3.1.1. Acanthostega

The largest preserved hyobranchial elements in Acanthostega gunnari are the ceratohyals (Fig. 2a). The left ceratohyal is well preserved in MGUH f.n.1227 and is exposed in ventro-lateral view. Coates & Clack (Reference Coates and Clack1991) suggested that it had rotated around 180° compared to its original orientation and swapped sides. However, the shape of the ceratohyal in Tiktaalik (Downs et al. Reference Downs, Daeschler, Jenkins and Shubin2008, fig. 6) and in Trimerorhachis (see below) suggests that the preserved orientation is correct. The ceratohyal is approximately 1.2 times longer than the ceratobranchials. Its proximal end is rounded and flattened, whereas the distal end is tapering. At mid-length, the anterodorsal margin bears a knob-like expansion with a distal “notch”, from which an anterodorsal crest runs towards the distal tip of the ceratohyal. A conspicuous ridge (medial ridge sensu Coates & Clack Reference Coates and Clack1991) extends parallel to the posteroventral margin from the expanded proximal blade in a distal direction. It exhibits fan-shaped striae in its proximal region that can be interpreted as muscle scars (Fig. 2a). This ridge is connected to the smaller anterodorsal crest via a short, transversally aligned crest.

Figure 2 Hyobranchial skeleton of stem-tetrapods: (a) Acanthostega gunnari MGUH f.n.1227, ceratohyal and ceratobranchials; (b) Adelogyrinus simorhynchus NMS.G.1889.101.17, hyobranchial skeleton and anterior part of interclavicle; (c–d) Adelospondylus watsoni NMS.G.1885.57.51, isolated ceratobranchial: (c) posterolateral view; (d) anterior view; (e–g) Greererpeton burkemorani: (e) CMNH 11090, basibranchial in ventral view; (f) CMNH 11130, basibranchial in ventral view; (g) CMNH 11219, dentigerous branchial platelets. Abbreviations: ac=anterodorsal crest; ant=anterior; cb=ceratobranchial; ch=ceratohyal; dist=distal; gr=groove; hb=hypobranchial; icl=interclavicle; k=knob; mr=medial ridge; ms=muscle scars; prox=proximal; tc=transversally aligned crest.

Anteromedial to the ceratohyals is a pair of small, squarish bones in MGUH f.n.1227. Their margins are ill-defined and the elements seem to be crushed, thus indicating a poor degree of ossification in the living animal. Their small size and anteromedial location indicate that these elements are hypohyals (see also Coates & Clack Reference Coates and Clack1991).

Coates & Clack (Reference Coates and Clack1991) did not comment on the number of ceratobranchials in Acanthostega, whereas Clack & Coates (Reference Clack, Coates, Hoch and Brantsen1993) restored its hyobranchial skeleton with three ceratobranchials per side. This was reproduced in Clack (Reference Clack2012). Re-examination of the gill skeleton showed that four pairs of ossified ceratobranchials are present in Acanthostega. As discovered by Coates & Clack (Reference Coates and Clack1991), their posterolateral surface bears a deep groove whose width corresponds almost to the width of the ceratobranchial and is visible along the whole length of the element (Fig. 2a). At their proximal end, the ceratobranchials bear two blunt processes and are slightly wider than at the distal tip. Distal (dorsal) to left ceratobranchial 4 in MGUH f.n.1227 is a short rod-like element that can be interpreted as epibranchial (see also Clack & Coates Reference Clack, Coates, Hoch and Brantsen1993 and Clack Reference Clack2012). Clack et al. (Reference Clack, Ahlberg, Finney, Dominguez Alonso, Robinson and Ketcham2003) discovered similar grooved branchial arches in the Devonian stem-tetrapod Ichthyostega.

3.1.2. Adelospondyli: Adelogyrinus

The ceratohyals are the largest and broadest elements of the hyobranchium (Fig. 2b) in the only known specimen of Adelogyrinus simorhynchus (NMS.G.1889.101.17). Their proximal and distal ends are broadened, and the shaft is curved and laterally convex and bears no processes. Five further elements can be interpreted as hypobranchials according to their location and orientation (Fig. 2b); thus, at least three pairs of hypobranchials were present in the living animal, as already reconstructed by Andrews & Carroll (Reference Andrews and Carroll1991). The first pair is the stoutest, with the distal end being broader than the proximal one, and the posterior edge being more concave than the anterior one. The second and third pairs are more slender, but have otherwise similar proportions.

Four or five long, slender ceratobranchials can be recognised distal to the hypobranchials (Fig. 2b). One element of the first pair is preserved dorsal to the first hypobranchials. The bone is bent and slightly broader proximally than distally. Crests or grooves are not visible. The second pair of ceratobranchials is preserved dorsal to the second pair of hypobranchials. These bones are straight, and the left one has a posterolateral groove that extends along almost its entire length; near the proximal end of the bone, the groove curves posteriorly and medially and becomes untraceable. One ceratobranchial of the third pair is preserved dorsal to the third pair of hypobranchials. The proximal portion of the element is concealed by the interclavicle. The posterolateral surface of the distal portion is slightly concave, and a distinct crest is visible laterally on the shaft that delimits a posterior groove on the ceratobranchial. Posterior to this element, a small part of a bone is visible, most of which is overlain by the interclavicle. It might represent the second element of the third pair of ceratobranchials. According to this description, Adelogyrinus has three pairs of ossified ceratobranchials (contra two pairs sensu Andrew & Carroll Reference Andrews and Carroll1991), and at least the second and third pairs of them bear posterolateral grooves.

3.1.3. Adelospondyli: Adelospondylus

An isolated, curved bone that is associated with Adelospondylus watsoni, specimen NMS.G.1885.57.51, is broadened at one end and tapers continuously towards the other end (Fig. 2c, d). It is ovate in cross-section. The surface of the more slender portion bears numerous striae that might represent muscle scars. The convex edge of this curved bone bears a groove that diminishes towards the broadened end. This bone, although conspicuously smaller, closely resembles the ceratobranchials in Dvinosaurus (see below). If the interpretation as a ceratobranchial is correct, then the broadened part represents the proximal end of the bone.

