Extant sea turtles develop and lay pliable (flexible) eggs, however, it is unknown if they inherited this reproductive strategy from their closer fossil relatives or corresponds to an evolutionary novelty. Here we describe the first undisputable gravid marine fossil turtle ever found, from the early Cretaceous of Colombia, belonging to Desmatochelys padillai Cadena and Parham, which constitutes a representative of Protostegidae. Using thin sectioning of one the eggs, as well as scanning electron microscopy coupled with elemental characterization, cathodoluminescence, and computer tomography we established that Desmatochelys padillai produced rigid eggs similar to the ones found in some extant and fossil freshwater and terrestrial turtles, and that at least 48 spherical eggs are preserved inside this gravid turtle. We performed also a phylogenetic analysis of several turtle taxa, including some of the extant marine species, plus the information from the gravid specimen of Desmatochelys padillai, and we found more support to previous ideas that egg and eggshell characters lack of strong phylogenetic signal amongst turtle taxa. We suggest that the development of rigid eggs in the marine extinct turtle Desmatochelys padillai resulted as an adaptation for eggs-embryos requirements dictated by the physical attributes of the nesting site
SIappendixFigS1
Supporting information Fig. S1. Additional views and location of the finding of Desmatochelys padillai FCG-CBP-75 specimen and its eggs, Testudoolithus osspp. (A). Map showing the location of Villa de Leyva town in Colombia (top) and the geology of the region (bottom), as well as the Monsalve hill where the fossil was found, figure modified from (1). B–C. Right lateroventral view of the posterior region of the shell, showing the hypoplastron (red) and portions of the ilium (green); (D–E). Cross view of one of the naturally fractured blocks that composed the fossil specimen, showing details of the hypoplastron, costal bone, a potential peripheral fragment (blue), and a totally recrystallized egg (green). Abbreviations: co, costal bone; hyp, hypoplastron; ili, ilium; pe, peripheral bone.
SIappendixFigS2
Supporting information Fig. S2. Detailed views of Testudoolithus osspp. eggshell morphology, and Desmatochelys padillai FCG-CBP-75 specimen. (A–B). One of the portions of the egg for which a thin section was made, showing well defined shell units, pores, and caverns, 4X magnification; (C–D). Close-up of one of the pores of the eggshell, showing also well define calcite crystals; (E–F). Close-up of the most posterior portion of the carapace of D. padillai FCG-CBP-75, showing the posterior almost parallel to the midline of the carapace projection of the naked costal rib of the most posterior costal bone; (G–H). Close-up of the most posterior portion of the shell of the extant sea turtle Caretta caretta NMW 31531(Natural History Museum, Vienna, Austria), showing the same pattern of naked costal rib posteriorly projected described in (G-H) for D. padillai. Abbreviations: ca, cavern; cr, costal rib; po, pore; rc, recrystallized calcite.
SIappendixVideoS1HorizontalView
Data Archiving Statement Video S1. Horizontal view of the specimen FCG-CBP-75
SIappendixVideoS2CoronalView
Data Archiving Statement Video S2. Coronal view of the specimen FCG-CBP-75
SIappendixVideoS3SagitalView
SIappendixFigS3
Supporting information Fig. S3. Comparison of the holotype of Desmatochelys padillai Cadena & Parham, 2015 and the D. padillai FCG-CBP-75 specimen described here in anterolateral views. A–B, D. padillai holotype exhibiting high keels along the midline of the carapace. C–D, D. padillai FCG-CBP-75 specimen exhibiting similar pattern of keels along the midline of the carapace.
SIappendixFigS4
Supporting information Fig. S4. Scanning electron microscope (SEM) images and elemental composition with energy-dispersive X-ray spectroscopy analyser (EDS) results for the middle portion of one of the eggshells of Testudoolithus oospp. A, SEM image of the calcite crystals; B, EDS elemental mapping at 15 kV for the region shown in A, predominantly calcium, carbon, and oxygen, consistent with crystals of calcium carbonate (calcite) and some traces of iron.
SIappendixFigS5
Supporting information Fig. S5. SEM images and elemental composition of the ventral (visceral) surface of one of the eggshells of Testudoolithus oospp., as well as the rock matrix that covers most of the shell bones and some of the eggs. A, ventral surface of the eggshell, showing small circular bumps interpreted as the organic cores of shell units; B, SEM and elemental composition analysis of a region of the ventral surface of the egg (see rectangle “B” in A), details of the composition are given in the table below A, indicating high carbon content. C, photograph showing the rock matrix covering most of the shell bones and some of the eggs; D, SEM and EDS analysis of the rock matrix (see rectangle “D” in D), details of the composition are given in the table below C, with absence of carbon and abundant contents of silica, aluminium, and iron.
SIappendixFigS6
Supporting information Fig. S6. Cathodoluminescence of the ventral surface of one of the eggs of Desmatochlys padillai exhibiting purple to cyan fluorescent colours corresponding to wavelengths between 400 and 500 nm.
