Abstract
Fossil amniotic eggs have great informative potential, especially regarding reproductive and evolutionary aspects of vertebrates. However, there are only few intact specimens or with fossilized embryos within, and the rare reported cases are mostly related to dinosaurs. In Brazil, the records of these ichnofossils are practically restricted to the Bauru Basin. This research aims to describe the first amniotic egg found in carbonate concretions in the Romualdo Formation, adding information to the study of these fossils and to the paleontological context of the basin. The specimen was collected at the Sítio Pé da Serra do Félix, in the Municipality of Simões, Piauí State, Brazil. The methodology employed was based on Scanning Electron Microscopy, Energy Dispersive Spectroscopy and Optical Microscopy techniques, as well as computed tomography analyses. The morphological and microstructural characteristics of the shell suggests the identification of the fossil as a crocodylomorph egg. This specimen differs from other fossil eggs assigned to the aforementioned group by its small size and considerably thick shell. The tomographic sections revealed possible basic structures of an embryo inside the egg, suggesting that this is the first fossilized egg with a crocodylomorph embryonic trace found in the world. Keywords: ichnofossil, amniotic egg, embryo, Crocodylomorpha, Aptian, Romualdo Formation. Ovos amnióticos fósseis possuem um grande potencial informativo, especialmente em relação aos aspectos reprodutivos e evolutivos dos vertebrados. Contudo, existem poucos exemplares intactos ou dotados de vestígios embrionários em seu interior e os raros casos registrados são em sua maioria relacionados a dinossauros. No Brasil, os registros desses icnofósseis estão praticamente restritos a Bacia Bauru. O presente trabalho objetiva descrever a primeira ocorrência de um ovo amniótico encontrado em concreções carbonáticas da Formação Romualdo, acrescentando informações ao estudo desses fósseis e ao contexto paleontológico da bacia. O espécime foi coletado no Sítio Pé da Serra do Félix, no Município de Simões, Piauí, Brasil. A metodologia empregada baseou-se nas técnicas de Microscopia Eletrônica de Varredura, Espectroscopia por Energia Dispersiva e Microscopia Óptica, bem como análises de tomografia computadorizada. As características morfológicas e microestruturais da casca sugerem que o fóssil se trata de um ovo de crocodilomorfa. O espécime analisado difere de outros ovos fósseis atribuídos ao referido grupo taxonômico pelo tamanho reduzido e casca consideravelmente espessa. As seções tomográficas revelaram possíveis estruturas básicas de um embrião no interior do material, sugerindo que se trata do primeiro ovo fossilizado de crocodilomorfa com restos embrionários do mundo. Palavras-chave: icnofóssil, ovo amniótico, embrião, Crocodylomorpha, Aptiano, Formação Romualdo.
Highlights
The formation of an ichnofossil involves three main factors: the producer, the preserved behavior pattern, and the substrate and sediment types that allowed its preservation
The dimensions of Museu Dom José (MDJ) Ic-069 are small compared to other crocodylomorph eggs from the Brazilian Cretaceous, such as a specimen from the Crato Formation (Magalhães-Ribeiro et al, 2011), or those described by Oliveira et al (2011) and Marsola et al (2016) from the Adamantina Formation, in the Bauru Basin
Its size resembles eggs from the Araçatuba Formation in the Bauru Basin ascribed to Mariliasuchus amarali (MagalhãesRibeiro et al, 2006), as well as from eggs reported by Novas et al (2009) from the Bolivian Cretaceous (Table 1)
Summary
The formation of an ichnofossil involves three main factors: the producer, the preserved behavior pattern, and the substrate and sediment types that allowed its preservation. The relevance of the study of an ichnofossil is the aid in paleoenvironmental and paleoecological interpretations of organic activities in a given geological time, as well as highlighting the behavior of different organisms and the sedimentation conditions in the past (Souto, 2017). As such, fossilized amniotic eggs are very important as they contribute to elucidate the reproductive and evolutionary aspects of vertebrates, especially if they harbor embryos. These fossils are very rare due to their fragility, and most of the record is of dinosaurs, with reports of nests, complete or fragmented eggs, and the occurrence of embryonic integument (Mikhailov, 1997; Chiappe et al, 1998; Chiappe et al, 2001; Grellet-Tinner et al, 2004, 2012; Srivastava et al, 2015). There are only occurrences of putative eggshells of uncertain taxonomic affinities from the Aliança Formation (Upper Jurassic, Jatobá Basin) in the Pernambuco State (Silva et al, 2012), of eggshells ascribed to dinosaurs from the Itapecuru Formation (Lower Cretaceous, Parnaíba Basin) in the Maranhão State (Vicalvi et al, 1993), and of a complete crocolylomorph egg from the Crato Formation (Lower Cretaceous, Araripe Basin) in the Ceará State (Magalhães-Ribeiro et al, 2011)
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