3.1.4. Colosteidae: Greererpeton

In Greererpeton burkemorani (CMNH 11090), the anterior part of the rod-like basibranchial attains more than five times the width of the slender shaft and is diamond-shaped (Fig. 2e). The ventral surface of the broadening is slightly convex, with a narrow concave part lateral to it. The basibranchial is not bent in lateral view. The shaft bears a ventral keel or crest, which diminishes anteriorly on the broadened portion. In CMNH 11130, it is clearly visible that the basibranchial is broadened also in its posterior part, attaining approximately two thirds the width of the anterior portion (Fig. 2f). In CMNH 11219, two or three short, stout rod-like bones are present posterolateral to the basibranchial and might represent remains of hypobranchials.

Dentigerous branchial platelets that were attached to the cartilaginous ceratobranchials in the living animal can be found in two adult skulls and a juvenile (CMNH 11219, 11320, 11132). These platelets are elongated, ovate in outline and bear one row of approximately 20 teeth at the bulge-like, presumably posterior margin (Fig. 2g). The internal surface of the platelets is concave. Due to incomplete preservation, it cannot be ascertained how many rows of platelets were present.

3.1.5. Colosteidae: Colosteus

Hook (Reference Hook1983, fig. 7a) figured short, rod-like elements in Colosteus scutellatus with slightly expanded ends that can be interpreted as hypobranchials. Furthermore, Hook (Reference Hook1983, fig. 12a, b) described elongated dentigerous branchial platelets that closely resemble those of Greererpeton, but are proportionally longer.

3.1.6. Baphetidae

Milner & Lindsay (Reference Milner, Milner and Walsh1998) provisionally designated a stout bone between the mandibular rami of Baphetes cf. B. kirkbyi as a ceratobranchial. However, since this bone is comparatively short and straight, this interpretation is doubtful. Clack (Reference Clack2003) tentatively described a possible ceratohyal in Kyrinion martilli. The bone has a slightly expanded proximal end, a short shaft and a blade-like expanded distal portion. Apart from articulatory facets at the distal and proximal ends, a further facet is present at the mid length of the bone.

3.2.1. Dvinosauria: Dvinosaurus

The basibranchial of D. primus is preserved in one specimen, PIN Am/40, and only its anterior part is exposed. It is a robust, rod-like bone and rounded-oval in cross section (Fig. 3a). The slightly expanded anterior portion is straight and becomes deeper than the shaft. In the same specimen, the first pair of hypobranchials is almost one third longer than the second pair (Fig. 3a). The first hypobranchials appear to be broader distally than proximally. Two pairs of ossified hypobranchials are preserved in the smaller skull of D. primus (PIN 2005/39). The first pair is slightly longer than the second pair (Fig. 3b). The bones are robust, round to ovate in cross section and slightly expanded at the ends. One pair of hypohyals is present in PIN Am/41, the right one being well preserved (Fig. 3a) and reaching about half the length of the first hypobranchial. It is rounded and rhombic in outline, with the medial portion being bulge-like, thickened and medially concave, whereas the lateral portion is thinner and laterally convex. Both portions are separated by a slight longitudinal depression.

Figure 3 Hyobranchial skeleton of dvinosaurian temnospondyls: (a–c) Dvinosaurus primus: (a) PIN Am/40, basibranchial, hypobranchials and hypohyals in ventral view; (b) PIN 2005/39, hypobranchials in ventral view; (c) PIN 2005/39, ceratohyal in lateral view; (d–f) Dvinosaurus campbelli, PIN 4818/410, isolated ceratobranchial: (d) posterolateral view; (e) posterior view; (f) anterior view; (g–i) Trimerorhachis insignis: (g) UCMP 105146, basibranchial and two right hypobranchials in ventral view; (h) UCMP 105175, three left hypobranchials in ventral view; (i) UCMP 105221, ceratohyal. Abbreviations: ant=anterior; bb=basibranchial; gr=groove; hb=hypobranchial; hh=hypohyal; lat=lateral; lp=lateral process; ms=muscle scars; ost=osteoderm; prox= proximal.

The ossified part of the right ceratohyal is well preserved in lateral and dorsal view in PIN 2005/39, whereas the left element has been removed since the descriptions of Sushkin (Reference Sushkin1936) and Bystrow (Reference Bystrow1938). It is a broad, stout bone with expanded proximal and distal ends (Fig. 3c). Its distal end is blade-like, rounded and laterally concave; this is the broadest part of the element. The concavity narrows and extends towards the proximal end. It is framed anterodorsally by a strong, bulge-like thickening, whereas the posteroventral margin is thinner. The distal end of the bulge-like thickening forms a lateral process (tuberculum ceratohyalis sensu Bystrow Reference Bystrow1938 and Shishkin Reference Shishkin1973) with unfinished surface. From the base of this process, a thinner process extends anterodorsally and forms the anterior edge of the posterior blade; dorsally (distally), it forms an elongate, unfinished surface.

The description of the four pairs of ceratobranchials by Sushkin (Reference Sushkin1936) and Bystrow (Reference Bystrow1938) was based on the skull PIN 2005/39. Since then, the left ceratobranchials have been removed, and the right ceratobranchials are in a very poor state of preservation. It is clearly evident that ceratobranchial 1 is the most robust branchial arch, but it cannot be determined if two grooves are present on it, as stated by Bystrow (Reference Bystrow1938). In PIN Am/41, three ceratobranchials are located on the left and at least two on the right side behind the skull, but they are very poorly preserved. The following description is based on well-preserved, isolated ceratobranchials of D. campbelli (mainly PIN 4818/410). The ceratobranchials are robust, curved elements that are well ossified (Fig. 3d–f). The proximal half is broad and anteroposteriorly flattened. The element tapers continuously towards its distal tip, with the proximal end attaining almost three times the width of the distal end. The posterolateral groove for the branchial artery (Sushkin Reference Sushkin1936; Bystrow Reference Bystrow1938; Schoch & Witzmann Reference Schoch and Witzmann2011) is clearly visible (Fig. 3d), but appears proportionally shallower and narrower than that of Acanthostega. The groove becomes shallower distally and disappears before reaching the distal tip. Proximally, the groove expands into a broad depression. The distal third of the anterior and posterior face possesses distinct muscle scars consisting of ridges and depressions (Fig. 3e, f), closely resembling those of Adelospondylus (Fig. 2d).

