutus also share the absence of outer ortho-or mesodentine layers, a condition which clearly distinguishes Machaeracanthus from the other acanthodians (Burrow et al. 2010b: 65). In this respect, we have to note here that the smooth longitudinal carination ornamenting the Machaeracanthus spines develop by the regular deposition of centrifugal dentine in constrast to the longitudinal ridges of other acanthodians (e.g., climatiforms, acanthodiforms) which develop by centripetal deposition of orthoor mesodentine (Denison 1979; Beznosov 2009). Interestingly, longitudinal striation with centrifugal dentine appears in dorsal spines of xenacanth sharks (cf. ...
We describe here a new machaeracanthid acanthodian (Machaeracanthus goujeti n. sp.), based on isolated spines, scales and scapulocoracoids from the Lower Devonian (Lochkovian-Pragian) of the Nogueras Formation, Celtiberia, Spain. The new taxon also includes a fragmentary spine and isolated scales from the Lower Devonian of northern Spain (Palencia and Cantabrian Mountains) and western France (Saint-Cenere) originally assigned to Machaeracanthus sp. The spines of M goujeti n. sp. comprise two morphotypes in agreement with the morphofunctional model of a pair of pectoral spines articulating with the pectoral girdle already indicated for M. hunsrueckianum Sudkamp 86 Burrow, 2007, M longaevus Eastman, 1907, and M. sulcatus Newberry, 1857. The morphology and size of the spines distinguish M goujeti n. sp. from the coeval species M bohemicus Barrande, 1872; the new species most closely resembles the younger species M per Newberry, 1857. The spines of M goujeti n. sp. consist of trabecular and lamellar dentine layers which form the wall of the central axis (pierced by a longitudinal pulp cavity) and lateral expansions. The most superficial layer of dentine is centrifugally deposited in the complete spine; this condition is found in fin spines of some chondrichthyans and contrasts with that observed in typical acanthodian fin spines where the exserted portion is ornamented with ribs of centripetally growing dentine. Very small spines and scapulocoracoids of M goujeti n. sp. described here, are the first report of juvenile specimens of a species of Machaeracanthus Newberry, 1857. The distal part of the juvenile spine lacks lateral expansions (keel and wing) and demonstrates the first stage in the development of the spine.
... The shark scales resemble those of a hybodontid/sphenacanthid and these plus the acanthodian support a late Famennian age; the palaeoenvironment might be marginal marine. The oldest known species of Acanthodes based on articulated specimens is A. lopatini , from the Tournaisian of Russia (Beznosov, 2009). While Acanthodes-type scales are recorded from many Middle–Late Devonian assemblages, their assignment to Acanthodes s.s. ...
Silurian vertebrate remains are rare in the Australasian region, mostly lacking from the end Ordovician to the mid-Ludlow presumably because of the purported Gondwana Ice Age. Thelodont, placoderm, acanthodian, ?stem actinopterygian, and probable chondrichthyan remains are known from eastern and western Australia and Irian Jaya. Of significance are the links between eastern Australia and China, with sinacanthid spines known only in these regions, and both having porosiform, but not punctatiform, poracanthodid acanthodians, while Western Australia, Iran and possibly Irian Jaya have similar thelodonts, all from shallow marine to evaporitic settings. Earliest Devonian (Lochkovian) vertebrate microfossils include placoderm taxa with circumArctic and Bohemian affinities, but post-Lochkovian marine assemblages comprise mainly endemic forms of turiniid thelodonts, placoderms, acanthodians, chondrichthyans, and sarcopterygians. Most of the Early Devonian marine assemblages from eastern Australia indicate tropical–subtropical depositional environments. From the Middle–Late Devonian, notable index taxa include Phoebodus spp. After the Frasnian-Famennian events, turiniids finally disappear in the Australian record, placoderms also become absent by the end of the period, acanthodians are increasingly dominated by acanthodiforms, and chondrichthyan and actinopterygian diversity increases. For the Carboniferous, vertebrate occurrences are early to mid-Mississippian, disappearing in the early Pennsylvanian in marine and non-marine environments.During the time span of IGCP 491, data on previously poorly known and many new taxa represented only by isolated remains have been analysed, and a wealth of new acanthodian taxa, a new Early Carboniferous tetrapod Ossinodus pueri, and several sarcopterygian taxa have been fully described.
