Although relationships among the major groups of living gnathostomes are well established, the relatedness of early jawed vertebrates to modern clades is intensely debated. Here, we provide a new description of Gladbachus , a Middle Devonian (Givetian approx. 385-million-year-old) stem chondrichthyan from Germany, and one of the very few early chondrichthyans in which substantial portions of the endoskeleton are preserved. Tomographic and histological techniques reveal new details of the gill skeleton, hyoid arch and jaws, neurocranium, cartilage, scales and teeth. Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms Gladbachus as a stem chondrichthyan and corroborates hypotheses that all acanthodians are stem chondrichthyans. The unfamiliar character combination displayed by Gladbachus , alongside conditions observed in acanthodians, implies that pre-Devonian stem chondrichthyans are severely under-sampled and strongly supports indications from isolated scales that the gnathostome crown group originated at the latest by the early Silurian (approx. 440 Ma). Moreover, phylogenetic results highlight the likely convergent evolution of conventional chondrichthyan conditions among earliest members of this primary gnathostome division, while skeletal morphology points towards the likely suspension feeding habits of Gladbachus , suggesting a functional origin of the gill slit condition characteristic of the vast majority of living and fossil chondrichthyans.

1. Introduction

The early evolution of the Chondrichthyes (cartilaginous fishes) has long been obscured by an impoverished fossil record [1–3]. This has only recently been improved through discoveries of partly articulated bodies [4–7] and braincases from the Lower and Middle Devonian [8–10], coupled with computed tomography (CT) [11–16]. The Lower and Middle Devonian record of chondrichthyans remains sparse, but the influx of high-quality data from slightly younger material, especially from the Upper Devonian and Carboniferous [14–17], combined with insights from earlier studies [18–21] has transformed our understanding of the early evolution of the crown group (Holocephali and Elasmobranchii). In a complementary manner, the origin of total-group Chondrichthyes has been amended by serial large-scale analyses of early gnathostome phylogeny [22–27], which consistently recover acanthodians [28] as stem chondrichthyans. However, because the acanthodian and chondrichthyan taxa included in these data matrices are largely unaltered from Brazeau [29] and Davis et al. [30], these results are not truly independent.

Here, we present a CT analysis and redescription of Gladbachus adentatus [31] from the Middle Devonian (Givetian approx. 385-million-year-old) of Germany, one of the earliest chondrichthyans known from articulated remains. Unlike Doliodus [1,4–7], the earliest and most completely described ‘unambiguous’ stem chondrichthyan [32], Gladbachus has never been included among acanthodians, but, like Doliodus, recent analyses and discussion [16,32] suggest that it might illuminate conditions bridging the acanthodian–chondrichthyan transition. Thus, a primary aim of the present work is to test the assumed chondrichthyan affinity of Gladbachus in light of the current acanthodians-as-stem-chondrichthyans hypothesis. Here, we have constructed a new early gnathostome database, with an augmented chondrichthyan component with taxa and characters from analyses by Pradel et al. [14], Coates et al. [16] and Coates & Tietjen [17]. This analysis presents the most detailed context, thus far, to reconstruct the evolutionary assembly of the chondrichthyan morphotype: a body plan that has persisted, more or less conservatively, for at least 370 Myr.

2. Material and methods

(a) Specimens

The subject of this study, Gladbachus adentatus [31] is known from a single, dorsoventrally compressed individual, UMZC (University Museum of Zoology, Cambridge, UK) 2000.32 [31,33], collected from the Lower Plattenkalk, Upper Givetian, Upper Middle Devonian, of Unterthal, Bergisch Gladbach (Germany). The specimen (electronic supplementary material, figure S1) consists of three pieces embedded within a rectangular slab of resin, with only the dorsal surface visible for direct inspection.

(b) Computed tomography, anatomical reconstruction and histological thin sections

Whole specimen scans: large-scale scans were completed by the high-resolution X-ray CT facility at the University of Texas at Austin (UTCT, www.digimorph.org).