3.2.2. Dvinosauria: Trimerorhachis

The basibranchial of Trimerorhachis insignis consists of a stout, short shaft and is broadened anteriorly and posteriorly (Fig. 3g). The ventral surface of the anterior broadening is convex and becomes slightly concave towards the lateral ends where the bone gets thinner. The shaft is dorsoventrally flattened in cross-section and bears no ventral crest. The posterior part of the basibranchial is extremely expanded, forming a triangular blade which is approximately double the width of the anterior broadening. The ventral surface of the posterior blade is concave. The posterior expansion is almost half the length of the complete basibranchial and is dorsoventrally thinner than the anterior portion. The basibranchial is not bent in a ventral or dorsal direction.

Three pairs of short hypobranchials are present, whose morphology is best visible in UCMP 105146 and 105175 (Fig. 3g, h). The anterior two pairs have hook-like proximal and distal expansions, and the third pair is smaller and only proximally broadened.

In UCMP 105221 is an ossified element that is approximately the same length as the basibranchial and is located anterior to the ceratobranchials. The bone is flattened, rod-like, slightly bent, and is expanded proximally into a blade that is more than double the width of the shaft (Fig. 3i). The distal end appears to taper in an acute apex. A sharp crest is aligned longitudinally along shaft and blade. This bone most probably represents a ceratohyal.

Ceratobranchials in Trimerorhachis were described by Olson (Reference Olson1979), Witzmann (Reference Witzmann2004) and Schoch & Witzmann (Reference Schoch and Witzmann2011). Olson (Reference Olson1979) suspected five pairs of ceratobranchials in Trimerorhachis, whereas Boy & Sues (Reference Boy, Sues, Heatwole and Carroll2000) regarded the first ceratobranchial in Olson's (Reference Olson1979, fig. 7) figure as a ceratohyal. According to Witzmann (Reference Witzmann2004) and Schoch & Witzmann (Reference Schoch and Witzmann2011), four pairs of ceratobranchials are present, each of which is a slender, slightly curved element with widened proximal end. A posterolateral groove runs from the proximal to the distal end. The grooves are broad, proportionally comparable to those of Acanthostega, but they are not as deep. Branchial platelets with three or four teeth at the posterior edge have been found associated with the ceratobranchials (Witzmann Reference Witzmann2004).

3.2.3. Dvinosauria: Thabanchuia

The basibranchial of Thabanchuia oomie consists of a short, stout shaft and is anteriorly and posteriorly distinctly expanded (Fig. 4a). The shaft is dorsoventrally flattened in cross-section and bears no ventral crest. The anterior expansion is bulge-like convex ventrally in its median part (as the anterior continuation of the shaft), but flattens increasingly in an anterior direction. It becomes dorsoventrally thinner laterally where it has a ventrally concave surface. The posterior expansion is dorsoventrally thinner with respect to the shaft, faintly concave ventrally and has a bifurcated posterior end with two small, blunt processes. The basibranchial is neither curved dorsally nor ventrally.

Figure 4 Hyobranchial skeleton of dvinosaurian and dissorophoid temnospondyls: (a–c) Thabanchuia oomie: (a) UCMP 42780 (holotype), basibranchial in ventral view; (b) UCMP 42781, ceratobranchials; (c) UCMP 42780 (holotype), clavicle in anterolateral view showing postbranchial lamina; (d–e) Isodectes obtusus, AMNH 11037: (d) basibranchial in ventral view; (e) dentigerous branchial platelets; (f) Apateon gracilis, MB.Am.406, ossified ceratobranchials with branchial teeth; (g–h) Micropholis stowi: (g) BSM 1934 VIII 43 (specimen B sensu Broili & Schröder, Reference Broili and Schröder1937), basibranchial and hypobranchials in ventral view; (h) BSM 1934 VIII 43 (specimen A sensu Broili & Schröder, Reference Broili and Schröder1937), ceratohyal; (i) Platyrhinops lyelli, MB.Am.23, ceratohyal of a large individual. Abbreviations: ant=anterior; bb=basibranchial; bt=branchial teeth; cb=ceratobranchial; cla=clavicle; gr=groove; hb=hypobranchial; icl=interclavicle; lat=lateral; lc=longitudinal crest; man=mandible; pbl=postbranchial lamina; pr=process; prox=proximal; sq=squamosal; trd=triangular depression.

At least three pairs of slender, slightly curved ceratobranchials are present (Fig. 4b). As stated by Warren (Reference Warren1999), the ceratobranchials appear hollow inside, indicating perichondral ossification of the elements, whereas the inner parts were still cartilaginous. They are somewhat expanded proximally and may possess a slightly bifurcated proximal end, reminiscent of the ceratobranchials of Acanthostega. Posterolateral grooves extend almost the entire length of the ceratobranchials, but diminish in depth proximally and distally.

The ascending process of the clavicle is clearly visible in UCMP 42781 and 42780. This process bears a broad lamina which extends medially and is anteriorly concave (Fig. 4c). This lamina can be interpreted as a postbranchial lamina.

3.2.4. Dvinosauria: Isodectes

In one specimen of Isodectes obtusus (AMNH 11037), the basibranchial is preserved as a slender, rod-like element with a ventrally concave anterior expansion that is framed by elevated lateral margins (Fig. 4d). A sagittal crest divides the anterior expansion into two equally sized areas. The basibranchial gets conspicuously deeper towards the shaft, where it is much deeper than wide and tapers ventrally to a thin, sharp crest. The posterior portion widens again and attains two thirds of the width of the anterior portion.

The dentigerous branchial platelets that were attached to the cartilaginous ceratobranchials in life are quadrangular, or only slightly longer than wide (Fig. 4e). One edge is bulge-like thickened and bears 5–6 teeth. Because of incomplete preservation, it cannot be ascertained how many rows of branchial platelets were present in Isodectes. Milner (Reference Milner1982) described a small larva of Isodectes with a rod-like basibranchial that has faintly expanded anterior and posterior ends, dentigerous branchial platelets and three pairs of external gills.

