Macroolithus
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Macroolithus Temporal range: Late Cretaceous |
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| A pair of Macroolithus yaotunensis eggs | |
| Eggshell classification | |
| Basic shell type: | Ornithoid |
| Morphotype: | Ornithoid-ratite |
| Oofamily: | †Elongatoolithidae |
| Oogenus: |
†Macroolithus Zhao, 1975 |
| Type oospecies | |
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Oolithes rugustus Young, 1965 |
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| Oospecies | |
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Macroolithus is an oogenus (fossil-egg genus) of dinosaur egg belonging to the oofamily Elongatoolithidae. The type oospecies, M. rugustus, was originally described under the now-defunct oogenus name Oolithes. Three other oospecies are known: M. yaotunensis, M. mutabilis, and M. lashuyuanensis. They are relatively large, elongated eggs with a two-layered eggshell. Their nests consist of large, concentric rings of paired eggs. There is evidence of blue-green pigmentation in its shell, which may have helped camouflage the nests.
Macroolithus eggs have been found containing oviraptorid dinosaur embryos resembling Heyuannia. Multiple other associations between oviraptorid and elongatoolithids (including other eggs containing embryos, parents brooding on nests, and a pair of shelled Macroolithus-like eggs preserved within an oviraptorid's pelvis) confirm that the parent of Macroolithus was an oviraptorid.
It is found in Upper Cretaceous formations of central and eastern Asia; fossils have been found in Mongolia, Kazakhstan and China. In the Nanxiong formation in Southern China, Macroolithus fossils range up to and possibly over the Cretaceous-Tertiary boundary, which is traditionally assumed to mark the extinction of the non-avian dinosaurs. Some paleontologists have interpreted the record of dinosaur eggs at this formation as supporting a gradual extinction event, rather than a sudden cataclysmic event. However, other paleontologists believe that these interpretations are merely based on artifacts of erosion and redeposition in the early Paleogene.
Macroolithus eggs are characterized by large size, measuring 16 to 21 cm (6.3 to 8.3 in) long, and by their particularly coarse ornamentation. Their microstructure is not well defined in the literature, but generally follows the typical elongatoolithid pattern: The eggshell is arranged into two structural layers (the mammillary layer and the continuous layer). The continuous layer forms the outer part of the eggshell; its eggshell units are fused together so that the layer appears to be a continuum. The inner layer, known as the mammillary, or cone, layer is made up of cone-shaped structures that form the base of the eggshell units. In Macroolithus, the continuous layer is two to three times thicker than the mammillary layer. The eggs have great diversity of pore structure. Since gas conductance is related to the pore size and density this could imply that the eggs were laid in variable environments.
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