Australia: The Land Where Time Began

A biography of the Australian continent 

Insects of the Cretaceous

In their book¹ the authors¹ visualise what they believe are the likely relationships between insects and dinosaurs, using fossil insects from the Cretaceous, suggesting how they may have affected populations of dinosaurs, basing their interpretations on the ecology and behaviour on the living insects that descended from those ancestral insects.

According to the authors¹ that when considering the Cretaceous insects from 100 Ma the current classification system of insects has not much significance from the family level down, as they are often what they have called chimaeras, with 2 or more morphological features found in insects of the present, making their classification difficult. The Cretaceous has proven to be a time of great structural and behavioural diversification of insect taxa, with habitat partitioning changing continuously. Any conclusions reached regarding these rare fossils has to be based on physical features of the insects as to obtain their DNA would destroy them, their evolution and extinction being restricted to speculation. The authors¹ suggest some of the shared characters may be the result of convergent evolution, while others result from spontaneous mutation which would be refined by adaptive radiation, and they also some of these chimaeras may be throwbacks to earlier lineages, possibly as a result of inhibitors being switched off during development that would allo latent DNA in the genome to be activated, thoough some may have features of 2 groups because they are intermediate between the both groups.

A chimaera from the amber found  in Burma was Hapsomella, a stone beetle with an ant-like appearance, but with both front legs having 6 segment instead of the 5 segments that are present in all extant insects. Among living arthropods the closest modern groups to have legs with more than 5 segments are spiders and mites, though this fossil was definitely an insect. Crustaceans, arachnids and millipedes all have more than 5 segments, but all known living insects are restricted to 5. In the Palaeozoic there are some insects that are known to have more than the standard 5 segments.

Also in amber from Burma is a tick Cornupalpatum, that is also a chimaera, that has small palps, sensory appendages, had 5 terminal claws. Such claws are found on some predatory mites from the present, where they are used for subduing prey, though they have not been seen on ticks from the present. Features of a mite being present on a tick is not usual, leading to the question of why they were there.

Aphids from Burmese amber also have a feature distinctive from all other known aphids both extant and extinct, they have 2 wings as opposed to 4 on all other known aphids. Most characters indicate they belong with extant aphids, they are aligned with the scale insects by their vestigial hind wings, male scale insects having rudimentary hind wings called hamulohalters. These hamulohalters appear to be similar to those present on flies, but their function is not known in scale scale insects. 

Sources & Further reading

1. Poinar Jr, George & Poinar, Roberta, What Bugged the Dinosaurs Insects, disease and Death in the Cretaceous, Princeton University Press.