Wednesday 3 April 2019

Sauropodomorph Dinosaur eggs from the Early Jurassic of South Africa, China, and Argentina.

The Amniotes, Vertebrate animals in which the egg and developing embryo are contained within a watertight membrane (the amnion), and which are therefore capable of laying their eggs out of water, first appeared during the Carboniferous Period, around 312 million years ago. However, while the adult forms of these animals are numerous in the fossil record from the Permian onwards, their eggs no not become numerous as fossils until the Middle Jurassic, from which point forward numerous eggs of Crocodiles, Birds, non-Avian Dinosaurs, Pterosaurs, Lizards, and Turtles, are known. The earliest known Amniote eggs date from the Early Jurassic Sinemurian Epoch (195-192 million years ago), when the eggs of three different Sauropodomorph Dinosaur species are known, from South Africa, China, and Argentina, though only one of these has previously been formally described, due to the poor preservation of many of the specimens.

In a paper published in the journal Scientific Reports on 14 March 2019, Koen Stein of Earth System Science at the Vrije Universiteit Brussel, and the Directorate of Earth and History of Life at the Royal Belgian Institute of Natural Sciences, Edina Prondvai of Evolutionary Morphology of Vertebrates at Ghent University, and the Lendület Dinosaur Research Group at Eötvös Loránd University, Timothy Huang of the International Center of Future Science, and Dinosaur Evolution Research Center at Jilin University, and the National Chung Hsing University, Jean-Marc Baele of the Department of Geology and Applied Geology at the University of Mons, Martin Sander of the Steinmann Institute of Geology, Mineralogy, and Paleontology at the University of Bonn, and the Natural History Museum of Los Angeles County, and Robert Reisz, also of the International Center of Future Science, and Dinosaur Evolution Research Center at Jilin University, the National Chung Hsing University, and the Department of Biology at the University of Toronto Mississauga, provide descriptions of all three Sinemurian Sauropodomorph egg types, and discuss the implications of these for our wider understanding of egg evolution in Dinosaurs.

The eggs of Lufengosaurus are found in the Lufeng Formation of Yunnan Province; these have been briefly described previously as part of a study of the neonate Dinosaurs found within them. The calcareous layer of shells of Lufengosaurus are extremely thin, ranging from 60-90 μm in thickness, and made up of wedge-and-crown shaped units similar to those seen in the eggs of modern Crocodiles. These units are radial in nature and made of calcite, with interlocking crystals, and are imbedded in a phosphorous-rich fibrous layer thought to represent the original eggshell membrane. This is topped by an even thinner (about 10 μm) crystalline phosphatic layer, which has a scalloped appearance, with low pits and ridges that do not appear to directly corelate with the borders of the units in the calcareous layer. These irregularities may relate to the ornamentation seen in later Dinosaur eggs, but they are so small in scale as to make them effectively invisible to the naked eye, making their purpose unclear. Pores in the eggshell are hard to define, they appear to be quite sparse in distribution, but their actual density could not be established.

(a) Section through nugget containing numerous Lufengosaurus eggshell fragments (plane polarized light). (b), close-up (plane polarized light) of a Lufengosaurus eggshell fragment, showing calcite crystals of the mammillary layer radiating from an organic core embedded in the eggshell membrane. (c) As in (b) under cross polarized light, highlighting the calcite crystals of a mammillary cone. (d) Different cross polarized light view with lambda waveplate. (e) Line drawing of (d). (f) Lufengosaurus cathodoluminescence view with 880 nm filter. Scale bars, (a) 1 mm, (b)–(f) 50 μm Abbreviations: cl, calcareous layer; em, eggshell membrane; ps, pore space; su, shell unit. Stein et al. (2019). 

Eggs attributed to the Sauropodomorph Dinosaur Massospondylus are known from a number of locations in South Africa. These eggs have a slightly thicker shell than those of Lufengosaurus, in the range of 80-100 μm, but have a similar structure, with radially arranged calcite crystals arising from a phosphorous rich membrane, and an outer layer which is rugged with tubercles and depressions. Pores are again sparse.

(i) Massospondylus eggshell fragment (ppl), showing wedges in the calcareous layer, and a homogenous eggshell membrane. (j), Line drawing of (i). (k) Massospondylus cathodoluminescence view with 880 nm filter. Scale bars, (i), (j) 100 μm, (k) 50 μm. Abbreviations: cl, calcareous layer; em, eggshell membrane. Stein et al. (2019).

