Science Literature

Harvestmen are said to be the third most-diverse arachnid order, and few will be unaware of these long-legged creepy-crawlies living somewhere near to our own homes. They belong to the order Opiliones and although the fossil record is understandably sparse (the exoskeleton is poorly mineralised), the fossil record is informative. Some of the earliest insects known are in intimate association with plant material - preserved in the Rhynie Chert. "Devonian (~410 Myr) harvestman fossils from the Rhynie Chert, while incomplete, preserve a three-dimensional internal anatomy suggestive of an essentially modern body plan." The opportunity has come to check out this suggestion, using high-resolution X-ray micro-tomography to examine specimens preserved in siderite nodules. The X-ray scans allow parallel slice images of the fossils to be recorded, from which three-dimensional, virtual models of the organisms are assembled.

"The international team, led by scientists at Imperial College London, took over 3000 X-rays of the harvestmen fossils from France, dating to the Carboniferous Period [. . .]
The scans were edited using computer software to produce highly detailed and accurate 3D models. They reveal 2 new species of ancient harvestmen, Macroglyon cronos and Ameticos scolos, which, unlike most land animals from this time, had bodies very similar to their modern relatives living today."

The probable appearance in life of A. scolos gen. et sp. nov. (above) and M. cronus gen. et sp. nov. (below). Scale bar, 5 mm. (Source here)

The most notable finding is that these insects are essentially modern. They belong to two identifiable suborders. The research paper provides a cladogram with the ancient harvestmen alongside modern forms: "comparable taxa of the modern Eupnoi and Dyspnoi". Dr Russell Garwood, currently based in the computed tomography lab at the Natural History Museum in London, is quoted as saying:

"It is absolutely remarkable how little harvestmen have changed in appearance since before the dinosaurs. If you went out into the garden and found one of these creatures today it would be like holding a little bit of prehistory in your hands. We can't yet be sure why harvestmen appear so modern when most land animals, including their cousins such as scorpions, were in such a primitive form at the time. It may be because they evolved early to be good at what they do, and their bodies did not need to change any further."

There is a problem with much of the terminology being used to describe fossils. I am thinking of words like: "primitive" and "modern" (also "stem" and "crown"). Much of this terminology is driven by a Darwinian perspective - a theoretical model of what the fossil record ought to be like. Cladism has imbibed this mindset, because it infers a lineage of organisms that can be reconstructed by the appearance of novel structures. Unfortunately, the fossil record is proving to be less and less Darwinian as we examine the details. We have modern body plans where there should be primitive body plans. We have primitive 'relict' species living on when they should have become extinct. We have to invoke 'convergence' to explain similar structures that do not fit the linear model. (But convergence is ubiquitous - here). Why should we live with such an unwieldy interpretative framework?

Most significant is the pattern of animal and plant radiation that we find in the fossil record. Darwin predicted a branching bush or tree - but this is not what we find! The Cambrian Explosion is the best rebuttal of this, for it demonstrates an initial burst of diversification and speciation followed by relative stasis (See here, here and here). This pattern of diversification suggests caution about using the word "primitive" or "modern" in an evolutionary sense. Are the characters under investigation part of the early diversification of an order or a Family? If the answer is 'yes', then we should be very cautious about placing an evolutionary interpretation on it - to say something is primitive then becomes little more than saying that it became extinct. This is one reason for documenting cases of stasis in the fossil record - there is far more evidence of modernity in the fossil record than is currently acknowledged by Darwinists. (For some recent blogs, try one, two, three, four and five).

Anatomically modern Carboniferous harvestmen demonstrate early cladogenesis and stasis in Opiliones
Russell J. Garwood, Jason A. Dunlop, Gonzalo Giribet, Mark D. Sutton
Nature Communications, 23 August 2011, 2, Article number: 444 | doi:10.1038/ncomms1458 (pdf here.)

Abstract: Harvestmen, the third most-diverse arachnid order, are an ancient group found on all continental landmasses, except Antarctica. However, a terrestrial mode of life and leathery, poorly mineralized exoskeleton makes preservation unlikely, and their fossil record is limited. The few Palaeozoic species discovered to date appear surprisingly modern, but are too poorly preserved to allow unequivocal taxonomic placement. Here, we use high-resolution X-ray micro-tomography to describe two new harvestmen from the Carboniferous (~305 Myr) of France. The resulting computer models allow the first phylogenetic analysis of any Palaeozoic Opiliones, explicitly resolving both specimens as members of different extant lineages, and providing corroboration for molecular estimates of an early Palaeozoic radiation within the order. Furthermore, remarkable similarities between these fossils and extant harvestmen implies extensive morphological stasis in the order. Compared with other arachnids - and terrestrial arthropods generally - harvestmen are amongst the first groups to evolve fully modern body plans.

See also:

Ancient harvestmen in 3D reveals early evolution, Natural History Museum Press Release (23 August 2011)