Scales, denticles, teeth and calcified cartilage were examined using synchrotron µCT: all data were collected at beamline 13-BM-D at the Advanced Photon Source at Argonne National Laboratory. Image reconstruction used the GSECARS tomography processing software (http://cars9.uchicago.edu/software/idl/tomography.html), which dark-current corrects and white-field normalizes acquired data prior to performing gridding-based image reconstruction. Further details are provided in the electronic supplementary material.

Histological thin sections: doubly polished thin sections were studied using a Zeiss Axioskop Pol microscope equipped with Nomarski DIC optics.

Anatomical reconstruction: Mimics v. 17 (biomedical.materialise.com/mimics; Materialise, Leuven, Belgium) was used for the three-dimensional modelling, including segmentation, three-dimensional object rendering, STL polygon creation and kinematics. 3D Studio Max (Autodesk.com/products/3ds-max; Autodesk, San Rafael, USA) was used for further editing of the STLs (colour, texture, lighting), kinematics and mirroring for the final restoration.

(c) Phylogenetic and phenetic analysis

The phylogenetic data matrix are developed from sources including iterations of the early gnathostome data matrix by Brazeau [29], Davis et al. [30] and Zhu et al. [22]; most recently updated by Lu et al. [23], Qiao et al. [26], Zhu et al. [27] and Burrow et al. [34]. Chondrichthyan content includes substantial additions of new data drawn from Pradel et al. [14,15], Coates et al. [16] and Coates & Tietjen [17], and observation of original specimens (electronic supplementary material).

Phylogenetic methods: the primary character matrix consists of 84 in-group taxa and two out-group taxa (Galeaspida and Osteostraci) coded for 262 characters. Character and taxon sampling sources and discussion are provided in the electronic supplementary material. Phylogenetic analyses used maximum-parsimony implemented in PAUP* 4.0.152 [35]. Nodal support was assessed via bootstrapping [36] and Bremer Decay Indices [37], carried out using AutoDecay [38] and PAUP*. Details of phylogenetic methods are provided in the electronic supplementary material. Character state transitions by node for the strict consensus cladogram of the MPTs were reconstructed in PAUP* assuming hard polytomies with DELTRAN [39] optimization (see Davis et al. [30], Coates et al. [16]).

A principal coordinate (PCO) analysis [40,41] was performed on the Hamming distance matrix [42] of the character data. Computed dissimilarity was restricted to characters coded for both taxa, and distances were normalized to the number of characters coded for both members in each taxon pair. For the PCO, all characters were treated as equally weighted and unordered.

3. Results

(a) Specimen description, including (b) results of computed tomography, anatomical reconstruction and thin section histology

Head length including the gill skeleton (figure 1a,b) is approximately 21 cm, and head plus trunk length as preserved with the caudal region mostly absent is approximately 60 cm (electronic supplementary material, figure S1), implying a total body length of approximately 80 cm. Figure 1. Gladbachus adentatus Heidtke & Krätschmer [31]. (a) Rendering of cranial and pectoral girdle remains in dorsal view and (b) ventral view; (c) mandibular tooth; (d) cranial roof scale; (e) branchial denticle; (f) trunk scale. All denticles and scales are rendered semi-transparent from micro-computed tomography scans. bhy, basihyal; chy, ceratohyal; hb, hypobranchial; mc, Meckel's cartilage; na, neural arches; nc, neurocranium; or, orbital ring; pop, postorbital process; pq, palatoquadrate; sco, scapulocoracoid.

Although considered one of the few ‘unambiguous sharks’ of the Lower and Middle Devonian [32], details of Gladbachus anatomy do not conform in a straightforward manner with contemporary models of early chondrichthyan anatomy. The internal skeleton consists of calcified cartilage with no perichondral bone, yet the cartilage surface lacks the tightly connected tesserae that is a hallmark of chondrichthyan skeletal anatomy [15,43]. Rather, most cartilage surfaces bear a mesh of continuously calcified ridges (electronic supplementary material, figure S2), broadly resembling the ‘wood-like’ [44] texture observed in some Mesozoic elasmobranchs. Discrete tesserae are visible only in the walls of the semicircular canals, but these are irregularly sized and shaped, with broad inter-tesseral spaces. Thin section histology and synchrotron microtomography show that poorly delineated tesserae are distributed elsewhere in the skeleton, but concealed beneath the continuously mineralized cartilage surface.