3.2.5. Further dvinosaurians

Kourerpeton bradyi is represented by a single specimen for which locality and geological age are unknown. It has four pairs of ceratobranchials preserved (Olson & Lammers Reference Olson, Lammers and Churcher1976, fig. 1). A revision of this taxon is currently being undertaken by Marcello Ruta and Andrew R. Milner. Berman (Reference Berman1973) described the only known specimen of Lafonius lehmani, which might be a larval or juvenile specimen. From the hyobranchial apparatus, Berman (Reference Berman1973) identified elongate dentigerous branchial platelets with space for approximately ten teeth at one margin (Berman Reference Berman1973, fig. 5e).

3.2.6. Zatracheidae: Acanthostomatops

Three or four pairs of ossified curved ceratobranchials are present in larval Acanthostomatops vorax that lack grooves and branchial teeth (Steen Reference Steen1937; Witzmann & Schoch Reference Witzmann and Schoch2006). A basibranchial and hypobranchials have not been found. In an adult specimen, Witzmann & Schoch (Reference Witzmann and Schoch2006) noted the absence of ossified ceratobranchials; instead, they identified at least four pairs of slender, linear bones ventral to the basal plate of the parasphenoid and interpreted them as part of a tongue supporting the hyobranchium.

3.2.7. Dissorophoidea: Micromelerpetontidae

From the larval hyobranchial apparatus of Micromelerpeton credneri, branchial platelets that are rounded rectangular and have up to six teeth are preserved (Boy Reference Boy1995). The platelets are strongly concave on the internal side that was attached to the four pairs of cartilaginous ceratobranchials. In a few individuals, two pairs of feebly ossified hypobranchials and ceratohyals (?hypohyals), but no basibranchials, are preserved (Boy Reference Boy1995, Reference Boy2003; Boy & Sues Reference Boy, Sues, Heatwole and Carroll2000). The hyobranchium of the adult, land-dwelling individuals is not known (Boy Reference Boy1995).

In contrast to Micromelerpeton, Branchierpeton amblystomum is known only from larval specimens. The basibranchial is a tiny rod-like element which is anteriorly and posteriorly slightly expanded (Werneburg Reference Werneburg1991, fig. 13d; pers. obs. MMG SaP179). The morphology of the branchial platelets corresponds to those of Micromelerpeton. One specimen of Branchierpeton (MNC-F 10368) has four pairs of ossified ceratobranchials preserved (described by Werneburg Reference Werneburg1998 as a larval Acanthostomatops vorax, but see assignment to B. amblystomum in Witzmann & Schoch Reference Witzmann and Schoch2006). The ceratobranchials are long, slender elements whose proximal ends are expanded (approximately twice the width of the shaft), whereas the distal ends are tapering and hook-like. The original bone surface is not preserved, but in places, the denser perichondral bone (i.e. the cortex) can be distinguished from the spongy endochondral bone.

3.2.8. Dissorophoidea: Branchiosauridae and Trematopidae

Branchiosaurids have a highly derived larval hyobranchial apparatus adapted to filter feeding on planktonic prey (Boy & Sues Reference Boy, Sues, Heatwole and Carroll2000; Schoch & Milner Reference Schoch and Milner2008). The often multi-ended branchial teeth are not associated with bony platelets, but are isolated and directly attached to the four pairs of (mostly) cartilaginous ceratobranchials. Six rows of teeth are present and face towards each other, forming a zipper-like structure in the three pairs of gill clefts (Boy & Sues Reference Boy, Sues, Heatwole and Carroll2000). Ossified elements of the branchiosaurid hyobranchium comprise a tiny, rod-like basibranchial, hypobranchials and ceratohyals (or hypohyals), similar to those of micromelerpetontids (see above) (Werneburg Reference Werneburg1991; Boy & Sues Reference Boy, Sues, Heatwole and Carroll2000). Few specimens of branchiosaurids with ossified ceratobranchials are known (Werneburg Reference Werneburg1991). In the specimen of Apateon gracilis (MB.Am.406), the slightly curved ceratobranchials are long, slender and not expanded at their ends (Fig. 4f). The larvae of trematopids also have a hyobranchial apparatus that closely resembles that of branchiosaurids and was adapted for filter feeding (Milner Reference Milner2007; Schoch Reference Schoch2009). The hyobranchium of metamorphosed branchiosaurids and trematopids is unknown.

3.2.9. Dissorophoidea: Amphibamidae

The basibranchial of Micropholis stowi is clearly evident in ventral exposure in individual B of specimen BSM 1934 VIII 43 (sensu Broili & Schröder Reference Broili and Schröder1937). It consists of a deep, narrow shaft that shows no posterior expansion, whereas the anterior portion is conspicuously expanded and attains more than five times the width of the shaft (Fig. 4g). The ventral surface of the anterior broadening is smooth. The basibranchial shows no ventral or dorsal curvature. Two pairs of robust hypobranchials are visible in individual B. They are about twice as broad as the basibranchial shaft and roundish in cross-section, and attain about 90% the length of the basibranchial. An ossified, short element anterolateral to the basibranchial fragment in individual A was interpreted as a hypohyal or a ceratohyal by Boy (Reference Boy1985). In the present study, this rod-like bone is interpreted as ossified part of a ceratohyal due to its elongate shape (Fig. 4h).

Pasawioops mayi has a very slender basibranchial whose anterior end is expanded and flattened (Fröbisch & Reisz Reference Fröbisch and Reisz2008). These authors further described one pair of hypobranchials, which are rod-like elements and are longer and more robust than the basibranchial.

Sigurdsen & Bolt (Reference Sigurdsen and Bolt2010, fig. 3b) reported feebly ossified, slender hyobranchial elements in a few specimens of Doleserpeton annectens. They consist of a short basibranchial bone (“basihyal”) and a pair of rod-like ceratohyals and one hypobranchial.

