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Re: Megarachne a eurypterid, not a spider



Off topic?
All eurypterids are bizzare forms. The eurypterids that I collected during my thesis work in the Ordovician of Ohio (Megalograptus sp.) lived only in energy restricted hypersaline environments and were obviously very specialized megapredators of their time. (Nicely clawed thank you!) They were up to 6 feet in length and certainly had their way with what ever crossed their path but they were also certainly quiet water marine (lagoonal). They also had very spade like chelicera (IV) probably modified for grasping/water movement though chelicera (I) should be homologous to fangs of some sort. No mention of Megarachne having swimming paddles (VI). It is interesting that Megarachne has a suture dividing the carapace as eurypterids usually have their prosoma as a single fused cephalon. It could be an adaptation to shedding the shell during molt. Also interesting is that the abstract talks about the "eye tubercule" as this indicates a single eye platform not a pair as in most eurypterids I am familiar with. No mention of a telson? Group it where you will but I suggest that such a large spider (or eurypterid) would be fragile and clumsy on land and would be better suited for a quiet water existence. Additionally, the large size goes along with the tendency toward large forms in a stable environment over time. The move from Marine to fresh water existence in eurypterids certainly occurred by the Pennsylvanian (if not much earlier). An 18 inch long, fresh water, Permo/Carb version of the bug is certainly plausible and they were certainly opportunistic colonizers. Sounds like a fresh water treat for some aquatic Permian reptile/amphibian to me.
Frank Bliss
MS Biostratigraphy
Weston, Wyoming.
On Feb 16, 2005, at 10:15 AM, Tim Williams wrote:


Thomas R. Holtz, Jr. wrote:

http://news.bbc.co.uk/2/hi/uk_news/england/manchester/4268363.stm

No mention of where the paper (if any) will be.

Here it is in FirstCite, at the Biology Letters website (http://www.journals.royalsoc.ac.uk):


Paul A. Selden, José A. Corronca, and Mario A. Hünicken (2005) The true identity of the supposed giant fossil spider _Megarachne_.

ABSTRACT: _Megarachne servinei_ from the Permo-Carboniferous Bajo de Véliz Formation of San Luis Province, Argentina (32° 17' S, 65° 25' E), was described as a giant mygalomorph spider ('tarantula') and, with its body length of 339 mm, the largest known spider ever to have lived on Earth. Its identification as a spider was based on interpretations of the shape of the carapace, the position of the eye tubercle, the anterior protrusion of the carapace as a pair of chelicerae, and the posterior circular structure as the abdomen. X-radiography revealed possible morphology hidden in the matrix: cheliceral fangs, sternum, labium and coxae, and so a reconstruction of _Megarachne_ as a giant spider was presented. Difficulties with the interpretation (unusual cuticular ornament, suture dividing the carapace and spade-like anterior border of the chelicera), together with non-preservation of synapomorphies of Araneae, provoked debate about its interpretation as a spider. Now, the holotype and a new specimen have become available for study. _Megarachne_ is shown to be a bizarre eurypterid ('sea-scorpion'), similar to rare forms known from Carboniferous rocks of Scotland and South Africa, and is the most complete eurypterid so far recorded from Carboniferous strata of South America.


The study refers _Megarachne_ to the eurypterid family Woodwardopteridae. _Megarachne_ is reconstructed as an aquatic sediment-feeder, although sedimontology would appear to rule out a marine habitat.