BR~ J. ENT. NAT~ HIST.; I : 1988

T1HE, ARACHNID FOSSIL .RECORD

PAUL S,E.LD ·E.N

Department of Extra-Mural Studies,. The University, Manchester Ml3 9PL.

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Ar ra,chnids. belong to the arthropod phylum Chelic ,erata which, :in addition to the scorpions, spiders, mites and others, e.mbraces th.e extinct eurypterids, the king­crabs (Xipbosura), and arguably the pycnogonids (sea-spid ,ers) .. Before pres .enting a review of the re.cent ,discoveries which extend a:nd m,odify th.e traditional picture of arachnid 1e,volutio ·n, let ·us first ]ook at th.e eurypterids and kin.g-crabs.

Th .e eurypterids, ors ,ea -scorpions, are known from rocks ranging from Ordo ,vician to Permian in age (Fig. 1) but are most abundant in upper Silurian strata in Britain, .sca .n:din.a.via and the USA. Indeed ., their distinctive appearan .ce (scorpion-like but with broad . swimming pad .dies in place of the last pair of legs) and abund .an.ce in Silurian limesto ·nes in the north-east USA has earne ,d 1one species, Eurypterw; remipes De K.ay the title of State F'ossil of New York. This part of the w 1orld has als,o yielded the giant .Buffalopterus w.hich at 2 m in length was the largest arthro ,pod which ever lived. Its fearsome, [ huge chelicerae exemplify just one of the methods of prey .... capture of these giant carnivores of mid-Palaeozoic seas. Though rar 1e .as fossils eurypterids are usually very well preserved, and on.e species ., Baltoeuryp'lerus tetra,gonophthalmus (Fischer), has provided a wealth of information about the mode of life ,and palaeoecology of these fascinating animals (Selden, 1981, 1984).

Traditionally allied with the eurypterids in the class Merostomata are the kin,g­crabs, typified by th,e living Limulus polyphemus. Th·e oldest xip·hosura ·ns ,a:re Cam ·brian in age, an ,d forms closely similar in morphol 1ogy and ecology to t:he living species ha·ve been f o·und in rocks as 0 1ld ras Carboniferous , he·nce Limulus is oiften d·ubb 1ed a 'living fossil'. The oldest .memb ·er of the superfamily Lirn.uloid.ea was described recently from Scotland (Waterston, 1985), ,and a slightly younger 'but giant form is n,ow known fro :m Weardale (Siveter & Selden ., 1987).

Scorpions have ,generally been considered th,e most primitive of arachnids; man.y autb .ors (e.g . Bristowe, 1958; Grasshoff, 1978) have considered . them as the sister ... group of the eurypterids, in whic .h case the eurypterids m.ust be included in the Ara .chnida, 0 1r the scorpions excluded from that grou .p .. Alternative opinions (Boudreaux ., 1979; We .ygoldt, 1980; Weygoldt & Paulus, 1979) place the eurypterids as 'the sister-gr 1oup of the a:ra.chnids (including scorpions); under either scheme, the Merostomata becom ·es an unnatural group.

Modern sco 1rpions are mainly tropical, nocturnal, terrestrial a:nimals .Particularly adapted to arid envir ,on.ments and are classified into less than a ,dozen families. However, fossil scorpions are kn.own from rock.s as old as late Silurian; at that ti.me they lived in water alongside their eury.pterid . .relatives, .a.n.d aquatic scorpions persisted possibly · into Meso .zoic times .. Air-breathing sc·orpions apparently ap,pe.ared in the early Carb ,oniferous ,, and scorpions w·ere at their most diverse in the

. .· .. - - . · .. · .: , , · . i . - 1 · - ·, '. 1 - •• , • _: .. · - i - . : ' .. · · '. - . · · .· . ·1· c·· b '"f h 2··5 f' ·1·· h b .. d (''K··''" II . ·w··'. - .. -ate .· ar .on1 erous w en _. .. am11.es . ave een recognize _ . , Je .. esv1g . aer1ng, 1986). Living scorpions can b,e se,arched for at night with a blacklight, since they fluoresce in ultra .vio 1let :li.ght . This is a property · of one lay·e.r in their exoskeleton: the hy·aline exocuticle " Hyaline exocuticle seems to be uniq .u.e to scorpions ,,, but is possibly prese ·nt 1n eurypterids and king~crabs also .. Scraps ,of sco 1rpion cuticle are abundant in coals and related sedim ,ents to the exclusion of all 1other arthropods, a phenomenon . which is .attributed to the inertness of the hyaline exocuticle (Bartram .et al., 1987) a.nd hel .ps to explain the relative . abundance of scorpions amongst arac ,hnid fossils ..

'16 :_· B:R . J. :ENT ~ NAT. HISTq I : 1988

Oth .er ,arac:hnids have ve,ry S'poradic fossil records (Fig.. 1),, though there is a general tren .d which can b 1e attributed almost entirely to the time . distribution of th,e rar ,e localities in which. they are found. s .pid,ers ,are abundant in some Tertiary (Palaeo ,g.ene and Neo ,gene) ambers (Oligoce ,ne, Baltic and, D ·ominican Republic for example), and are known from shales of the same age in Colorado 1

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Fig. 1.. Known stratigrap .hic ranges of th ,e major groups of che licerates, from . published and unp ·ub,Iished data .. S01lid lines indicate fairly continuous occurren ,ce, dashed lines indicate data .poor or lacking .. Solid circles mark only or most impo ,rtant occu:rren,ces, open circle indicates unce.rtain occurrence.

