Cranial Morphology of the Carboniferous-Permian Tetrapod Brachydectes newberryi (Lepospondyli, Lysorophia): New Data from µCT

Abstract

Lysorophians are a group of early tetrapods with extremely elongate trunks, reduced limbs, and highly reduced skulls. Since the first discovery of this group, general similarities in outward appearance between lysorophians and some modern lissamphibian orders (specifically Urodela and Gymnophiona) have been recognized, and sometimes been the basis for hypotheses of lissamphibian origins. We studied the morphology of the skull, with particular emphasis on the neurocranium, of a partial growth series of the lysorophian Brachydectes newberryi using x-ray micro-computed tomography (μCT). Our study reveals similarities between the braincase of Brachydectes and brachystelechid recumbirostrans, corroborating prior work suggesting a close relationship between these taxa. We also describe the morphology of the epipterygoid, stapes, and quadrate in this taxon for the first time. Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium. Morphology previously considered indicative of taxonomic diversity within Lysorophia may reflect ontogenetic rather than taxonomic variation. The highly divergent morphology of lysorophians represents a refinement of morphological and functional trends within recumbirostrans, and is analogous to morphology observed in many modern fossorial reptiles.

Fig 1. Stratigraphic and geographic context of specimens studied.

A, geographic location of specimen localities; B, stratigraphic position of localities, after [48]. 1, Humboldt, NE, localities (undivided); 2, Eskridge, KS locality. Light gray indicates Upper Carboniferous surface deposits; dark gray indicates Lower Permian surface deposits.

Fig 2. Volume renders of selected skulls studied, showing representative size variation in specimens, dorsal view.

A, DMNH 47854; B, DMNH 521121; C, KUVP 49541; D, UNSM 32149; E, comparison of skull sizes. Scale bar equals 1 cm.

Fig 3. External skull morphology of Brachydectes newberryi KUVP 49541, rendered from μCT.

A, dorsal view; B, anterior view; C, right lateral view; D, occipital view; E, palatal view, with lower jaws removed. Scale bar equals 5 mm. Abbreviations: ang, angular; ap, median ascending process of supraoccipital; bo, basioccipital; cbic, foramen serving the cerebral branch of the internal carotid artery; ch, choana; d, dentary; en, external naris; eo, exoccipital; eoc, exoccipital condyle; fm, foramen magnum; lac, lacrimal; mf, mandibular fenestra; mx, maxilla; op, opisthotic; pal, palatine; pmx, premaxilla; porp, postorbital process; ppl, postparietal lappet; preart, prearticular; prf, prefrontal; psph, parasphenoid; pt, pterygoid; q, quadrate; qsp, stapedial process of quadrate; qtr, trochlea of quadrate; rap, retroarticular process; sa, surangular; so, supraoccipital; sq, squamosal; sqpf, posterior flange of squamosal; t, tabular; v, vomer. Scale bar equals 1 cm.

Fig 4. Reconstruction of the skull of Brachydectes newberryi.

A, lateral view; B, dorsal view; C, palatal view. Image not to scale.

Fig 5. Braincase morphology of Brachydectes newberryi, DMNH 521121, 3D renders segmented from μCT.

A, left lateral view; B, right medial view; C, ventral view; D, dorsal view; E, posterior view. Scale bar equals 5 mm. Abbreviations: bo, basioccipital; eo, exoccipital; eoc, exoccipital condyle; fme, foramen metoticum; n.V, foramen serving the trigeminal nerve (undivided); n.XII, foramen serving the hypoglossal nerve; op, opisthotic; os, orbitosphenoid; pa, pila antotica; po, prootic; psph, parasphenoid; so, supraoccipital.

Fig 6. Braincase of Brachydectes newberryi, KUVP 49541, 3D renders segmented from μCT.

A, dorsal view; B, anterior view; C, left lateral view; D, left anterolateral view; E, ventral view; F, occipital view. Scale bar equals 5 mm. Abbreviations: ap, median ascending process of the supraoccipital; bo, basioccipital; boc, basioccipital cotyle; bptp, basipterygoid process of parasphenoid; cbic, path of the cerebral branch of the internal carotid artery; cp, cultriform process of the parasphenoid; crbo, conical recess of the basioccipital; eo, exoccipital; eoc, exoccipital condyle; fenv, fenestra vestibularis; fm, foramen magnum; fme, foramen metoticum; hf, hypophyseal fossa; mcv, foramen serving middle cerebral vein; n.I, foramen serving olfactory nerve; n.II, foramen serving optic nerve; n.V, foramen serving trigeminal nerve (undivided); n.VII, foramen serving facial nerve; n.XII, foramen serving hypoglossal nerve; op, opisthotic; os, orbitosphenoid; pa, pila antotica; pbic, sulci serving the palatal branch of the internal carotid artery; po, prootic; psph, parasphenoid; psphe, presphenoid; so, supraoccipital; vs, vidian sulcus.

Fig 7. Braincase morphology of Bracydectes newberryi UNSM 32149, 3D renders segmented from μCT.

A, dorsal view; B, left anterolateral view; C, left lateral view; D, occipital view; E, right lateral view; F, left posterolateral view; G, ventral view. Scale bar equals 5 mm. Abbreviations: ap, median ascending process of the supraoccipital; bo, basioccipital; crbo, conical recess of the basioccipital; eo, exoccipital; eoc, exoccipital condyle; fm, foramen magnum; fme, foramen metoticum; hf, hypophyseal fossa; lap, lateral ascending process of the supraoccipital; mcv, foramen serving the middle cerebral vein; n.I, foramen serving the olfactory nerve; n.V, foramen serving the trigeminal nerve (undivided); n.VII, foramen serving facial nerve; n.XIII, foramen serving hypoglossal nerve; op, opisthotic; os, orbitosphenoid; pa, pila antotica; po, prootic; pop, paroccipital process; psph, parasphenoid; psphe, presphenoid; so, supraoccipital.