The dermal skeleton includes no large plates. Head scales (figure 1d) are mostly larger than trunk scales (figure 1f), and the lateral line runs between scales. However, scale shape, composition and histology are remarkable, as they resemble conditions observed in ‘placoderms’ [33,45], and differ markedly from polyodontode scales like those of Doliodus [4,46] and mongolepids [47,48]. In Gladbachus, scale and branchial denticle crowns consist of overlapping, mono-layered, cellular dentine tubercles (electronic supplementary material, figure S3), lacking neck and basal canals. The standard, total-group chondrichthyan scale growth pattern of areally apposed odontodes [32] is absent, as are growing monodontode scales [49], and the non-growing placoid scales characteristic of modern chondrichthyans. Instead, the reconstructed growth pattern of Gladbachus scales is linear and bidirectional. Most unusually for a chondrichthyan, the cranial scales are asymmetric, with irregular and inconsistent shapes. Fin spines, and spines associated with girdles and the flank region, are completely absent.

The anterior section of the braincase is not preserved (figure 1a,b), thus evidence of a precerebral fontanelle is unknown, contra Heidtke & Kratschmer [31]. The right postorbital process includes traces of a jugular canal, and a groove on the posterior surface, probably for articulation with the upper jaw (electronic supplementary material, figure S4). The basicranium is compressed against the subjacent visceral arches, and too poorly preserved to demonstrate the presence or absence of a ventral cranial fissure, or canals for all or part of the dorsal aorta network. Reconstructions of the vestibular, semicircular canals and ampullary spaces (electronic supplementary material, figure S4c) demonstrate that the otic capsules were large and widely separated across the midline. In extant gnathostomes, this degree of lateral separation is manifest only in embryonic forms, and resemble adult conditions observed in ‘placoderms’ [50–53]. A pair of ring-shaped structures flanking the dorsal ridge, next to the anterior lip of the persistent otico-occipital fissure probably represents endolymphatic duct openings. This location for the endolymphatic ducts is consistent with the absence of an endolymphatic fossa or single, median endolymphatic foramen, which characterizes all Recent and fossil conventional chondrichthyans.

The jaws, hyoid arch and gill skeleton are exceptionally complete (figures 1a,b and 2; electronic supplementary material, figure S5), providing ready comparison with recently described in-group [15,54] and out-group [55] examples. The mandibular arch morphology (electronic supplementary material, figure S6) is more conventional than previously understood. In contrast with previous descriptions [31,56], there is no palatal symphysis. The difference in length between the upper and lower jaws is considerable (figure 2e), and comparable to conditions in Acanthodes [30,57], implying that a significant portion of the upper dentition was borne on the underside of the neurocranium; presumably on the internasal plate (cf. Ptomacanthus [29]; Doliodus [5]). Notably, the scales and teeth (mixed) bordering the gape are preserved as continuous, subparallel bands spanning the inter-orbital space (figure 1a,b). The palatoquadrate is generally comparable to that of an early, conventional chondrichthyan (e.g. Orthacanthus [58]). The well-developed otic process bears a broad posterodorsal rim; the palatine process is broad and short, but there is no evidence of a flange or process contributing to a palatobasal articulation (figure 2a). Preserved most completely on the left side of the specimen, the anterior of the palate is thrust beneath the postorbital process (figure 1a). The section of jaw visible in front of the preserved portion of the braincase is the anterior extremity of Meckel's cartilage. The posterior portion of Meckel's cartilage is also exposed on the dorsal surface of the specimen, but rotated through 90°, such that the dorsal surface is compressed against the mesial surface of the palatoquadrate. Figure 2. Gladbachus adentatus Heidtke & Krätschmer [31]. (a) Palatoquadrate; (b) Meckel's cartilage; (c) jaws, hyoid arch and gill arches restored, dorsal view; (d) gills reconstructed ventral view; (e) reconstruction of articulated mandibular arch, left lateral view. adf, adductor fossa; bhy, basihyal; cbr, ceratobranchial; chy, ceratohyal; cop, copula; ebr, epibranchial; end, endolymphatic duct; fm/oc, foramen magnum/occipital cotylus; gl, glenoid; hb, hypobranchial; hy, hyomandibula; mp, mesial process; opr, otic process; pbr, pharyngobranchial; pop, postorbital process; ppr, palatine process; pq, palatoquadrate; q, quadrate condyle, unmineralized site of; ssc, semicircular canal network; ocf, otico-occipital fissure.

