The material from Kansas studied here comprises two specimens belonging to the Kansas University Natural History Museum, Lawrence, Kansas, USA (KUNHM 22060, KUNHM 21894). A map showing the sources of this material is provided in Hamel and Poplin (2008). One specimen, OKM 38, comes from Oklahoma, belongs to the American Museum of Natural History, New York, and was loaned by Dr John Maisey (AMNH, New York). Images of the original nodules are provided in Pradel et al. (2009) and Pradel (2010).

The Kansas nodules crop out approximately 150 km southeast of Kansas City, between Lawrence and Baldwin, in about thirty different localities. Stratigraphically, they occur at the limit between the marine Haskell Limestone Member and the overlying Robbins Shale of the Stranger Formation, deposited in brackish water (see Pradel, 2010, for details on the paleoenvironment and the taphonomy). This sequence is part of the Douglas Group, dated as Late Virgilian, Upper Pennsylvanian (Ca 305–299 Myr).

The Oklahoma specimen was collected by Dr. Royal Mapes (Geology Department, Ohio University, Athens, Ohio, USA) from the Coffeyville Formation (dated as Pennsylvanian, Missourian, Ca 307 Myr), at a roadcut in Tulsa County, Oklahoma (Center of NW sec. 2., T. 18 N, R. 12 E, Sapulpa North 7½ Quadrangle).

KUNHM 22060 and OKM 38 display scattered elements of the pectoral girdle and fins, whereas the pectoral girdle and fin of KUNHM 21894 are well preserved, almost in natural position. Most of the illustrations are therefore based on the latter specimen.

OKM 38 has been scanned by μCT scan at the University of Texas High-Resolution X-ray CT Facility, Austin, USA. Scan parameters were as follows: 1024 × 1024 16-bit TIFF images. II, 180 kV, 0.12 mA, no filter, air wedge, no offset, slice thickness 2 lines (=0.04872 mm), S.O.D. 70 mm, 1400 views, 2 samples per view, inter-slice spacing 2 lines (=0.04872 mm), field of reconstruction 22.8 mm (maximum field of view 23.17 mm), reconstruction offset 8000, reconstruction scale 3200. The scan is acquired with 19 slices per rotation and 15 slices per set. Flash and ring-removal processing are based on correction of raw sinogram data by Alison Mote using IDL routines “RK_SinoDeSpike” and “RK_SinoRingProcSimul,” both with default parameters. There was no gantry tilt. The first four duplicate slices of each rotation are deleted, except for slices 1–4 and the rotation correction processing has made using the IDL routine “DoRotationCorrection.” Total no of final slices = 649.

KUNHM 21894 was imaged using X-ray based synchrotron microtomography in absorption contrast (SR-μCT) (Tafforeau et al., 2006) on beamline ID19 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Scan parameters were as follows: monochromatic X-ray beam of 60 keV energy. The detector was a FReLoN (Fast Readout Low Noise) (Labiche et al., 2007) CCD camera coupled with an optical magnification system, yielding an isotropic pixel size of 30.3 μm. 1200/180° projections with 0.4 s of exposure time. Data were reconstructed using the filtered backprojection algorithm (PyHST software, ESRF). Reconstructed slices were converted from 32 bits to 8 bits in order to reduce the data size for 3D processing.

Because KUNHM 22060 is much more mineralized than other specimens, absorption contrast was not sufficient to observe all the features present inside the nodules. Consequently, a more powerful technique based on propagation phase contrast was used on that specimen on the beamline ID19 of the ESRF (Tafforeau et al., 2006). That scan was done with a voxel size of 7.46 μm at an energy of 60 keV, with a propagation distance of 900 mm. In order to obtain 2.9 cm of lateral field of view, the scan was made in half-acquisition geometry, with the centre of rotation on one side of the field of view and a 360 degrees rotation. That technique allows one to nearly double the lateral field of view. We used 5000 projections over 360 degrees with an exposure time of 0.7 s for each of the 9 scans performed, each of them covering 5.8 mm vertically. Reconstruction was performed using an adapted filtered backprojection algorithm, allowing reconstruction with half-acquisition and local tomography (PyHST software, ESRF). After reconstruction, all the scans were concatenated in order to generate a single slices stack in tif 16 bits mode. A second version of that dataset with a 2 × 2 × 2 binning in 8 bits was then computed in order to reduce the data size for the general anatomy investigations.