Clack & Milner (Reference Clack and Milner2010) detected an elongated, flattened bone in the anterior region of the palate of a small and a larger specimen of Platyrhinops lyelli and interpreted the bones as ceratohyals. The small specimen (MB.Am.331) bears two rod-like bones ventral to the vomers. As Clack & Milner (Reference Clack and Milner2010) pointed out, it cannot be decided if the two elements represented one element that is broken, or two separate bones. The bones are slender and show no differentiation, except for the fact that the anatomical right element bears a wide longitudinal groove all along its length. In the larger specimen (MB.Am.23), the element found by Clack & Milner (Reference Clack and Milner2010) is broader and flattened and is slightly curved (Fig. 4i). It bears a longitudinal, sharp crest on its (presumed) ventrolateral side, dividing the surface into equal dorsal and ventral parts. At the (presumed) proximal portion, the crest is divided and frames a notch-like, triangular depression. In the (presumed) distal third of the bone, the medial crest has diminished. There is a pointed elevation or process in the distal third of the (presumed) anterior margin. At least the bone in the larger skull (MB.Am.23) can be interpreted as ceratohyal with some confidence, because of the flattened, albeit slender morphology and its anterolateral location ventral to the skull.

Bourget & Anderson (Reference Bourget and Anderson2011) detected a pair of poorly preserved, flattened elements in Rubeostratilia texensis that resemble the described possible ceratohyal in the small specimen of Platyrhinops. However, their orientation and location ventral to the parasphenoid might indicate that they represent hypobranchials.

3.2.10. Eryopidae: Onchiodon

In larvae of Onchiodon labyrinthicus, Boy (Reference Boy1990) and Witzmann (Reference Witzmann2005) described the rod-like basibranchial that has flattened anterior and posterior ends. The anterior end is more expanded than the posterior one. The basibranchial is unknown in adult O. labyrinthicus and O. thuringiensis (see Werneburg Reference Werneburg2007), but Werneburg (Reference Werneburg1997, figs 1d, 4b) described and illustrated the basibranchial in adult specimens of O. frossardi. In this form, the expanded anterior end of the basibranchial has approximately four times the width of the rather short shaft, and the posterior end attains almost the width of the anterior end. There is no evidence of downwards curvature of the anterior portion of the bone. The dentigerous branchial platelets of Onchiodon larvae correspond in morphology and arrangement to those of micromelerpetontids (Boy Reference Boy1990). Four pairs of slender ceratobranchials without grooves are ossified in a number of specimens of O. labyrinthicus, consisting of spongy bone, to which the branchial platelets are attached (Witzmann Reference Witzmann2005).

3.2.11. Temnospondyli incertae sedis: Iberospondylus

The basibranchial of Iberospondylus schultzei (PU-ANF 14) is a robust, rod like element with a distinctly expanded, flattened anterior end (Fig. 5a, b). This anterior portion is curved ventrally and has two concave areas on its ventral surface, separated in the midline by a shallow longitudinal crest. The slender shaft is deeper than wide in cross section. The posterior end is flattened again and is expanded, but attains less than half the width of the anterior portion.

Figure 5 Hyobranchial skeleton of Iberospondylus and stereospondylomorph temnospondyls: (a–b) Iberospondylus schultzei, PU-ANF 14: (a) basibranchial in ventral view; (b) basibranchial in ventrolateral view; (c) Sclerocephalus haeuseri, SMNS 81791, basibranchial in ventral view; (d) cross-section of the anterior portion in (c); (e) Sclerocephalus nobilis, NHMM-PW 2005/2, basibranchial with possible hypobranchial fragment in ventral view; (f) Sclerocephalus haeuseri, UHC-P 0489, parasphenoid with exoccipitals, basibranchial and hypobranchials in ventral view; (g) Glanochthon latirostris, MB.Am.211, basibranchial with possible hypobranchial fragments in ventral view; (h) Glanochthon angusta, GPIT Am 34, basibranchial in ventral view; (i) Glanochthon latirostris, MB.Am.224, imprint of cartilaginous ceratobranchial with ossified dentigerous branchial platelets; (j–k) Archegosaurus decheni: (j) MB.Am.953, basibranchial in ventral view; (k) IGS U II 1/2, imprints of cartilaginous ceratobranchials with ossified dentigerous branchial platelets; (l) Lydekkerina huxleyi, BSM 1934 VII 44, basibranchial in ?ventral view; (m–p) Trematolestes hagdorni, SMNS 81790: (m) basibranchial in ventral view; (n) basibranchial in lateral view; (o–p) possible hypobranchials; (q) Callistomordax kugleri, SMNS 90700, dentigerous branchial platelets; (r) Metoposaurus diagnosticus SMNS 5143, basibranchial in ventral view; (s) Stanocephalosaurus birdi (Wellesaurus peabodyi), UCMP 36040, basibranchial in ventral view. When not otherwise indicated, anterior is to the top. Abbreviations: ant=anterior; bb=basibranchial; bp=branchial platelets; cb=ceratobranchial; eo=exoccipital; hb=hypobranchial; prox=proximal; ps=parasphenoid; us=unfinished surface.

3.2.12. Stereospondylomorpha: Sclerocephalus

The ontogeny of the basibranchial in Sclerocephalus haeuseri was described by Boy (Reference Boy1972, Reference Boy1974, Reference Boy1988) and Schoch (Reference Schoch2003), highlighting the proportional broadening of the anterior end during growth. In adult specimens, the basibranchial morphology resembles closely that of Iberospondylus, as described above (Fig. 5c, d). The ventral curvature of the expanded anterior portion and the two concave areas, however, are not apparent in larvae and juveniles. The only known specimen of S. nobilis (NHMM-PW 2005/2) has a large and massive basibranchial element (Fig. 5e). Its anterior expansion is further ossified anteriorly; thus, the ventral curvature is more pronounced and the bone has a shovel-like appearance. The shaft is stout and ventrally convex, and bears a sharp crest more posteriorly. Interestingly, the flattened and slightly expanded posterior portion of the basibranchial has an unfinished surface not only at its posterior edge, but at least also on the complete ventral surface (the lateral and dorsal sides are concealed). Apart from S. nobilis, this structure is also evident in specimen BSM AS I 575 of S. haeuseri.

In one specimen of S. haeuseri (UHC-P 0489), one pair of short, rod-like bones is preserved anterolateral to the basibranchial ventral to the base of the cultriform process (Fig. 5f). They are interpreted here as the anterior pair of hypobranchials, but might alternatively represent ossified hypohyals. In S. nobilis, a rod-like bone articulates with the right posterolateral edge of the expanded anterior portion of the basibranchial and might represent a part of a hypobranchial (Fig. 5e).