The hyoid arch (figures 1a,b and 2c,d) is morphologically distinct from the gill arches. There is no interhyal, and both the large and well-mineralized ceratohyal and slender first ceratobranchial articulate with a broad basihyal. The five gill arches (electronic supplementary material, figure S7) are positioned caudal to the braincase, as in non-holocephalan chondrichthyans. Epibranchials are present on the first four arches, with anteriorly directed simple pharyngobranchials (with no suprapharyngobranchials) present in the first three gill arches (figures 1a and 2c) as in osteichthyans and Ozarcus [15]. A pair of short, laterally directed cartilages medial to the base of the second gill provide the only evidence of hypobranchials. The fifth arch ceratobranchials are unusually broad, nearly rectangular and keeled along the anterior margin. Remarkably, and uniquely, these resemble the posteriormost ceratobranchials of Paraplesiobatis, a Lower Devonian ‘placoderm’ [55] (electronic supplementary material, figure S8). The ceratobranchials of Gladbachus' fourth and fifth gill arches articulate with a large basibranchial copula, which is separated from the basihyal process by a large gap. This revised description provides the first accurate association of dorsal to ventral parts of each arch.

Although reported and named as toothless, Gladbachus possesses a dentition of small, mono-, bi- and tri-cuspid teeth lining the jaws, with branchial denticles lining gill arches I–IV (figure 1a–c; electronic supplementary material, figure S3). The teeth are individually separate, and despite a suggestion of lingual to labial alignment, there is no trace of whorl-like families as in conventional, non-holocephalan, chondrichthyans.

(c) Phylogenetic analysis and principal coordinates analysis

Phylogenetic analysis of the dataset returned 249 600 most parsimonious trees (TL = 691, CI = 0.396, RI = 0.785, RCI = 0.311). The strict consensus cladogram of MPTs (figure 3a; electronic supplementary material, figure S11) strongly corroborates recent phylogenetic hypotheses of early gnathostomes [23–27], reconstructing all taxa usually referred to as acanthodians in a paraphyletic assemblage branching from the chondrichthyan stem. Here, Gladbachus is also recovered as a stem chondrichthyan, as the sister taxon to a poorly resolved set of climatiid acanthodians and conventional chondrichthyans (including crown clade Chondrichthyes). A monophyletic group uniting diplacanthid, ischnacanthid and acanthodid acanthodians forms a clade that is the sister group of all other total-group chondrichthyans, including Gladbachus. Figure 3. Phylogenetic placement of Gladbachus adentatus Heidtke & Krätschmer [31]. (a) parts (i) and (ii) join at arrowheads. Strict consensus, complete tree. (b) Strict consensus of chondrichthyan total-group obtained from reduced taxon set; arrowhead joins arrowhead of (a) part (i). Branch colours: black, stem-group gnathostomes; green, Osteichthyes; magenta, acanthodid stem Chondrichthyes; red, non-acanthodid stem Chondrichthyes; purple, Holocephali (crown Chondrichthyes); blue, Elasmobranchii (crown Chondrichthyes). Circles mark nodes with bootstrap support greater than 50% and/or decay values greater than 1; filled circles mark nodes with bootstrap support greater than 75% and/or decay values greater than 3. (c) Phenetic results: PCO 1 (18.1% explained variance) is plotted on the vertical axis and PCO 2 (9.6%) is plotted on the horizontal axis. (d) PCO 1 (vertical) versus PCO 3 on the horizontal axis (8.1%). Details of the PCO analysis are presented in the electronic supplementary material. The four traditionally named groups (placoderms in purple, acanthodians in green, chondrichthyans in blue, osteichthyans in red) cluster in distinct non-overlapping regions on the first three PCO dimensions. Relevant stem chondrichthyan taxa are indicated in each plot.

