Among living Vertebrate groups, Lizards show the most diverse range of reproductive strategies, with species known that reproduce sexually and parthanogenically (check spelling – a form of asexual reproduction in which the female fertilizes her own eggs, rather than producing clones as in some Insects and Plants), as well as egg laying and viviparous (live-birthing) Lizards being known. The majority of Lizards produce soft-shelled eggs, with casings with a leathery or parchment-like casing, similar to the eggs of Turtles and Crocodiles, however some members of one group, the Geckoes, produce calcified eggs, similar to those of Birds. Calcified Lizard eggs are known in the fossil record from the Early Cretaceous onwards, and have generally been referred to the Gekkota, however there is no strong basis for this assumption, as these eggs have not been found in close association with adult Lizards nor produced examinable embryos, so the possibility that members of other Lizard groups produced calcified eggs in the past cannot be ruled out.
In a paper published in the journal PLoS One on 15 July 2015, Vincent Fernandez of the European Synchrotron Radiation Facility and the Evolutionary Studies Institute at the University of the Witwatersrand, Eric Buffetaut of the Laboratoire de Géologie de l’Ecole Normale Supérieure at the Centre National de la Recherche Scientifique, Varavudh Suteethorn of the Palaeontological Research and Education Centre at Mahasarakham University, Jean-Claude Rage of the Sorbonne Universités and the Muséum national d’Histoire Naturelle, Paul Tafforeau, also of the European Synchrotron Radiation Facility, and Martin Kundrát of the Subdepartment of Development and Evolution at Uppsala University and the Institute of Physiology of the Academy of Sciences of the Czech Republic, describe a series of calcified Lizard eggs with preserved embryos from the Early Cretaceous Sao Khua Formation of Sakhon Nakhorn Province in northeast Thailand.
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 is 1 cm. Fernandez et al. (2015).
The eggs were collected during official field campaigns of the Royal Thai Department of Mineral Resources and examined at the European Synchrotron Radiation Facility in Grenoble, and have subsequently been placed in the collection of the Sirindhorn Museum in Phu Kum Khao. When collected the eggs were thought to have been produced by a Theropod Dinosaur, but examination with phase contrast synchrotron microtomography (multiple X-rays used to build up a three dimensional computer model of the internal structure of an object) revealed the presence of disarticulated embryonic Lizard skeletons in the eggs.
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 is 5 mm. Fernandez et al. (2015).
The eggs were all crushed, but are interpreted to have been about 18 mm in height and 11 mm in maximum diameter, giving them a volume of about 1.15 cm3. The shells of the eggs appear to comprise a single layer of calcitic material overlaying an inner layer of fibrous material. The surface ornamentation of the eggs is nodular, with two distinct sizes of nodules, one tall and one smaller. Beneath these nodes are funnel-shaped canals, with their tips opening at the tips of the nodes and with wider depressions on the inner surface; these are interpreted as pore-canals (which would have allowed the living egg to breath and regulate moisture). The calcite layer is comprised of large crystals arranged in a columnar manner, arranged in a fan-shaped pattern around the pores/nodes.
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. 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). Note the fibrous layer (f) underlining the eggshell. (D) Close up from (C). Scale bars in (A, B) are 500 μm. Fernandez et al. (2015).
Computerised tomography enabled the reconstruction of two embryos of slightly different ages, both apparently the same species of long-snouted, large-braincased Lizard. A number of features of the skulls and teeth of these specimens suggest that they are members of the Platynota, the group of Anguimorph Lizards that includes the modern Monitor and Bearded Lizards as well as the extinct Mosasaurs, and definitely not to the Gekkota. This is the first time that a calcified egg has been shown to have been to have been produced by a non-Gekkotan Lizard.
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 are 1 mm. Fernandez et al. (2015).
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