Fig 8. Selected otic elements of the left otic capsule of Brachydectes newberryi, UNSM 32149, in medial view.

Scale bar equals 1 mm. Abbreviations: amascc, fossa enclosing ampulla of the anterior semicircular canal; ascc, sulcus for anterior semicircular canal; crint, crista interfenestralis; fme, foramen metoticum; hscc, sulcus for horizontal semicircular canal; lr, lagenar recess; op, opisthotic; po, prootic.

Fig 9. Morphology of the stapes in small, subadult, and skeletally mature Brachydectes newberryi.

A-E, DMNH 521121, left stapes in A, dorsal; B, left lateral; C, posterior; D, anterior; and E, ventral view; F-J, KUVP 49541, right stapes in F, dorsal; G, right lateral; H, posterior; I, anterior; and J, ventral view; K-O, UNSM 32149, left stapes in K, dorsal; L, left lateral; M, posterior; N, anterior; and O, ventral view. Scale bar equals 1 mm. Abbreviations: col, columellar process of stapes; dp, dorsal process of columella of stapes; stf, stapedial foramen.

Fig 10. Selected skulls of Brachydectes newberryi in palatal view.

A, KUVP 49539; B, UNSM 32149. Scale equals 5 mm. Abbreviations: ch, choana; mx, maxilla; pal, palatine; psph, parasphenoid; pt, palatine; q, quadrate; v, vomer.

Fig 11. Right palatal and suspensorial elements of Brachydectes newberryi, UNSM 32149.

A, lateral view; B, anterior view; C, posterior view. Scale equals 5 mm. Abbreviations: ape, ascending process of epipterygoid; dlpt, dorsal lamina of pterygoid; epi, epipterygoid; prpt, palatine ramus of pterygoid; pt, pterygoid; stp, stapedial process of quadrate.

Fig 12. Right lower jaw of Brachydectes newberryi, KUVP 49541, rendered from μCT.

A, lateral view; B, dorsal view; C, medial view; D, ventral view. Scale equals 5 mm. Abbreviations: af, articular fossa; ang, angular; cor, coronoid; d, dentary; mf, mandibular fenestra; mnf, mental foramen; ms, medial shelf of prearticular; rap, retroarticular process; sa, surangular; sym, mandibular symphysis.

Fig 13. Cranial endocast of Brachydectes newberryi, KUVP 49541, rendered from μCT.

A-B, position of cranial endocast within complete skull, A, dorsal view; B, lateral view; C, endocast in dorsal view; D, endocast in right lateral view; E, endocast in left lateral view; F, endocast in ventral view; G, endocast in endocast in left posteroventral oblique view; H, left anteroventral oblique view. Scale equals 5 mm. Abbreviations: cbic, passage of the cerebral branch of the internal carotid artery; fme, passage of the foramen metopticum; hsc, horizontal semicircular canal; hyp, hypophyseal fossa; lag, fossa enclosing lagena; mcv, passage serving middle cerebral vein; n.I, passage of the olfactory nerve; n.II, passage of the optic nerve; n.V, passage of the trigeminal nerve; n.VI, passage of the facial nerve; n.XII, passage of the hypoglossal nerve; psc, posterior semicircular canal;; stym, fossa possibly accommodating scala tympani; ut, utricular fossa.

Fig 14. Left otic endocast of Brachydectes newberryi, KUVP 49541, rendered from μCT.

A, left lateral view; B, medial view; C, dorsal view. Scale equals 5 mm. Abbreviations: aasc, ampulla of the anterior semicircular canal; amhsc, ampulla of horizontal semicircular canal; asc, anterior semicircular canal; ds, dorsal sulcus; fo, surface of fenestra vestibularis; hsc, horizontal semicircular canal; lag, fossa accommodating lagena; mcv, passage of middle cerebral vein; psc, posterior semicircular canal; stym, fossa possibly accommodating the scala tympani,

Fig 15. Atlantoccipital articulation of Brachydectes newberryi, and comparison with selected early tetrapods.

A-B, atlantoccipital articulation of Brachydectes newberryi, KUVP 49541, in right lateral view; A, rendered from μCT; B, interpretive drawing; C, Araeoscelis gracilis, after [81], occipital view; D, Quasicaecilia texana, after [43], occipital view; E, Brachydectes newberryi, occipital view; F, Acheloma dunni, after [79], occipital view; G, Hynobius amjiensis, occipital view. A-B, scale bar equals 5 mm; C-G, not to scale. Abbreviations: atc, centrum of atlas; atn, neural arch of alas; bo, basioccipital; boc, basioccipital condyle or cotyle; eo, exoccipital; eoc, exoccipital condyle or shelf; op, opisthotic; pat, proatlas; soc, supraoccipital.

Fig 16. Rendered μCT volumes of selected elements of the axial skeleton of Brachydectes newberryi.

A-B, dorsal vertebrae and ribs of a small specimen of Brachydectes newberryi, DMNH 51121, left lateral view; A, volume render; B, interpretive drawing; C, external and D, internal, view of four dorsal ribs of the largest specimen of Brachydectes newberryi, UNSM 32149. Abbreviations: al, rib alae; na, neural arch; pl, pleurocentra; r, rib. Scale bar equals 1 cm.