The widely discussed Lower and Middle Devonian sharks Doliodus and Pucapampella branch from close to the apex of the chondrichthyan stem. Pucapampella is recovered in an uncertain position relative to several ‘acanthodian’ genera and the clade of conventional chondrichthyans. Doliodus is recovered as a sister taxon to conventional chondrichthyans.

Contra Qiao et al. [26] and Zhu et al. [27], Ramirosuarezia is not recovered among ‘acanthodian’ stem chondrichthyans, but rather, is nested among stem gnathostomes, with other taxa, such as Qilinyu, Entelognathus and Janusiscus branching crownward of the paraphyletic placoderms.

Within the chondrichthyan crown group, xenacanths and ctenacanths (sensu lato) form a clade branching from the elasmobranch stem. A further, poorly resolved cluster, including Homalodontus, Tristychius, Acronemus and hybodontids, branches from more crownward nodes, suggesting successive sister groups to the elasmobranch crown and close relatives. Holocephalans include the symmoriids, corroborating the arrangement found in Coates et al. [16].

A phylogenetic analysis on a reduced sample of chondrichthyans, focusing on the relationships among stem members (figure 3b), recovered 24 MPTS, with a mostly resolved set of relationships among acanthodians along the chondrichthyan stem. In this reduced analysis, Gladbachus again branches from within the ‘acanthodians’, suggesting that the position of Gladbachus within this paraphyletic assemblage is not the result of noise introduced into the dataset by the large chondrichthyan sample.

PCO analysis of the character data recovers all four of the traditional gnathostome divisions (‘placoderms’, ‘acanthodians’, osteichthyans and chondrichthyans) as discrete clusters in the space defined by the first three PCO axes (figure 3c,d). Notably, Gladbachus clusters with chondrichthyans in the PCO, despite its phylogenetic position among ‘acanthodians’. Gladbachus, Doliodus and Pucapampella each occupy positions in PCO space between conventionally defined chondrichthyans and ‘acanthodians’; however, these three genera are all significantly closer in PCO space to chondrichthyan taxa than to ‘acanthodians’ (t-tests of the inter-taxon distances in PCO space for Gladbachus, Doliodus and Pucapampella yield p-values of 1.39 × 10−6, 0.01 and 8.65 × 10−8, respectively).

4. Discussion

(a) Tree shapes and implications for evolutionary timescale

Owing principally to its plesiomorphic scale conditions and the absence of a dentition consisting of toothwhorls, Gladbachus is reconstructed close to the base of chondrichthyan total-group (figure 3a,b), removed from Doliodus and Pucapampella, which have traditionally been ascribed to the Chondrichthyes, but interleaved among taxa normally referred to as ‘acanthodians’. Accordingly, despite a phenetic similarity to conventionally defined chondrichthyans, phylogenetically, Gladbachus is an acanthodian-grade stem chondrichthyan.

Support for the acanthodian branching pattern is weak (electronic supplementary material, figure S11), but consistent with recent analyses [23–27]. Recent reconstructions have recovered traditional acanthodian family-level sets: acanthodids, ischnacanthids, diplacanthids and climatiids (electronic supplementary material, figure S9). In all of these trees, the climatiids group with conventional chondrichthyans, echoing results of Brazeau [29] and Davis et al. [30]. Diplacanthids, ischnacanthids and acanthodids fall into one of two arrangements: (i) as successive sister groups to more crownward taxa [25,27], or (ii) as in the present analysis, a monophyletic clade [23,24]. Here, we propose resurrecting the term Acanthodii to define the diplacanthid–ischnacanthid–acanthodid clade.

