This study resolves novel characters that may inform blattopteran phylogeny, but a revision of this populous assemblage and the position of A. eggintoni will require analysis of multiple taxa, and is beyond the scope of this paper; we focus instead on mode of life.
The antennae preserve a scape, pedicel and flagellum. They are filiform, and while probably incomplete, long annuli suggest the flagellum could comprise significantly fewer pseudosegments than found in crown-group cockroaches, which can possess in excess of 150. One is preserved parallel to the long axis of the organism, with the other at 70° to this. This flexibility suggests that A. eggintoni had well-developed sensory abilities, using its antennae to sweep a broad sensory arc as it moved, in a manner analogous to extant cockroaches.
The mandibles of A. eggintoni
are closely comparable to the mouthparts of crown-group cockroaches, which are generalists, with great dietary versatility. The asymmetry preserved (left mandible larger) is often seen in extant cockroaches (Zhuzhikov 2007
), the left overlapping the right, allowing the teeth to interdigitate (e
). Archimylacris eggintoni
has four sizeable denticles in the incisor region, an arrangement also seen in Blaberus atropos
, fig. 4d
), but simpler to that in pest species such as Periplaneta americana
, which has several additional denticles (Zhuzhikov 2007
, fig. 4c
). Blaberus atropos
is often found in decaying litter, epiphytes, hollows of trees and inside rotting logs (Bell et al. 2007
). This—coupled with the Coal Measures environments in which the Blattoptera are typically found—suggests that A. eggintoni
and other Carboniferous forest-dwelling stem-group Dictyoptera were saprophagous/detritivorous.
The limbs display adaptations that are indicative of rapid movement in extant cockroaches (i.e. a cursorial habit), including flattened, thin podomeres (Full & Tu 1990
), a long femora and tibiae in all limbs (Gullan & Cranston 2005
) and a low angle between the limbs and the body (Kram et al. 1997
). Motion studies (Frazier et al. 1999
) have shown that the five tarsomere arrangement is highly advantageous for rapid traversing of irregular terrain. The proportionally longer hind limbs of A. eggintoni
are also present in pest species such as P. americana
. This species takes a bipedal stance when running, overcoming limitations imposed upon stride length by shorter anterior limbs (Full & Tu 1990
); P. americana
is one of the fastest invertebrates known, relative to mass (Bell et al. 2007
). The specializations towards cursorial habit seen in A. eggintoni
suggest that it too was capable of very rapid movement.
Pretarsal claws are used in crown-group cockroaches only for climbing rough surfaces (Bell et al. 2007
), suggesting that A. eggintoni
was capable of venturing beyond the relative safety of the leaf litter. This is supported by the presence of euplantulae (f
), used in modern forms for climbing smooth vertical surfaces such as those of plants (Bell et al. 2007
). These structures tend to be lost by pure leaf-litter dwelling cockroaches, but retained by those which perch, forage or oviposit in leaves and on plants (e.g. Deans & Roth 2003
The posterior of a flattened body is preserved (c
). This, in conjunction with the low-angle limbs, creates a flat insect that could fit easily into crevices, suggesting a cryptic habit similar to that of crown roaches. Cerci (paired tail appendages) are known in all crown-group cockroaches, and their presence in primitive winged insects, e.g. the Palaeoptera, and Polyneoptera, suggests they are plesiomorphic to the Dictyoptera. Their absence here could result from decay. Duncan et al. (2003)
present a series of cockroach decay experiments in which the disarticulation of cerci precedes that of the limbs, which themselves precede the loss of the ovipositor. The left hindleg in this fossil is missing, suggesting that partial limb decay is recorded, and hence that decay has progressed beyond the loss of the cerci. The remaining limbs appear well preserved, suggesting that the absence of an ovipositor is not taphonomic. A more plausible explanation for its absence is that the specimen is a male.
This study demonstrates the use of µCT-based approaches in the study of siderite-hosted fossils; otherwise unobtainable characters are resolved that—in insects—provide additional characters to wing venation for building phylogenies. The resolution of appendage morphology can provide new insights into the mode of life of Carboniferous organisms. Archimylacris eggintoni
shows a high degree of specialization early in the evolution of insects. It was likely to have been a fast runner in life, but also had the ability to climb plant surfaces. Its diet was probably comparable to that of modern forest cockroaches. Its anatomical—and by inference functional and ecological— similarity to crown-group cockroaches (e.g. Bell et al. 2007
, figs 1.4, 2.7A, 2.8 and 3.7) is apparent; this emphasizes the cockroach-like nature of the dictyopteran stem group, highlighting in contrast the derived and specialized nature of the mantids.