New light shed on the oldest (?flying) insect

["Tim Williams" <twilliams_alpha@hotmail.com>]

A very interesting article appears in tomorrow's _Nature_, which might give 
some hope to those who argue for an origin of avian flight long, long before 
the time of _Archaeopteryx_.

Engel, M.S. and Grimaldi, D.A. (2004).  New light shed on the oldest insect. 
 Nature 427: 627-630.

Abstract: "Insects are the most diverse lineage of all life in numbers of 
species, and ecologically they dominate terrestrial ecosystems.  However, 
how and when this immense radiation of animals originated is unclear.  Only 
a few fossils provide insight into the earliest stages of insect evolution, 
and among them are specimens in chert from Rhynie, Scotland's Old Red 
Sandstone (Pragian; about 396?407 million years ago), which is only slightly 
younger than formations harbouring the earliest terrestrial faunas.  The 
most well-known animal from Rhynie is the springtail Rhyniella praecursor 
(Entognatha; Collembola), long considered to be the oldest hexapod.  For 
true insects (Ectognatha), the oldest records are two apparent wingless 
insects from later in the Devonian period of North America.  Here we show, 
however, that a fragmentary fossil from Rhynie, Rhyniognatha hirsti, is not 
only the earliest true insect but may be relatively derived within basal 
Ectognatha.  In fact, Rhyniognatha has derived characters shared with winged 
insects, suggesting that the origin of wings may have been earlier than 
previously believed.  Regardless, Rhyniognatha indicates that insects 
originated in the Silurian period and were members of some of the earliest 
terrestrial faunas."

Later on, the authors make an oblique reference to the origin of avian 
flight:

"The metapterygotan structure of Rhyniognatha's mandibles has profound 
implications regarding insect evolution.  Current fossil evidence indicates 
that insect wings originated in the Early Carboniferous period, some 90, 170 
and 270 Myr before pterosaurs, birds and bats, respectively, and these 
structures are believed to have led largely to the spectacular 
diversification of insects.  Fossil insects are completely absent from the 
Late Devonian and Early Carboniferous, and a significant diversity of 
palaeopterous and neopterous species appeared suddenly in the earliest Late 
Carboniferous."

...and:

"A Devonian origin of winged insects is highly relevant to several current 
hypotheses on the origin of wings.  One hypothesis is that metabolically 
expensive insect flight evolved in hyperoxic atmospheres similar to what 
occurred in the Carboniferous and Permian, but the oxygen content of 
Devonian atmospheres was far less, approximately 15%.  Also, the paranotal 
theory hypothesizes that insect wings evolved from lateral extensions of the 
thorax, called paranotal lobes ... originally used for controlled gliding 
similar to what modern silverfish are capable of.  Under this hypothesis, 
paranotal lobes presumably evolved later into broader, hinged structures 
capable of powered flight, analogous to the transitional forms seen with 
Archaeopteryx and modern birds and with 'flying lemurs' (Dermoptera) and 
bats."



Tim

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