Support for the chondrichthyan crown clade is strong (figure 3a), introducing new data for the elasmobranch branch and corroborating the topology found in Coates et al. [16]. A time-calibrated phylogeny using the strict consensus tree (figure 4) places the origin of the crown group at least as early as the end-Middle Devonian. The initial evolutionary radiation of crown chondrichthyans is primarily post-Devonian, forming a significant component of the vertebrate recovery after the end-Devonian Hangenberg extinction [60,61], which is evident from faunas recorded at Lower Carboniferous localities such as Glencartholm [62], Bearsden [62] and Bear Gulch [63,64]. Figure 4. Early chondrichthyan phylogeny: simplified strict consensus of phylogenetic analysis results calibrated against Ordovician–Carboniferous chronostratigraphic chart. Consensus computed from matrix with 86 taxa and 262 characters. Taxon bar colour: black, non-chondrichthyan; magenta, acanthodid stem chondrichthyan; red, non-acanthodid stem chondrichthyan; purple, holocephalan crown chondrichthyan; blue, elasmobranch crown chondrichthyan. Bar length corresponds to earliest occurrence stage duration. Brown bar signifies the total range of chondrichthyan-like scale-based taxon. Timescale (Ma) from Cohen et al. [59]. Complete cladogram shown in figure 3a.

Conventional chondrichthyan conditions, exemplified by Doliodus [4], are present by the middle-Lower Devonian (Pragian: approx. 410 Ma), and a minimum date for the origin of the chondrichthyan total-group is currently tethered to the late Silurian (Ludlow: approx. 423 Ma) by the earliest well-preserved osteichthyan (Guiyu) [65]. However, the earliest ‘acanthodian’ stem-chondrichthyan body fossils (Nerepisacanthus) are only slightly younger (Pridoli: approx. 419 Ma) [66], and a wide variety of ‘acanthodians’ are known from the Lower Devonian (e.g. Ptomacanthus, Brochoadmones, Cassidiceps and Promesacanthus [29,67–70]). Such diversity, first apparent in the Lochkovian (figure 4) supported by a taphonomically biased record of articulated specimens, implies either a sudden radiation in the early Lower Devonian or a severely under-sampled history of Silurian stem sharks. The latter hypothesis is supported by isolated scales scattered through the Middle Ordovician to Silurian [48,49], including strikingly characteristic, classically defined ‘acanthodian’ scales from the Rhuddanian (Llandovery approx. 440 Ma) of the Siberian Platform [71]. In the present phylogenetic context, we prefer to combine the micro- and macro-/articulated fossil records, which strongly suggest that the chondrichthyan total-group, and, therefore, the gnathostome crown node, dates to at least the earliest Silurian, approximately 440 Ma. Thus, the early history of chondrichthyans consists of two phases (figure 4): a Silurian–Devonian evolutionary radiation of micromeric, acanthodian-like taxa, and a subsequent Carboniferous radiation of the crown clade, initially dominated by holocephalans [16,64].

(b) Palaeobiological inferences

Gladbachus adds to an increasingly populated chondrichthyan stem lineage that also includes Acanthodes [24,30], Ptomacanthus [29,67], Pucapampella [8–10] and Doliodus [4–7]. The resultant data on early chondrichthyan morphological diversity captures endoskeletal details comparable to the content of early osteichthyans, contributing to a more balanced interpretation of the initial gnathostome radiation. However, there is no straightforward emerging sequence of character acquisition for the chondrichthyan crown group (crown clade apomorphies are listed in electronic supplementary material, figure S10). The current analysis highlights conflicting patterns of character-state distributions, implying repeated and convergent evolution of chondrichthyan-like specializations among the earliest total-group members. For Gladbachus, PCO analysis clearly identifies the chondrichthyan-like nature of its body plan (figure 3c), but this stands in marked contrast to its reconstruction as representative of a previously unrecognized ‘acanthodian’ lineage (figures 3a,b and 4). Gladbachus approaches a quantifiably defined shark space, but does so from a phylogenetically distinct origin (electronic supplementary material, figure S10).