Larvae of Sclerocephalus haeuseri possess three or four rows of dentigerous branchial platelets that are more elongate than those of micromelerpetontids and bear between five and ten teeth (Boy Reference Boy1988; Schoch Reference Schoch2003). These platelets are not confined to larval specimens, but occur also in at least some adults (Schoch & Witzmann Reference Schoch and Witzmann2009a).

3.2.13. Stereospondylomorpha: Glanochthon

In Glanochthon latrirostris, the basibranchial has not been found in adults (Boy Reference Boy1993; Schoch & Witzmann Reference Schoch and Witzmann2009b), but is clearly visible in the juvenile MB.Am.211 (Fig. 5g). Its anterior portion is deeper and about 2.5 times wider than the shaft and slightly curved ventrally. The shaft is round in cross-section and bears a faint ventral ridge. The posterior portion is again deeper and expanded, attaining approximately 80% of the width of the anterior portion. Posterolateral to the basibranchial are two short, poorly ossified rods that might represent hypobranchials. The basibranchial is preserved in an adult specimen of G. angusta (GPIT.Am.34) and is more slender and less robust than that of Sclerocephalus (Fig. 5h). The anterior expansion is flattened and is about 3.5 times wider than the shaft, whereas the posterior extension attains only twice the width of the shaft. A ventral curvature is not determinable. Two anterior concave areas on the ventral surface are faintly indicated.

Four rows of dentigerous branchial platelets (sometimes associated with imprints of cartilaginous ceratobranchials) are preserved in small specimens and are retained at least in medium-sized skulls (Schoch & Witzmann Reference Schoch and Witzmann2009b). They are elongate and have space for more than 20 teeth (Fig. 5i).

3.2.14. Stereospondylomorpha: Archegosaurus

The ontogeny of the basibranchial in Archegosaurus decheni was documented by Hofker (Reference Hofker1926) and Witzmann (Reference Witzmann2006) and resembles that of Sclerocephalus. In adults, the expanded anterior portion is six times wider than the slender shaft and is curved ventrally (Fig. 5j). The posterior portion is only slightly expanded (1.6 times the width of the shaft), but has become conspicuously deeper.

The imprints of cartilaginous ceratobranchials have distally and proximally expanded ends (Fig. 5k). Imprints of two short rod-like bones posterior to the parasphenoid might represent hypobranchials 1 and 2, and a short element anterolateral to them might be a ceratohyal (Hofker Reference Hofker1926, fig. 8). The morphology of the denticulate branchial platelets and their arrangement correspond to those of Glanochthon and are present in larvae and retained at least in middle-sized specimens. Some of the platelets may bear an ornamentation of concentric rings which may represent growth rings (Witzmann Reference Witzmann2006).

3.2.15. Stereospondylomorpha: basal stereospondyls

Van Hoepen (Reference Van Hoepen1915) and Schoch (Reference Schoch2002) described elongate branchial platelets in Uranocentrodon senekalensis that were arranged in at least three rows. Each platelet has space for more than 12 teeth. A rod-like bone that is located next to the cultriform process of the parasphenoid in Lydekkerina huxleyi (BSM 1934 VIII 44) has an anterior expanded portion that attains more than five times the width of the slender shaft (Fig. 5l). The surface of the anterior expanded portion is slightly convex. Posteriorly, the bone widens again and attains three times the width of the shaft. Because of its location and morphology, the element can be interpreted as the basibranchial.

3.2.16. Stereospondylomorpha: Lapillopsidae

Among this presumably semi-terrestrial group of stereospondyls, a specimen of Lapillopsis nana is preserved with a slender, rod-like bone ventral to the parasphenoid (Yates Reference Yates1999, fig. 7). Yates (Reference Yates1999) suggested that it might represent a part of the hyobranchium and could be a possible hypobranchial (probably because of its oblique orientation with respect to the longitudinal axis of the skull). However, due to its isolated nature ventral to the parasphenoid, it might also represent a slightly rotated basibranchial.

3.2.17. Stereospondylomorpha: Trematosauroidea

In the type specimen of Trematolestes hagdorni (SMNS 81790), a long, very slender bone is located approximately in the midline of the skull between the orbits (Fig. 5m, n). Its shaft is round-ovate in cross-section and the caudally oriented end is flattened and slightly expanded with a concave surface. The opposite end is less expanded, and has become deeper rather than flattened. Because of its morphology and position, Schoch (Reference Schoch2006) regarded this element as the basibranchial, and this interpretation is followed here. The element has probably rotated around 180° and the broader portion represents the anterior end. Posterior to the basibranchial is a pair of short bones (Fig. 5o, p). Because of their position and length, these bones might be hypobranchials. More posterior, lateral to the left clavicle, is a bone fragment that might represent the expanded proximal end of a ceratobranchial.

In the stem-metoposaurid Callistomordax kugleri, several long-ovate branchial platelets with space for approximately 15 teeth at one edge are preserved (Fig. 5q). Additionally, a slightly curved bone fragment with a longitudinal groove is present that probably represents a part of a ceratobranchial.

In specimen SMNS 5143 of Metoposaurus diagnosticus, a rod-like bone is preserved ventral to the cultriform process (Fig. 5r). Its shaft is stout, dorsoventrally flattened, and the anterior and posterior ends are slightly broadened and rounded. The anterior end is somewhat deeper than the shaft and bears a pitted surface. A longitudinal crest is visible in the anterior half of the bone. In the posterior half, two crests converge towards the posterior end. Morphology and location of the bone strongly suggest the interpretation as a basibranchial.

3.2.18. Stereospondylomorpha: Capitosauroidea

In two specimens of Stanocephalosaurus birdi (Wellesaurus peabodyi; UCMP 56098, 36040), a stout linear bone is visible that is located next to the cultriform process of the parasphenoid and oriented parallel to it. It is rather undifferentiated and slightly expanded anteriorly and posteriorly (Fig. 5s) and probably represents the basibranchial. Warren & Hutchinson (Reference Warren and Hutchinson1988, fig. 6) described a basibranchial in Rewanobatrachus (Parotosuchus) aliciae. This bone has a long, slender shaft and is slightly expanded anteriorly and posteriorly. In Parotosuchus wadei, Damiani & Warren (Reference Damiani and Warren1997, fig. 2) found thin rod-like bones between the clavicles and the posterior end of the mandibles. They tentatively interpreted these bones as ribs, but did not rule out the possibility that they might represent ossified ceratobranchials.