Segmentation and 3D rendering were performed with MIMICS^® 13.1 software (Materialise Inc. NV, Leuven, Belgium) and with Volume Graphics VG Studio MAX^© 2.0 × 64.

Many of the illustrations in this work are images captured from MIMICS and VG Studio MAX, respectively in isosurface and volume renderings. Adobe Photoshop CS3 extended v.10.0.1 was used to optimize the contrast of the MIMICS and VG Studio MAX 3D renderings and make the final illustration.

Systematic paleontology

Class CHONDRICHTHYES Huxley, 1880

Order INIOPTERYGIA Zangerl and Case, 1973

Family SIBYRHYNCHIDAE Zangerl and Case, 1973

Genus INIOPERA Zangerl and Case, 1973

Type species: Iniopera richardsoni Zangerl and Case, 1973.

Etymology: from the Greek inion (= nape) and pera (= leathery pouch).

Iniopera sp. indet.

Examined material: KUNHM 22060, KUNHM 21894 and OKM 38 (deposited in the AMNH).

The pectoral girdle of sibyrhynchids consists of a pair of prominent, elongated and robust scapulocoracoids, a pair of small and thin triangular suprascapular cartilages and an unpaired intercoracoid basal plate (sc, supsc, intc, Fig. 1). The pectoral girdle and left pectoral fin of KUNHM 21894 are preserved, almost articulated, in a natural position (Fig. 1A). The scapular region of the scapulocoracoids lies immediately posterior to the neurocranium, or braincase (br, Fig. 1A), whereas the rest of the element is preserved almost horizontal, beneath the ventral surface of the braincase, with its anteroventral end reaching the mid-length of the braincase. Nevertheless, the scapulocoracoids may have been slightly displaced during fossilisation and were in fact more vertical in life (Fig. 1B). The general outline of the scapulocoracoid is L-shaped, with its ventral half bending forward, so that its anteroventral end was probably situated ventral to the otico-occipital region of the braincase, and its anteriormost and dorsalmost parts stood approximately at a right angle to one another. The anteroventral end of the scapulocoracoids are not fused together at the ventral midline, contrary to what is observed in most extant chondrichthyans (Compagno, 1999 and Didier, 1995). In contrast, the ventral end of the scapulocoracoids articulates with the intercoracoid cartilage (see below), along the posterolateral sides of the latter.

The suprascapular elements (see below) are situated dorsal to the scapular region of the scapulocoracoids.

The pectoral fin consists of a single robust proximal basal element (basipterygium), which articulates with the dorsolateral part of the scapulocoracoid, and bears a series of distal pectoral radials along its dorsal edge (bas, fr1, frp, Fig. 1A). The anteriormost pectoral fin distal radial is much larger than the posteriorly following ones, as in Iniopera richardsoni (Zangerl and Case, 1973: fig. 78).

In none of the Paleozoic chondrichthyans are the coracoids fused ventrally (Zangerl, 1981), and this condition is commonly supposed to be plesiomorphic for chondrichthyans (Stahl, 1999). An unpaired cartilage situated between the ventral extremities of the two coracoids is present in some modern hexanchiforms, such as Notorynchus (Haswell, 1884 and Kälin, 1931, Parker, 1891), which are considered as primitive neoselachians (Maisey et al., 2004). In all modern holocephalans (Chimaeroidei), the scapulocoracoids are fused ventrally. Nevertheless, a possible Jurassic Chimaeroidei (Didier, 1995), Ischyodus schuebleri, possesses an intercoracoid cartilage that articulates with the two scapulocoracoids ventrally (Heimberg, 1947: fig. 6a). The latter displays exactly the same outline as the intercoracoid cartilage of the Sibyrhynchidae Iniopera sp., which can be resolved as a stem holocephalan (see above; Janvier, 1996, Pradel, 2010 and Zangerl, 1981). The Sibyrhynchidae Sibyrhynchus also shows an unpaired intercoracoid cartilage, as also does the Iniopterygidae Promexyele (Zangerl and Case, 1973). Another putative stem-holocephan, the eugeneodontid Ornithoprion (Grogan and Lund, 2004 and Janvier, 1996), also possesses an intercoracoid cartilage (Zangerl, 1966 and Zangerl, 1981).

