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).
See also…
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