Geraridae

Reconstruction of Gerarus danielsi, from Burnham (1983).

Belongs within: Orthoptera.

The Geraridae are a group of insects known from the Upper Carboniferous. Members of the type genus, Gerarus, had an expanded spinose prothorax; prothoraces of other genera are unknown (and the monophyly of the larger Geraridae was regarded as questionable by Béthoux & Nel 2002).

All about Gerarus
Published 13 May 2009

There can be no doubting that the fossil record has provided us with knowledge of some extremely cool organisms. The funny thing is, not all of these extremely cool organisms are very well known. Most popular books on extinct animals tend to select from the same relatively small pool—dinosaurs, ammonites, maybe a trilobite or two. But there are other organisms that one would think would be the stuff of celebrity, but which get almost no screen-time at all. Take Gerarus, for instance. Gerarus is one of the most abundant of Carboniferous insects—specimens have been recovered from almost all major terrestrial deposits of this time, including localities such as Mazon Creek in Illinois and Commentry in France (Béthoux & Briggs 2008). It’s a fairly large insect—some species had wingspans of over ten centimetres. But, beyond all this, the really awesome thing about this critter was that it the unholy offspring of a mantis and a medieval mace. Gerarus was the proud owner of an inflated thorax, liberally studded with prominent don’t-f***-with-me spines up to a millimetre in length. Wings of different individuals were notably variable in their venation patterns, suggesting relaxed selectional pressure, and this together with the shift in weight that would have resulted from the hypertrophied thorax suggest that Gerarus was probably not a very active flier (Béthoux & Nel 2003). Instead, it would have clambered on vegetation like a stick insect, relying primarily on its spines to dissuade potential predators. If that wasn’t enough, it could escape by jumping and using its wings as passive gliding planes.

The aforementioned variability of Gerarus and the other members of the family Geraridae led to earlier authors describing nearly every specimen as a separate species. Many of these were synonymised by Burnham (1983) in her review of the family, but it is quite possible that the group is still over-split. Initially, gerarids were included in the “Protorthoptera”, an unabashedly paraphyletic or polyphyletic grouping of Palaeozoic polyneopteran-grade insects that was believed to be ancestral to such modern groups as cockroaches, crickets and stick insects, and possibly even to all other recent neopteran insects. When some more specific affinity was hypothesised, it was usually to the Orthoptera (crickets and grasshoppers). This hypothesis was challenged by Kukalová-Peck and Brauckmann (1992), who identified an expanded clypeus in Gerarus (the clypeus is the front part of an insect’s head). This, together with certain features of the wing venation, lead them to position Gerarus closer to the Paraneoptera, the group including Psocoptera (booklice) and Hemiptera (bugs). Even more notably, they also identified exites on Gerarus‘ legs.

Figure of Gerarus danielsi specimen from Kukalová-Peck & Brauckmann (1992), as reproduced in Béthoux & Briggs (2008), showing exites attached to the legs.

Kukalová-Peck is best known for her theories on the origin of insect wings. Many fossil arthropods, and modern crustaceans, possess branched legs, and Kukalová-Peck holds that ancestral insects also possessed such legs, with the wings developed from side-branches (exites) that have become dissociated from the legs and moved closer to the top of the thorax. This contrasts with the earlier idea that insect wings were derived from dorsolateral lobes of the thorax itself. Kukalová-Peck’s model has certainly got some points in its favour—it avoids the difficulty of a transition from a fixed lateral lobe to a mobile, articulated wing, and genetic studies have shown that similar genes are involved in the development of Drosophila wings as in that of crustacean gills (which are undoubtedly derived from exites). Kukalová-Peck also identified the presence of exites in a number of fossil insects as further support for her model (Kukalová-Peck 1987).

However, there are a couple of stumbling blocks. Firstly, those fossil insects on which exites have been identified are phylogenetically nested among modern insects with unbranched legs, which would require the convergent loss of exites in a number of independent lineages (not impossible—exite loss seems to be directly connected to adaptation to life on land for arthropods). Secondly, and perhaps more damningly, some of Kukalová-Peck’s reconstructions have been accused of (shall we say) a certain excess of imagination. Béthoux & Nel (2003) re-interpreted the wing venation of Gerarus, and found that it did not possess the features cited by Kukalová-Peck & Brauckmann (1992) as indicating paraneopteran relationships. That still left the expanded clypeus and the exites, but those little details were re-interpreted by Béthoux & Briggs (2008) as artefacts seemingly produced by over-enthusiastic preparation. The current indication is that Gerarus is a member of the Panorthoptera, the clade including Orthoptera plus the extinct orders Titanoptera and Caloneurodea. A close relationship between Geraridae and Titanoptera, enormous grasshopper-like insects, was popular for a while, but was rejected by Béthoux (2007)*. The exact affinities of Gerarus still await elucidation.

