The Aves Prevented the Evolution of Giant Flying Insects during the Mesozoic

Scientists at the University of California (Santa Cruz) have set about analysing the fossil record of flying insects in a bid to determine the affect on insect size as a result of the evolution of other flying creatures such as the birds and the Pterosaurs.  The largest flying insects in the fossil record are found in Carboniferous aged strata, they include examples of the giant dragonfly Meganeura which had a wingspan of seventy-five centimetres.

Many palaeontologists believe that large flying insects could evolve during the Carboniferous because the atmospheric levels of oxygen were very high.  Scientists have suggested that the oxygen levels reached around 28-30% in the atmosphere, much higher than today’s figure of approximately 21%.  The denser air would have made powered flight easier and the high concentration of oxygen would have permitted giant flying insects to get enough oxygen to their flight muscles through their spiracles.  Oxygen concentration level is a key physiological factor in the control of insect body size, particularly in groups that have high oxygen demands, such as those that undertake powered flight.

Fossil Dragonfly Wingspan Compared to Extant Species

Giant flying insects of the Palaeozoic

Picture Credit: Wolfgang Zessin

The picture shows the fossilised impression of a Carboniferous-aged dragonfly (Stephanotypus schneideri) compared to the wing of the largest type of dragonfly alive in more recent times.

However, in this study, the research team postulate that the relationship between insect body size and atmospheric oxygen levels is disrupted in part, and that during the Mesozoic and the later Cenozoic, despite fluctuations in oxygen levels and an increase in atmospheric O2 concentration, large flying insects did not evolve.

To read more about the research identifying the relationship between oxygen levels and insect body size: High O2 levels led to Super-sized Flying Insects

Matthew Clapham, an assistant professor of Earth and planetary sciences at the University of California (Santa Cruz) and Jered Karr, a graduate student compiled a huge database of fossilised insect wing measurements.  They were then able to use this data, in conjunction with information about prehistoric atmospheric oxygen levels to plot insect size and O2 concentrations.  Their paper has been published this week in the scientific journal “The Proceedings of the National Academy of Sciences”.

Commenting on the results of the study, assistant professor Matthew Clapham stated:

“Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years.  Then right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down.  And this coincides really strikingly with the evolution of birds.”

The analysis shows that insect body size becomes less dependent on atmospheric oxygen concentration, it seems that other factors begin to impact on this relationship.  With the evolution of the birds, insects were no longer aerial masters and the need for maneuverability or the need to be small may have taken over as the driving force of insect evolution with oxygen levels becoming less pivotal.

Interestingly, the analysis provided only weak support for an effect on insect size with the evolution of the Pterosaurs in the Triassic.  The first Pterosaurs are believed to have evolved around 225 million years ago.  The fossil record shows that there were larger flying insects in the Triassic than in the Jurassic but the lack of Early Jurassic aged insect fossils prevents the scientists from making any firmer conclusions about the impact of Pterosaur predation on flying insects.  A drop in global oxygen levels during this Triassic/Jurassic part of the Mesozoic further complicates the analysis.

Another transition in insect size occurred more recently at the end of the Cretaceous period, between 90 and 65 million years ago. Again, a shortage of fossils makes it hard to track the decrease in insect sizes during this period, and several factors could be responsible.  These include the continued specialisation and rapid diversification of neornithes (modern birds) the evolution of bats, and the mass extinction event at the end of the Cretaceous.

When asked to provide an explanation for the decline in insect size towards the end of the Mesozoic, Clapham said:

“I suspect it’s from the continuing specialisation of birds.  The early birds were not very good at flying but by the end of the Cretaceous, birds did look quite a lot like modern birds.”

The researchers stressed that this study focused on changes to the maximum size of insects over time.  The average insect size in any geological period was much more difficult to calculate due to a bias for larger insect remains to be preserved in the fossil record.  Larger insects are more likely to be preserved as fossils and found by fossil hunters than smaller insects.

Commenting on this aspect of the research, Matthew added:

“There have always been small insects.  Even in the Permian when you had these giant insects, there were lots with wings a couple of millimetres long.  It’s always a combination of ecological and environmental factors that determines body size, and there are plenty of ecological reasons why insects are small.”

This study that suggests a decoupling of insect size and atmospheric O2 concentrations in favour of other factors such as the emergence of vertebrates capable of powered flight has implications for scientists trying to predict the consequences of extinction rates in our own time.

If the competition from the Aves (birds) was removed due to extinction then if the Earth’s oxygen levels were to rise, those organisms left could well face the possibility of sizeable flying insects evolving once again.  The insects could once again become the masters of the air.

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