... Also from the Carboniferous Mazon Creek faunal assemblage, abundant specimens of a larval fish, Esconichthys, were identified as a dipnoan with a delayed rate of ossification [82]; however, this identification is doubtful because Esconichthys lacks the characteristic dentition that devel-ops early in the ontogeny of dipnoans. Immature (larval and/or juvenile) fish associated with adult forms have been reported for Devonian heterostracans [47,48,[83][84][85][86][87][88][89], Devonian anaspids [11,90], Devonian osteostracans [11,19,49], Silurian thelodonts [46], Carboniferous [79,[91][92][93][94][95], Jurassic [96] and Cretaceous [97,98] chondrichthyans, Devonian placoderms [11,28,33,58,[99][100][101][102][103][104][105][106][107][108][109][110][111], Devonian [11,105,112,113], Carboniferous [114,115] and Permian [115] acanthodians, Carboniferous [13,26,116,117], Permian [118,119], Triassic [31,37,64,[120][121][122][123][124], Jurassic [12,125], Cretaceous [7,12,14,[126][127][128][129], Paleocene [130], Eocene [12,30,[131][132][133][134][135][136], Oligocene [130,137], Miocene [138][139][140][141] and Pliocene [142] actinopterygians, and Devonian [8,11,23,27,29,36,38,105,143,144], Carboniferous [145,146] and Triassic [147,148] sarcopterygians (Fig. 2). Fossilized ontogenies have been reported in the literature Phylogenetic and geological distribution of the fossilized ontogenies of fishes based on published works treated in this paper. ...
One of the properties of fossils is to provide unique ontogenies that have the potential to inform us of developmental patterns and processes in the past. Although fossilized ontogenies are fairly rare, size series of relatively complete specimens for more than 90 fish species have been documented in the literature. These fossilized ontogenies are known for most major phylogenetic groups of fishes and have a broad stratigraphic range extending from the Silurian to the Quaternary with a good representation during the Devonian. Classically, size series have been studied in terms of size and shape differences, where subsequently allometric changes were used as indicators of heterochronic changes in Paleozoic placoderms and sarcopterygians. Quantitative analyses of fossilized ontogenies of dipnoans have been interpreted in terms of morphological integration and fluctuating asymmetry. Recently, reconstructed sequences of ossification have been used to identify recurrent patterns of similar development in actinopterygians and sarcopterygians in order to infer phenotypic developmental modularity and saltatory pattern of development. Phylogenetic and temporal landmarks are put forward for some of the major developmental patterns in the evolution of fishes.
Acanthodians are a poorly understood paraphyletic grade of extinct Palaeozoic fishes. They play an increasingly prominent role in our understanding of vertebrate evolution as part of the chondrichthyan stem‐group even though their evolutionary history is scarce. The limited preservation of their mostly cartilaginous skeleton largely results in a bias towards isolated remains such as fin spines and scales. Here, we quantify the quality of the acanthodian fossil record by using a variation of the Skeletal Completeness Metric (SCM), an approach that calculates how complete the skeletons of individuals are compared to their theoretical complete skeleton. A novel Soft Tissue Completeness Metric (STCM) is introduced to estimate the percentage of soft body tissue preserved as an alternate measurement of completeness. Completeness scores for >1600 specimens comprising >300 taxa obtained from museum collection visits and literature surveys were assembled into a database. Acanthodian completeness peaks in the Lower–Middle Devonian, Pennsylvanian, and earliest Permian. There is no correlation between acanthodian taxonomic richness and completeness. Acanthodians show a significantly lower completeness distribution than many tetrapod groups, but a similarly low distribution to bats. Skeletons deposited in freshwater are significantly more complete than in marine environments where sea level significantly negatively correlates with observed completeness. Our assessment reveals only weak spatial biases influencing the acanthodian fossil record while environmental biases are much higher. This quantified evaluation of acanthodians provides a foundation for further assessments of the likely influence of character absences from morphological datasets on estimates of early chondrichthyan and, therefore, early gnathostome evolution.
