Fig 1.
Material and geological settings.
A, map of Thailand showing outcrops of the Sao Khua Formation (in green) and B, close-up on north-eastern-Thailand with location of Phu Phok; C, and photograph of 4 of the eggs from Phu Phok (SK1-1, SK1-2, SK1-3 and SK1-4). Scale bar, 1 cm.
Fig 2.
Three-dimensional rendering of two fossil eggs and their enclosed embryonic bones from Phu Phok.
A, SK1-2. B, SK1-1. Colours: red, skull and mandible; yellow, vertebrae; grey, ribs; green, pectoral and pelvic girdle; blue, limbs. Scale bar, 5 mm.
Fig 3.
Skull and mandible of the anguimorph embryos from Phu Phok.
A,B, skull, dorsal (A) and lateral (B) views. C, left mandible, lateral view. Colours: yellow, SK1-1; green, SK1-2; red, absent or incomplete bone replaced by symmetrical reconstruction. Anatomical abbreviations: a, angular; ar, articular; c, coronoid; d, dentary; e, epipterygoid; ec, ectopterygoid; end, calcified endolymph; eo, exoccipital; f, frontal; j, jugal; m, maxilla; mf, mental foramen; op, opisthotic; p, parietal; pbs, parabasisphenoid; pf, postfrontal; pl, palatine; po, postorbital; pr, prootic; prf, prefrontal; pt, pterygoid; q, quadrate; s, stapes; sa, surangular; sm, septomaxilla; soc, supraoccipital; sq, squamosal; v, vomer. Scale bars, 1 mm.
Fig 4.
Skull and mandible of the anguimorph embryos from Phu Phok.
A, B, skull, ventral (A) and medial view of a sagittal section (B) views. C, left mandible, medial view. Colours: yellow, SK1-1; green, SK1-2; red, absent or incomplete bone replaced by symmetrical reconstruction. Anatomical abbreviations: a, angular; aif, anterior inferior alveolar foramen; ar, articular; bo, basioccipital; c, coronoid; d, dentary; e, epipterygoid; ec, ectopterygoid; end, calcified endolymph; eo, exoccipital; f, frontal; j, jugal; m, maxilla; mc, Meckelian canal; mfo, mandibular fossa; op, opisthotic; p, parietal; pbs, parabasisphenoid; pl, palatine; po, postorbital; pr, prootic; prf, prefrontal; pt, pterygoid; q, quadrate; s, stapes; sa, surangular; sm, septomaxilla; soc, supraoccipital; sp, splenial; v, vomer. Scale bars, 1 mm.
Fig 5.
Anguimorph cranial and post-cranial material from Phu Phok.
A, segments of the sclerotic ring, lateral view. B, Calcified endolymph from the left sacculus. C, tooth from the anterior part of the dentary in cranial, labial and rostral views; D, presacral vertebra, ventral and dorsal views. E, caudal vertebral centrum, dorsal and lateral views. F, two consecutive presacral vertebrae from Phu Phok, attributed to an anguimorph, ventral view. G, caudal vertebrae, near the sacral region, lateral view. H, reconstruction of part of the pectoral girdle, ventral view. I, reconstruction of the pelvic girdle, lateral view. Colours: yellow, SK1-1; green, SK1-2; red, absent or incomplete bone replaced by symmetrical reconstruction. Anatomical abbreviations: cl, clavicle; co, coracoid; fra, autotomous fracture plane; icl, interclavicle; il, ilium; is, ischium; prc, procoracoid; pu, pubes; sc, scapula. Scale bars, 1 mm.
Fig 6.
Comparison of the ossification extension of several postcranial components from the embryos SK1-1 and SK1-2.
The vertebral elements compared (both pre- and post-sacral), as well as the rib, are the largest ones from the eggs SK1-1 and SK1-2. In the pectoral girdle, SK1-1 shows an advance degree of ossification notably in the extension of the procoracoid, the ventral margin of the glenoid fossa and the blade of the scapula. Ossification toward epiphyses of the humerus is more advanced in SK1-1. Scale bar, 1 mm.
Fig 7.
Eggshell morphology and microstructure of the eggs from Phu Phok.
A, 3D rendering of a portion of the surface of the eggshell of SK1-2 showing the distribution of nodes. B, tomogram of SK1-1 showing two eggshell fragments that slid in the egg, outer surfaces oriented to the top of the figure. The inner half of both shell fragments is displayed in darker shades of grey indicating the shell is less dense than the whiter outer half. Unlike micrographed thin sections (Fig 7), the funnel-shaped depression (d) do not seem to be obstructed. The pore canals (p) are highlighted by the edge interference resulting from the phase contrast effect (black and white fringes). C-D, SEM photographs of an eggshell fragment showing the fan-shaped pattern of crystal at the level of a surface node (n). Not the fibrous layer (f) underlining the eggshell. D, close up from C. Scale bars (A, B), 500 μm.
Fig 8.
Micrographed radial thin section of egg SK1-5.
A-C, close up on a radial section at the level of a tall ornamentation node (n) in non-analysed polarized light (A) and analysed polarized light (B and C). The funnel shaped depression (d) exhibit similar interference colours than the innermost part of the shell. The depression tapers toward the outer surface into a very narrow pore canal (p). D, flat portion of the eggshell in analysed polarized light showing large crystals (c) with a columnar extinction. E, flat portion of the eggshell in transmitted light, showing the eggshell underlined by a fibrous layer (f). F, close up on the fibrous layer. The outer surface of the eggshell is positioned on the top part of each panels (top-right in panel C). Scale bars, 100μm.
Fig 9.
Known eggshell types across a simplified time-calibrated lepidosaur phylogeny based after morphological studies [28,31].
A, Phu Phok embryos are tentatively placed in an unresolved trichotomy with shinisaurids and varanoids. B-E, schemas of known lepidosaur eggshell types: semi-rigid, loosely connected calcite columns embedded in the shell membrane [36,47] (B, modified from Packard et al. [35]). Examples: Tuatara (Rhynchocephalia: Sphenodon punctatus)[35,36] and Bearded lizard (Agamidae: Pogona barbata)[47]; leathery, shell membrane often covered with thin calcitic elements [8]. Examples: the wall lizard (Scincomorpha: Lacerta lepida)[9], zebra-tailed lizard (Iguania: Callisaurus draconoides)[38]; (C); rigid, well-connected adjacent calcitic columns covering a thin shell membrane [6,7,9]. Example: Gekko gecko[7]; (D); Phu Phok, similar to the rigid type, developed in an undulatory pattern, covering a thin structure interpreted as the shell membrane (E). Abbreviations: a, amorphous layer; c, calcite component; p, pore canal; sm, shell membrane.