Introduction

The Fox Mesa microvertebrate site represents an unusual concentration of eggshell and vertebrate bone (some embryonic), located in the Late Jurassic Morrison Formation of the Big Horn Basin, Wyoming. Because such sites are uncommon in the Morrison Formation, Fox Mesa affords a rare opportunity to examine reproductive behavior in this ancient ecosystem.

Different types of archosaur eggshells can be distinguished based on their crystalline structure and layering, the orientation and type of pore canals, and surface ornamentation. We analyzed eggshell fragments from the Fox Mesa site to determine whether they were made by a single taxon, its potential identity, and the nature of the original concentration

Materials & Methods

The site was mapped into blocks, and a detailed stratigraphy of it was done.

The eggshells were sorted by block, embedded in epoxy, and then thin sectioned.

The exposed surfaces of the epoxy blocks were polished and etched in acetic acid, and then examined under environmental scanning electron microscope (ESEM). The thin sections were viewed under polarized, transmission, and cathodoluminescent light microscopes.

The bones were picked under a binocular microscope and sorted by block, identifiability, as well as ontogenetic stage. They were then photographed and examined for diagnostic features.

The teeth were photograhed under ESEM, and one was embedded in epoxy and sectioned to study the microstructure.

Results

Eggshell

The eggshell exhibits the two layered structure of a theropod egg, with the possibility of a third layer typical of a more derived theropod egg. Eggshell structure is consistent regardless of where at the site it was found, meaning all of the eggs are of the same type, and thus laid by the same taxon. The eggs also show both oblique and vertical tubular pore canals.

Bone

The most predominant single type of bone fragment at the site displays the grain and morphological features diagnostic of embryonic bone. When taken together, the only pieces of embryonic bone that are identifiable are all assignable to Theropoda. This strongly suggests that all of the embryonic bone is derived from one theropod taxon. The high level of pneumaticity in this early ontogenetic stage also suggests coelurosaurian affinities.

Teeth

The embryonic teeth come in two forms: serrated and unserrated. The serrated are diagnostic to Theropoda, and possibly to Coelurosauria based on the proportions and root constriction. The unserrated teeth appear to be germ teeth of the same taxon.

Discussion

Morrison Formation coelurosaurs are very poorly known, and the reproductive strategies of Morrison Formation dinosaurs are utterly unknown. The myriad eggshells and stratigraphy indicate more than one egg-laying event occured, and the two thin eggshell layers as well as the fragility of eggshells themselves further suggest that each layer represents a single event. This raises the possibility of group nesting among early coelurosaurs. The low level of transport as well as the unique assemblage may also represent a new type of microsite.

Conclusions

• All of the eggs were laid by one taxon
  
  
• The embryonic/newly hatched bones and teeth
  belong to the same taxon as the eggs
  
  
• That taxon is a member of Theropoda
  
  
• That taxon is possibly a member of Coelurosauria
  
  
• That nesting may have occured in a communal
  setting among this early coelurosaur.

Acknowledgements

I would like to thank Scott Whittaker for the teaching me how to use the ESEM and letting me spend countless hours asking him for more help, Steve Jabo for helping me in every way possible, from prepping embryos to acid-etching eggshells, Jonathan Wingerath for teaching me how to thin section fossils, Tim Rose for help with the cathodoluminescense, Peter Kroehler for his tireless work on the site and his generous donation of an ostrich egg, Erik Kvale for discovering the site, Mike Brett-Surman for his previous work on the site, the RTP for fieldwork support, Cliff and Row Manuel for making that fieldwork comfortable, Mary Sangrey for organizing the whole RTP, the Dick Benson Endowment for financing my tenure here, and many others who made all of this possible.