3.2.19. Stereospondylomorpha: Brachyopoidea

The small and only known specimen of the brachyopoid Platycepsion wilkinsoni has four ossified ceratobranchials and an ossified part of a possible ceratohyal is present anterior to them (Watson Reference Watson1956; Warren & Marsicano Reference Warren and Marsicano1998). Warren et al. (Reference Warren, Rozefelds and Bull2011) found fragments of ossified, grooved ceratobranchials in a specimen of Bothriceps australis.

3.2.20. Stereospondylomorpha: Gerrothorax (Plagiosauridae)

The view of Jenkins et al. (Reference Jenkins, Shubin, Gatesy and Warren2008) is followed here that other Gerrothorax species cannot be differentiated from G. pulcherrimus. Therefore, G. “rhaeticus” from Sweden (Nilsson Reference Nilsson1946) is treated here as the same taxon as G. pulcherrimus from Greenland (Jenkins et al. Reference Jenkins, Shubin, Gatesy and Warren2008) and Germany (Hellrung Reference Hellrung2003). In the following, the partially articulated hyobranchial apparatus SMNS 83866, MGUH 28919 and NRM-PZ B.18b will be described separately.

The Stuttgart specimen SMNS 83866 is an almost complete skeleton of Gerrothorax, in which large parts of the hyobranchial apparatus are preserved in ventral view (Fig. 6a). The large paired elements in the anterior part of the hyobranchium are elongated and rectangular in outline. The anatomical right element appears to have a concave posteromedial edge, whereas this region is obscured in the opposing element. Poor preservation precludes recognition of more anatomical details. In contrast to Hellrung (Reference Hellrung2003), who reconstructed these elements as hypohyals, they are interpreted here as the first pair of hypobranchials for the following reasons. First, the ceratobranchials and not the ceratohyals are located distal to these elements; second, comparison with tetrapodomorph fishes (Jarvik Reference Jarvik1954, Reference Jarvik1963; Downs et al. Reference Downs, Daeschler, Jenkins and Shubin2008), Acanthostega (Coates & Clack Reference Coates and Clack1991; this study), other temnospondyls (Bystrow Reference Bystrow1938; this study) and extant salamanders (e.g. Stadtmüller Reference Stadtmüller, Bolk, Göppert, Kallius and Lubosch1936; Deban & Wake Reference Deban, Wake and Schwenk2000) shows the hypohyals to be mostly rather small elements proximal to the ceratohyals. If this interpretation is correct, then three ossified pairs of hypobranchialia are present in this specimen, the posterior two pairs being more slender and rod-like. Additionally, a fragment of the basibranchial can be identified between the hypobranchial elements (contra Hellrung Reference Hellrung2003). It appears to be rod-like and slender. Two ceratobranchials can definitely be identified posterior to the hypobranchials (see also Hellrung Reference Hellrung2003), and a bone fragment directly posterior to the first hypobranchial probably represents a further ceratobranchial. Thus, probably at least three ossified ceratobranchials are present in this specimen. Their following morphological description is supplemented by the isolated finds SMNS 83370 and 83382. The ceratobranchials are widened proximally in a triangular extension, and taper continuously in a distal direction (Fig. 6a–c). The shaft is slender and may be slightly curved, and broadens again somewhat at its distal end. With the exception of short striae and tiny ridges, the dorsomedial surface of the ceratobranchials is smooth. In contrast, the ventrolateral surface is rather complex. A broad, shallow ridge extends from the proximal end distally. It divides the proximal triangular expansion into two concave fields. The broad ridge increases distinctly in height at the transition from the triangular expansion to the shaft and tapers into a rather sharp ventral crest, so that the shaft becomes deeper than wide. Two grooves extend along the lateral face of the shaft, a posteroventral and an anteroventral one (Fig. 6c). The posterior of these furrows represents the distal continuation of the posterior concave field; it is rather shallow and located immediately posterior to the apex of the ridge. Anterior to the crest is a deeper furrow that represents the distal continuation of the anterior concave field. Branchial platelets and teeth are not visible. Each ceratohyal has an elongate, oval shape and is the longest preserved element of the hyobranchium. The anterolateral edge is slightly convex, whereas the posteromedial edge appears slightly concave. Approximately the proximal half of the ventrolateral surface consists of a distinct depression with longitudinal striae at the proximal end. The distal end of the element bears similar longitudinal striae, but no depression is visible.

Figure 6 Hyobranchial skeleton of plagiosaurid stereospondyls: (a–f) Gerrothorax pulcherrimus: (a) SMNS 83866, partial hyobranchial apparatus in ventral view; (b) SMNS 83382, isolated ceratobranchial, lateral view; (c) SMNS 83382, isolated ceratobranchial, cross-section of distal end; (d) MGUH 28919: partial hyobranchial apparatus in ventral view; (e) NRM-PZ B.18, four ceratobranchials and fragmentary ceratohyal; (f) NRM-PZ B.18, dentigerous branchial platelets; (g) Plagiosuchus pustuliferus, SMNS 84794, dentigerous branchial platelets. Abbreviations: ant=anterior; bb=basibranchial; bp=branchial platelets; cb=ceratobranchial; ch=ceratohyal; cla=clavicle; gr=groove; hb=hypobranchial; ost=osteoderm; ppl=palatal platelets; prox=proximal; ps=parasphenoid; tre=triangular expansion; ven=ventral.

