Scott Richard Anderson A primitive ant brood chamber with evidence of
brood care in Burmese amber (Lower Cretaceous) -
implications for brood care as the facilitating factor
for true eusociality and dominance of ants
Abstract: Ants are one of the most successful and ecological dominant organisms on Earth, owing their success and dominance
to their advanced social structure, eusociality. While many new discoveries of primitive ants and studies have occurred, the origins
of the true ants and their evolution of eusociality remains largely unexplained. Until now, evidence of eusociality in the primitive
ants has been based on morphological features (presence of different castes and metapleural gland) with inference of the critical
requirement of brood care. For the first time, direct evidence of brood care is observed in a Cretaceous ant specimen. A primitive
ant of undetermined subfamily (though not Sphecomyrminae) occurs in a Burmite specimen along with nest material, an
ant egg and food for ant brood (arthropod prey and ant eggs - oophagy). While this specimen containing an ant brood chamber
answers questions as to the origin of eusociality in primitive ants, observations of this specimen compared to other primitive ants
(specifically Sphecomyrminae) raises many new questions. Most of these questions center on: If primitive ants were eusocial, why
did one lineage become extinct (Sphecomyrminae) while others survived and later explosively diversified into the dominant organisms
that they are today? Interpretations of general morphological features of the worker caste coupled with their social roles
allows for the postulation that brood care was the facilitating factor that helped establish the dominance of particular ant lineages
originating in the Cretaceous. This non-Sphecomyrminae worker ants generally appears to be larger and more graceful (exhibits
very long legs and slim body) with smaller eyes and simple mandibles, suggesting adaptation to specialized brood care within
the nest. In contrast, Sphecomyrminae generally have stouter bodies and bigger eyes (compared to this new specimen) and
likely development of non-traditional social roles, suggesting that they are better adapted to hunting and scavenging and activities
outside the nest (and brood). While oophagy probably occurred in the specimen herein presented, it is also known to be common
in many primitive ant lineages, thus providing an advantage to these non-Sphecomyrminae ants as well as an engine for evolutionary
change. Concluding that more advanced social structure was attained compared to their counterparts (Sphecomyrminae),
these non-Sphecomyrminae lineages were able to form more complex nests with larger populations. With these social and
perhaps evolutionary advantages, these non-Sphecomyrminae lineages were poised to explode in diversity and numbers during
the early ant radiations of the ever increasingly diversifying Cretaceous forests becoming the superorganisms that they are today.
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