Among the other Paleozoic chondrichthyans, some taxa, such as stethacanthids, symmoriids and primitive caseodontid, (Coates and Sequeira, 2001 and Zangerl, 1981), display large, paired and ventral procoracoid cartilages at the ventral end of the scapulocoracoids. According to Coates and Gess (2007), these procoracoid cartilages are homologous with the separate coracoid cartilages of xenacanths (Heidtke and Schwinde, 2004). In the Iniopterygidae Papilionichthys and Rainerichthys, the ventral surface of each pectoral girdle is linked to the branchial apparatus through paired, elongate cartilages, although Grogan and Lund (2009) interpreted these elements as hypobranchials. It was also suggested that the unpaired intercoracoid plate of the other iniopterygians is connected to the branchial apparatus (see above; Zangerl, 1981).

In acanthodians, a pair of procoracoids is present in the endoskeletal pectoral girdle (Miles, 1973). In acanthodian taxa in which the dermal skeleton of the pectoral girdle is more developed, such as the Climatiidae and Diplacanthidae, there is one or two median, unpaired dermal lorical plate (Gardiner, 1984 and Miles, 1973).

In osteichthyans and placoderms, there is neither evidence of an endoskeletal intercoracoid, except in the lungfish Ceratodus (Goodrich, 1930), nor evidence of endoskeletal procoracoids. Nevertheless, the endoskeletal scapulocoracoid is generally reduced, and a dermal interclavicle is present between the two clavicles in osteichthyans (Janvier, 1996), whereas the anterior and posterior median ventral plates are wedged between the anterior ventrolateral plates in placoderms (Goujet, 1984). Gardiner (1984) homologized the ventral median dermal plates of osteichthyans, placoderms and acanthodians.

The interclavicle is obviously not homologous to an endoskeletal intercoracoid element. Nevertheless, in some taxa, the interclavicle lies ventral to, or fuses with unpaired, median endoskeletal element. In some tetrapods, a dermal interclavicle extends along the ventral surface of the endoskeletal sternum (Goodrich, 1930). In addition, according to Patterson (1977), the urohyal of teleosteans is actually derived from the fusion of the dermal interclavicle and the endochondral urohyal.

These observations suggest that the endoskeletal and dermal pectoral shoulder of gnathostomes are primitively separated ventrally by either unpaired or paired endoskeletal intercoracoid elements, or dermal bones, respectively. The median endoskeletal intercoracoid cartilages may have been lost in most derived chondrichthyans, so that the coracoid part of the two scapulocoracoids meets ventrally. Moreover, Parker (1891) interpreted the intercoracoid element of Notorynchus as a sternal element, which is divided into two parts, a pre-omosternum and a post-omosternum. Zangerl and Case (1976), suggested that the “sternum” of chondrichthyan was primitively paired (the condition seen in some Iniopterygidae, stethacanthids, symmoriids, caseodontids and xenacanths), and that the unpaired “sternum” of Notorynchus is the result of a secondary fusion of the originally paired elements. The intercoracoid cartilage of Iniopera sp. may provide support for this interpretation, as it is perfectly symmetrical and displays a pair of symmetrical foramina. One may suppose that this cartilage results from the fusion of two embryonic structures, ventral to the embryonic coracoids.

