Incubation of Continuoolithus canadensis eggs from the late Cretaceous Two Medicine Formation of Montana

Abstract

Incubation strategies of extinct organisms are largely speculative. Most inferences of nesting behavior are based on comparison with closely related extant taxa. To date the most useful evidence for support of a particular incubation strategy includes water vapor conductance (determined from fossil eggshell) and sedimentary structures, associated with eggs or eggshell, that are interpreted as fossil nests. One such sedimentary structure (MOR 3062) from the Upper Cretaceous Two Medicine Formation of Montana, is described along with the microstructure of the associated eggs and eggshell (oospecies Continuoolithus canadensis). The green nest trace truncates red mudstone and differs in sedimentary attributes. Geochemical analysis indicates that the host substrate was highly oxidized compared to the in-filled nesting structure. This is probably due to presence of decaying vegetative debris within the structure. A Continuoolithus canadensis egg removed from MOR 3062, along with Continuoolithus canadensis eggs from the nearby site TM-006, have a water vapor conductance value of 119 mgH ²O/ (day Torr). This equates to a 4.1 times greater mean water vapor conductance in Continuoolithus canadensis than a bird egg of the same mass. American alligator (Alligator mississippensis) eggs have a water vapor conductance rate 4.2 times greater than a bird egg of the same mass. Strong similarities exist between relative water vapor conductance of Continuoolithus canadensis and modern alligators. In addition, characteristics of MOR 3062 suggest the eggs were buried in a mixture of sediment and vegetative debris: therefore, I conclude that Continuoolithus Canadensis eggs were incubated underground, possibly with the aid of heat produced by rotting vegetation.