The character combination observed in Gladbachus, alongside the array of contrasting conditions observed in Early Devonian acanthodians (figure 4), defies conventional hypotheses of morphologically segregated acanthodian and chondrichthyan morphotypes, reinforcing the hypothesis that pre-Devonian stem-chondrichthyan diversity is fundamentally under-sampled. Reasons for the absence of substantial Silurian remains of crown gnathostomes are unclear, although restricted environmental specificity has been conjectured [72]. Gladbachus is a morphotypic outlier, in the sense that although phylogenetically placed within the acanthodian grade, it lacks fin spines, its scales lack synapomorphies shared with any acanthodian subgroup and its estimated body length (electronic supplementary material, figure S1) is two to three times greater than contemporary or earlier ‘acanthodians’, with the notable exception of gyracanthids [61]. Furthermore, several features of the skeletal morphology suggest that Gladbachus was a continuous ram suspension feeder [73], somewhat like modern basking sharks (Cetorhinus). The head, including the gill skeleton, accounts for approximately 25% of estimated total body length, the reconstructed oral aperture is likely to have been in a near-perpendicular plane to the direction of forward movement, the dentition is minimal, and the lower jaw is long and slender (figure 2b,c). To the best of our knowledge, this is the earliest combination of such features known in any jawed vertebrate, adding to an emerging picture of total-group chondrichthyans as early, nektonic specialists, in contrast with the reconstructed demersal habits of their heavily skeletonized osteichthyan and ‘placoderm’ contemporaries [74]. Aspects of this character suite occur repeatedly among stem chondrichthyans, suggesting that the familiar gill slit condition of sharks might originate from such early, and apparently multiple, natural experiments in suspension feeding.

5. Conclusion

Gladbachus offers a glimpse of early chondrichthyan diversity yet to be discovered. Significantly, Gladbachus scales, if discovered as isolated specimens, would be unrecognizable as chondrichthyan in the new, total-group sense, unlike an increasing variety of Silurian and Ordovician [47–49,75] scale-based taxa assigned with increasing confidence to the chondrichthyan total-group. Insights offered by Gladbachus and other early chondrichthyans suggest that the morphological disparity in the early members of the chondrichthyan total-group was probably substantially greater than that which is observed in the more-or-less stable shark morphotype which has persisted from the Middle Devonian through to the present. Accordingly, the importance of Gladbachus lies in its apparent morphological incongruence with its phylogenetic position, hinting at multiple paths leading to the modern shark-like body plan.

Data accessibility

Data are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.84mh3 [76].

Authors' contributions

M.I.C. and J.A.F. conceived the idea and designed the research. K.E.C. provided additional input. I.J.S. and P.S.A. provided all thin section histological analyses, related figure preparations and comparative scale data. K.E.C. and M.I.C. completed initial CT renderings. M.K.T. generated present CT renderings and produced the figures. K.E.C. provided comparative developmental data. M.L.R. and P.J.L.R. conducted the synchrotron CT scanning. J.A.F. and M.I.C. conducted phylogenetic analyses. J.A.F. conducted PCO analysis. The manuscript was drafted by M.I.C. with significant input from J.A.F., I.J.S. and K.E.C.

Competing interests

The authors declare no competing interests.

Funding

This work was supported by grants DEB-0917922 and DEB 1541491 from the National Science Foundation (USA) (to M.I.C.). GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1128799) and Department of Energy- GeoSciences (DE-FG02-94ER14466). The Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, is operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

Acknowledgements We thank J.A. Clack and M. Lowe for specimen loan, U. Heidtke for provision of materials recording early preparation of specimen and K. A. Hardy for assistance with CT renderings. We thank the GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory for access and support to complete significant portions of this work.

Footnotes

Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare.c.3952948.