The sibyrhynchid Iniopera sp. possesses a pair of suprascapular cartilages dorsal to the scapulocoracoids. Suprascapular cartilages were already reported in two Iniopterygidae, Rainerichthys and Papilionichthys, although in larger numbers (Grogan and Lund, 2009). The presence of suprascapular cartilages in other fossil chondrichthyans is poorly documented, but they are present in xenacanths (Zangerl, 1981), and in the hybodont Lissodus cassangensis (Maisey, 1982). Among extant chondrichthyans, only batomorphs possess suprascapular cartilages, which articulate with the neural arches of the vertebrae, or directly with the synarcual (Capetta, 1987).

The endoskeletal shoulder girdle of some acanthodians, such as Acanthodes, displays a pair of suprascapular elements. Nevertheless, these bones are absent in other forms, such as climatiiforms (Janvier, 1996).

In osteichthyans, the endoskeletal pectoral girdle is tripartite. It ossifies, however, as a single element, except in primitive living teleosts, in which a dorsal mesocoracoid is separated from a medial scapula that is also distinct from the ventral coracoid (Gardiner, 1984). Gardiner (1984) supposed that the primitive number of ossifications in the endoskeletal shoulder girdle is three in osteichthyans. Placoderms never display any endoskeletal element dorsal to the scapular blade.

The phylogenetic signal of endoskeletal suprascapular elements is thus unclear in gnathostomes.

It was suggested that the suprascapular cartilages of iniopterygians link the pectoral girdle to the rear of the neurocranium (see above; Grogan and Lund, 2009). This condition is unique among chondrichthyans, since the pectoral girdle is always separated from the rear of the neurocranium in other chondrichthyans, even in batomorphs, xenacanths and Lissodus cassangensis, which possess suprascapular cartilages, and this could represent a synapomorphy of iniopterygians.

A series of dermal bones links the dermal pectoral girdle to the dermal bones of the head in osteichthyans and placoderms, suggesting that a dorsal connection between the pectoral girdle and the head could be plesiomorphic for gnathostomes.

All iniopterygians known to date possess a pectoral fin that articulates with the dorsal part of the scapulocoracoid, and their scapular region is thus extremely reduced. Among extant and fossil chondrichthyans, this feature is unique and could be a non-ambiguous autapomorphy of iniopterygians. The position of the attachment of the pectoral fin is located at the level of about one-third to mid-length of the scapulocoracoid in most of the Paleozoic chondrichthyans, such as symmoriiforms, xenacanths, ctenacanths, eugeneodontiforms, as well as in hybodontiforms and neoselachians (Lane and Maisey, 2009 and Zangerl, 1981). The position of this fin attachment is much more variable among the holocephalans. The condition in many stem-holocephalans, such as Debeerius (Grogan and Lund, 2000), is almost similar to that in, e.g., hybodontiforms and neoselachians. However, the pectoral fin articulation is situated slightly more dorsally in Harpagofututor (Lund, 1982), and much farther ventrally in, e.g., Squaloraja, Ischyodus and modern chimaeroids (Stahl, 1999). The dorsal position of the pectoral fin attachment relative to the scapulocoracoid may thus be considered as a derived condition for chondrichthyans.

All iniopterygians possess a monobasal pectoral fin with a single, proximal, enlarged basal element (basipterygium), which articulates with the scapulocoracoid, and which is variously sized according to the genus (see above; Grogan and Lund, 2009 and Zangerl and Case, 1973). The distal radials of the pectoral fin are attached all along the distal edge of this basal element.

A monobasal pectoral fin may represent a plesiomorphic condition for gnathostomes, since various stem gnathostomes display a single basal element, as in, e.g., the osteostracan Escuminaspis, and some placoderms (Goujet, 2001, Goujet and Young, 2004 and Janvier et al., 2004). Nevertheless, the presence of three or more proximal basals supporting distal radials is supposed to be a feature of crown-gnathostomes (Coates, 2003, Goujet, 2001, Goujet and Young, 2004, Janvier et al., 2004 and Mabee and Noordsy, 2004). Proximal premetapterygial and metapterygial radials, either or not forming a basal plate, are present in most Paleozoic chondrichthyans (e.g., symmoriiforms, primitive xenacanths, hybodonts, eugeneodontids), and a monobasal pectoral fin, like that of iniopterygians, may therefore represent a derived condition among chondrichthyans.