*Some day I may do a review of Béthoux (2007), a paper which may or may not constitute a glimpse into the fiery depths of hell. Right now, I haven’t the strength.

Systematics of Geraridae

Characters (from Burnham 1983): Head small and probably mobile; antennae filamentous. Prothorax elongate, flask-shaped, and distinctively spinose. Fore and hind wings similar in size and shape, but markedly different in venation. Forewing length 35-55 mm, apparently not coriaceous; costal area broad in basal region, SC simple, terminating in C; R parallel to SC, terminating at wing apex; RS originating from base of R near midpoint of wing; M either anastomosing with RS for a short distance or connecting to it by a cross-vein; CUA strongly developed, arising from base of M; CUP forked, arising independently from wing base. Hind wing length 30-48 mm; costal area not as broad as in forewing; SC simple, terminating in C; R parallel to SC, terminating at wing apex; RS pectinate, arising from R near wing base; M forked, arising near base of RS; CUA and CUP simple and parallel to one another; CUA arising from base of RS near M, CUP arising independently from wing base; anal area unusually reduced; cross veins abundant in both fore and hind wings. Legs cursorial; tarsi five-segmented. Abdomen cylindrical.

<==Geraridae [Gerarida, Gerarina, Gerarodea]
    |--Genentomum Scudder 1885R02, BN02
    |    `--G. validum Scudder 1885BN02
    |--Progenentomum Handlirsch 1906R02, BN02
    |    `--P. carbonis Handlirsch 1906BN02
    |--Nacekomia Richardson 1956R02, BN02
    |    `--N. rossae Richardson 1956BN02
    |--Anepitedius Handlirsch 1911R02, BN02
    |    `--A. giraffa Handlirsch 1911BN02
    |--Osnogerarus Kukalová-Peck & Brauckmann 1992BN02
    |    `--*O. trecwithiensis Kukalová-Peck & Brauckmann 1992BN02
    |--Cantabrala Kukalová-Peck & Brauckmann 1992R02, BN02
    |    `--C. gandli Kukalová-Peck & Brauckmann 1992BN02
    `--Gerarus Scudder 1885BN02
         |--*G. vetus Scudder 1885BN02
         |--G. bruesi (Meunier 1909) [=Archaeacridites bruesi, Sthenaropoda bruesi]BN02
         |--G. collaris Handlirsch 1906BN02
         |--G. danielsi Handlirsch 1906BN02
         `--G. fisheri (Brongniart 1885)BN02

*Type species of generic name indicated

References

Béthoux, O. 2007. Cladotypic taxonomy applied: titanopterans are orthopterans. Arthropod Systematics and Phylogeny 65 (2): 135–156.

Béthoux, O., & D. E. G. Briggs. 2008. How Gerarus lost its head: stem-group Orthoptera and Paraneoptera revisited. Systematic Entomology 33 (3): 529–547.

[BN02] Béthoux, O., & A. Nel. 2002. Venation pattern and revision of Orthoptera sensu nov. and sister groups. Phylogeny of Palaeozoic and Mesozoic Orthoptera sensu nov. Zootaxa 96: 1–88.

Béthoux, O., & A. Nel. 2003. Wing venation morphology and variability of Gerarus fischeri (Brongniart, 1885) sensu Burnham (Panorthoptera; Upper Carboniferous, Commentry, France), with inferences on flight performance. Organisms Diversity & Evolution 3 (3): 173–183.

Burnham, L. 1983. Studies on Upper Carboniferous insects: I. The Geraridae (order Protorthoptera). Psyche 90 (1–2): 1–57.

Kukalová-Peck, J. 1987. New Carboniferous Diplura, Monura, and Thysanura, the hexapod ground plan, and the role of thoracic side lobes in the origin of wings (Insecta). Canadian Journal of Zoology 65: 2327–2345.

Kukalová-Peck, J., & C. Brauckmann. 1992. Most Paleozoic Protorthoptera are ancestral hemipteroids: major wing braces as clues to a new phylogeny of Neoptera (Insecta). Canadian Journal of Zoology 70: 2452–2473.

[R02] Rasnitsyn, A. P. 2002. Infraclass Gryllones Laicharting, 1781. The grylloneans (=Polyneoptera Martynov, 1938). In: Rasnitsyn, A. P., & D. L. J. Quicke (eds) History of Insects pp. 254–262. Kluwer Academic Publishers: Dordrecht.

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