Shells assigned to the Sauropodomorph Mussaurus are known from the Laguna Colorada Formation of Argentina, again associated with embryo material but never before formally described. These shells are not well preserved, but the one examined appeared to follow the same pattern as the other two species, with a thick (150-180 μm) phosphatic layer interpreted as a membrane, with sparse preserved calcite crystals apparently radiating from growth points.

(g) Mussaurus eggshell, showing thick eggshell membrane, and distorted calcareous layer. (h) Line drawing of (g). Scale bars 100 μm. Abbreviations: cw, crystal wedges of calcareous layer; em, eggshell membrane. Stein et al. (2019). 

Fragments of all three Sauropodomorph egg types retain their curvature, suggesting that the shells had a rigid structure, though all three have very thin mineralised shells. This contrasts with later Dinosaur eggs, which are widespread from the Middle Jurassic onwards, which all have thicker shells (as do modern Birds). This is particularly curious as the later eggs come from a wide variety of Dinosaur species, including Theropods, Ornithischians, and Sauropods (which were presumably more closely related to the Early Jurassic Sauropodomorphs than they were to the other two groups). However, beyond the differences in thickness, all these Dinosaur eggs are essentially similar in structure, with shells made up of interlocking calcite crystals which radiate from points of growth on the underlying membrane.

Eggshell membrane and porosity in Massospondylus eggs. (a) Nest of Massospondylus eggs with preserved embryos. Note the presence of numerous cracks in the eggs, likely caused by postmortem crushing of the thin but hard eggshell. Eggshell membrane is exposed in egg number 4, just beneath the skull, and in egg number 7, just beneath the right scapula. (b) CT scan of a complete egg in a, showing the eggshell (es) and the detached preserved eggshell membrane (em). (c) Outer surface SEM image of a Massospondylus eggshell fragment showing rare small and irregularly shaped pores occurring in random patterns (red arrows). (d) Enlarged view of boxed area in (c). Stein et al. (2019).

Mineralised Crocodile eggs also become widespread in the Middle Jurassic; these are similar in nature to Dinosaur eggs in their mineralogy, but their structure is somewhat different, which suggests they may have a common mineralised ancestry, but that it was long before the appearance of their eggs in the fossil record. Turtle eggs appear at about the same time, but are aragonitic in structure, and Turtles are generally thought to have evolved mineralised eggs separately.

Since strongly mineralised eggs have a good preservational potential, Stein et al. suggest that the absence of such eggs in the fossil record before the Middle Jurassic probably suggests that they were not being produced. This raises the question of why several different Archosauromorph lineages should suddenly start to produce mineralised eggs in a very short period, all over the world. Stein et al. reason that mineralised eggs have a structural and protective benefit that would have been just as useful before the Middle Jurassic as it was after, and that therefore some ecological constraint must have prevented them from doing so up until this point.

Reconstruction of a basal Sauropodomorph egg showing detail of the eggshell. Eggshell units (esu) form the calcareous layer (cl) and are embedded with organic cores in the eggshell membrane (em). Stein et al. (2019).

Stein et al. theorise that the most likely explanation for this may have been atmospheric oxygen levels. The Earth’s atmosphere is thought to have contained about 32-33% oxygen during the Carboniferous, with oxygen levels falling steadily during the Permian and Triassic, eventually falling to around 15% in the Early Jurassic, then rising to around 20% in the Middle Jurassic, a level which has remained more-or-less constant till today. Modern Archosaurs are known to produce weakly mineralised eggs if they are suffering from oxygen starvation, which suggests that high levels of oxygen are needed for mineralised egg production, with the possibility that oxygen levels in the Triassic and Early Jurassic were too low for mineralised egg production, but that once sufficient oxygen became available, multiple Archosauromorph lineages quickly developed the ability to produce such eggs.

Ancestral state reconstruction of calcareous layer thickness to egg mass ratios. Note that the root was set to represent the hypothesized ancestral flexible shelled condition. Nodes represent (a), Archosauromorpha, (b), Archosauria (c), Ornithodira, (d), Dinosauria, (e), Birds. Note the independent acquisitions of thick eggshell in Choristoderes (represented by Hyphalosaurus), Chelonians, Crocodiles, Pterosaurs and several Dinosaur clades, as well as reversals in Chelonians. From the Sinemurian (199 million years ago) onwards, eggshells (e.g. Testudoflexoolithus and Lourinhanosaurus) show a significant calcareous layer thickness increase corresponding with atmospheric oxygen increase. Stein et al. (2019).

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