In the Iniopterygidae, the pectoral basipterygium also supports an axis made up by small cartilages at its posterodistal corner, which bears distal radials (Grogan and Lund, 2009 and Zangerl and Case, 1973), whereas this feature is absent in the Sibyrhynchidae (see above; Zangerl and Case, 1973). An axis of small cartilages, which articulates with a proximal basal cartilage of the pectoral fin distally, and which bears, in some genera, distal radials, is a widespread feature among Paleozoic chondrichthyans (e.g., the symmoriiform Cobelodus; Zangerl and Case, 1976; the xenacanth Orthacanthus; Hampe and Heidtke, 1997; the eugeneodontid Fadenia; Bendix-Almgreen, 1975; the stem holocephalans Traquairus; Lund and Grogan, 1997). Consequently, the presence of this pectoral axis (as in the Iniopterygidae) may be considered as a plesiomorphic condition for chondrichthyans.

Zangerl (1981) suggested that the single basal element of the iniopterygian pectoral fin may actually be a compound element. The shape of the basipterygium of the specimens studied here, along with its topographic relation to the distal radials compared to extant chimaeroids, may provide support for this assumption. The pectoral fin of crown holocephalans displays a distinctive endoskeletal pattern; i.e. the pectoral fin is dibasal, with a propterygium and a metapterygium that bear all the distal radials. This condition is supposed to be derived for chondrichthyans (Coates and Sequeira, 2001). In addition, in extant species, such as Callorhinchus milii and Harriotta releighana, only the small propterygium articulates with the scapulocoracoid. Moreover, the anteriormost distal radials of extant chimaeroids fuse to form a more robust element, which articulates with the propterygium, whereas the posteriorly following distal radials articulate with the metapterygium (Didier, 1995). The anteriormost distal radial of sibyrhynchids and iniopterygids (see above; Zangerl and Case, 1973), as well as that of many other stem holocephalans (Stahl, 1999), is also more robust than the following posterior ones. In addition, the shape of the enlarged anteriormost distal radial of iniopterygians suggests a fusion of two adjacent distal radials. This robust distal radial articulates with the anterior part of the single basipterygium, which is larger and more complex than its posterior part, at any rate in sibyrhynchids (the morphology of the basipterygium of iniopterygids is poorly known). The following posterior distal radials are supported by the posterodistal edge of the basipterygium. One may therefore infer that the single basipterygium is in fact derived from the fusion of the pro- and metapterygium, and that only the propterygial part of the basipterygium bears the anteriormost fused distal radials, as in extant chimaeroids. The condition in other stem holocephalans is unclear. For instance, it was suggested that Traquairus (Lund and Grogan, 1997) and Debeerius (Grogan and Lund, 2000) possess a dibasal pectoral fin, as in extant chimaeroids, plus a more or less segmented metapterygial axis posteriorly. The metapterygium and the metapterygial axis support distal radials. Nevertheless, in both cases, the cartilage that Lund and Grogan (1997) and Grogan and Lund (2000) have interpreted as the propoterygium, and its position relative to the scapulocoracoid and the proximal basal cartilage of the pectoral fin, are not clearly visible in the specimens (Grogan and Lund, 2000: p. 228, fig. 12A; Lund and Grogan, 1997: p. 482, fig. 3.4). It is possible that it rather represents an enlarged anteriormost distal radial supported by a single basal cartilage, which articulates with the pectoral girdle anteriorly, and is followed by a metapterygial axis posteriorly. In such a case, the condition in these two taxa is the same as that described above for iniopterygians. Consequently, one may suppose that the plesiomorphic condition for holocephalans is the presence of a single basipterygium, which articulates with the scapulocoracoid. The basipterygium articulates posteriorly with a well-defined metapterygium in extant holocephalans, and a metapterygial axis in some stem holocephalans.