The Copenhagen specimen MGUH 28919 is a rather small, apparently juvenile specimen from Greenland. Several ossified elements of the hyobranchium are articulated and exposed in ventral view (Fig. 6d). The large paired elements regarded as hypohyals by Jenkins et al. (Reference Jenkins, Shubin, Gatesy and Warren2008) are interpreted here as hypobranchials 1 for the same reasons as the hypohyals sensu Hellrung (Reference Hellrung2003) in SMNS 83866 (see above). Consequently, the posterior following, much narrower element can be regarded as second hypobranchial, and the third hypobranchial is not preserved. The ventral surface of the first hypobranchial bears a ridge along the shaft and bifurcates at the distal broadening of the bone. The anterolateral edge of the bone bears a notch-like depression. If the designation as first and second pair of hypobranchials is correct, then the median, unpaired element can be interpreted as basibranchial rather than a basihyal sensu Jenkins et al. (Reference Jenkins, Shubin, Gatesy and Warren2008). It is quite short and only slightly broader anteriorly than posteriorly. It is dorsoventrally flattened in cross-section. At least three, but probably four ceratobranchials are preserved which correspond in outline to the ceratobranchials described in the Stuttgart material. On the anatomical right side, ceratobranchial 1 is preserved only in its proximal part. Ceratobranchial 2 is straight, grooved and has a trough in its proximal end (Fig. 6d). Ceratobranchials 3 and 4 are only fragmentarily preserved, but at least ceratobranchial 3 appears to be slightly curved. On the anatomical left side, three grooved ceratobranchials are preserved (probably ceratobranchials 1–3, not shown in the Figure). Whereas the anteriormost element is rather straight, the posteriorly following ones are increasingly curved and are becoming shorter, similar to NRM-PZ B.18b (see below). The ceratohyals are the broadest elements, but in contrast to SMNS 83866, they are shorter than the ceratobranchials, probably because of incomplete ossification. They have a slightly convex lateral and a concave medial margin. The medial margin is distinctly thickened, and the proximal portion bears a notch-like depression whose length accounts for approximately one quarter of the length of the ceratohyal. In its distal part, the bone bears smaller, paired depressions.

The Stockholm specimen NRM-PZ B.18b is very poorly preserved, and only the outline of the hyobranchial elements, not their surface structure, can be seen (Fig. 6e). The first ceratobranchial is rather straight, whereas the posteriorly following three elements get increasingly curved. The length of the first ceratobranchial cannot be ascertained, but ceratobranchials 2–3 get increasingly shorter. Associated with ceratobranchials 2 and 3 are ossified branchial platelets of rectangular outline, with space for 3–4 posterior teeth (Fig. 6f). Judging from its position lateral to ceratobranchial 1, an elongate, very poorly preserved element can best be interpreted as ceratohyal (see also Nilsson Reference Nilsson1946). The elements interpreted by Nilsson (Reference Nilsson1946) as three hypobranchials are too fragmentarily preserved to draw any conclusions.

3.2.21. Stereospondylomorpha: Plagiosuchus (Plagiosauridae)

Next to the clavicular stem in an articulated specimen of Plagiosuchus pustuliferus (SMNS 84794) are several branchial platelets bearing curved teeth at one edge (Fig. 6g). Because of poor preservation, they are of ill-defined outline and it cannot be ascertained if they are quadrangular or rather elongate. Interestingly, they bear an ornament of fine, curved ridges and furrows. Four pairs of grooved ceratobranchials are ossified in Plagiosuchus (Schoch & Witzmann Reference Schoch and Witzmann2011, fig. 4g, h). Due to poor preservation, it cannot be determined if the ceratobranchials have the widened proximal ends that are visible in Gerrothorax.

3.3.1. Lysorophia

The extensive hyobranchial apparatus of lysorophians was described in detail by Wellstead (Reference Wellstead1991). Hypohyals and ceratohyals are more flattened than the rather cylindrical, stout four pairs of hypobranchials and the four pairs of ceratobranchials. Some specimens of Brachydectes elongates bear “accidental bones” in the first branchial arch between the hypobranchials and ceratobranchials, the homology of which is uncertain (Wellstead Reference Wellstead1991). The ceratobranchials bear no grooves.

3.3.2. “Microsauria”

“Microsaurs” are probably not a monophyletic assemblage (Marjanović & Laurin Reference Marjanović and Laurin2013, and references therein) and are therefore written in quotation marks. Microbrachis pelikani is the only lepospondyl in which branchial platelets were described (Carroll & Gaskill Reference Carroll and Gaskill1978; Olori Reference Olori2011). According to Olori (Reference Olori2011), the platelets are round or ovate and may bear three denticles. In some platelets, the bone surface is concave, probably representing the side that was attached to the ceratobranchials as in colosteids and temnospondyls (see above). In specimen AMNH 2557, posterolateral to two preserved rows of branchial platelets is a wrinkled structure that extends mediolaterally. Because of its location and alignment, this structure might represent preservation of external gills proper.

The hyobranchial apparatus of Pantylus cordatus is well ossified and was described by Romer (Reference Romer1969, fig. 16) and Carroll & Gaskill (Reference Carroll and Gaskill1978) in a specimen which has been sectioned since then. Three pairs of elongate bones are present that are blade-like, expanded in their proximal portion and continue distally into a narrower stem. At about midlength, these bones curve in a dorsomedial direction. The anteriormost of these bones appears to bifurcate distally. Distal to them are located three pairs of stout, broadened bones. Because the proximal bones are proportionally very long and curved and the distal ones are rather short, the hyobranchial elements in Pantylus can best be designated as ceratobranchials and epibranchials sensu Reilly & Lauder (Reference Reilly and Lauder1988). The basibranchial or parts of the hyoid arch are not preserved.

Further microsaurs with hyobranchial remains are Pelodosotis elongatum, Tambaroter carolli and Hapsidopareion lepton. Narrow, rod-like hyobranchial elements of uncertain affinities (? ceratohyal and hypobranchials) are present in Pelodosotis and Tambaroter (Carroll & Gaskill Reference Carroll and Gaskill1978; Henrici et al. Reference Henrici, Martens, Berman and Sumida2011), whereas in Hapsidopareion, a short, robust bone posterior to the articular and stapes was interpreted as a probable hyoid element by Carroll & Gaskill (Reference Carroll and Gaskill1978).

3.3.3. Nectridea, Aïstopoda and Acherontiscidae

No unambiguous remains of hyobranchial elements have been demonstrated so far in nectrideans (Bossy & Milner Reference Bossy, Milner and Wellnhofer1998) or in aïstopods (Carroll Reference Carroll and Wellnhofer1998; Anderson et al. Reference Anderson, Carroll and Rowe2003). Carroll (Reference Carroll1969) interpreted four disarticulated, flattened bones of rectangular outline just behind the skull of Acherontiscus caledoniae as elements of the hyobranchial apparatus, but a more precise assignment is not possible.