. BR .. J~ ENT '. NAT4 HJST+, I : 1988 17

are more :similar to thos ,e of today · than to Meso·.zoic and Palaeozoic faunas .. The end­Cretaceous mass e.xtinctio .n brou .ght the Mesozoic ·era to a close .and the fau .nal and . floral changes a.cross this bound .ary ar,e great« Unfortunately, Mesozoic ara.ch·nids are exceptionally rare, so we cannot be sur 1e whether the same is true of them. Incidentally, C:retaceous ambers are k·nown from Cana .da. and the Lebanon whic.h contain spide ·r inclusi 1ons, so it may not be very long before we have e:nou.gh well preserved specimens to test this hypothesis.

In the last few years, four spider specimens from ,early Cretaceo ·us lithographic limestone of the Sierra d,e Monts.e.ch in north-east Spain have been discovered. T'his to,cali.ty has Jong be,en fa:mou.s for its bea.utifully· preserv 1ed p1lants, insects, am .... phibians, and especially bird feathers. , Three of the spiders a.re mature males with their ,distinctively elab.,orate palps, and the fourth has characters which could place it in a modern family. A major problem with these Mesozoic spiders is that the details n.eeded to ·place the ·m in the classification sche·me for Re,cent spiders are not always preserve ,d. In a Jurassic example (Es.kov, 1984), a c.om.bination of characters of numerous living families was o·bserved which result 1ed in the setting up of a new 'extin 1ct' family ..

Travelling further back in time we pass the end-Permian e.xtinction event, the most severe of a.II, and com,e to 1 the late Carboniferous .. Iron.stone nodules in coal measure rocks of Europe and. North America have yielded an. abundant terrestrial and near-h f . I ct·· · . •111· d · . h · d · h ·ct s _ore -auna, 1nc u, 1n.g crustaceans, insects, m1111pe .es, xip ,osurans a·n arac .. 1n1> s.

Particularly interesting are the 20 or so specimens of ricinulei 1ds .. Ricinuleid.s are charming little thick -skinned arac .hnids which cre·ep amongst leaf litter und ,er logs and in. caves in the American . and West African tro _pics. There are ab 1out 50 living sp,ecies in three genera. The C,ar·bonife-ro·u.s ricinuleids ar1e re :markably similar t10 the living forms; an interesting difference being that th 1e fossils show two pairs of eyes in the position on the living animals wh.ere vestigial eye-spots are present (Seld.en, 1986).

Another group of .arachnids relatively well-represented in Coal Me.asure rocks are the extinct tri .gonotarbids. T'hese animals are v·ery clo 1se to spiders in their morphology, 'but lac.k spin.ning organs. Trigonotarbids are also a major compo ·nent of the, terrestrial faunas of thr 1ee important Devonian localities: th·e Rhynie chert of Aberd ,eenshire; Alken -an- ,der-Mosel, Germany; and Gilboa, New y ·ork .. Hirst (1922) described the Rhynie fauna, a bog habitat of early vascular plants rapidly engulfed in hot, siliceous waters from ne.arby vo.lcanic eru ·ptions. The Rhy·nie ch.ert contains the oldest terrestrial fauna known, and inclu .des a springtail, a mite, numerous trigonotarbid.s, and a :possible spider (see Rolfe, 19·80 for review of this and other early terrestrial fau·nas). The Alken fa·una includ·es both terrestrial (trigonotarbid, myriapod) and amphibio 1us (eurypterid) e.lements (St0rmer, 197'l)-16) ..

More recently, palaeob 1otanists Bonamo and Grierson were dissolving grey siltstones from the Devonian of Gilboa, New York for early vascular ·plants w·hen they came across an extremely varied fauna of early land animals (Shear et al., 1984; Shear, 1986). Again ., trigonotarbids are v,ery commo ,n (S,hear & Seld 1en, 1986), but there is also evidence of insects, myriapods (the earliest centiped .e) ., scorpions, a·nd mites~ The fossils are beautifully preserve .d , .and when mounted. in balsam and vie.we,d in transmitted light:, minute details of setae, trichob 1othria, slit sensillae, lyriform and other s,ense organs are clearly · visible (see Shear et al., 19188).

On,e element of the, fa.un.a caused the greatest mo .dification to the a.rachnid fossil record ·when first discovered .. Two beautiful little cre .atures, describe ·d by the assistant ·who found them . as "little dragons', turned out to1 be pseud .oscorpio 1ns.

18 BR. J. ENT. NAT . HIST.~ I : 1988

Virtually identic ,al in all aspects to living pseudoscorpions, the find of these Devonian forms pushes the fossil reco ,rd of the group back tenfold.

Exciting thoug.h these finds of ral'e, earliest terrestrial arachnids are, their impo1rtance to Zoo1logy extends beyond that to Arachnology alone .. T'hey help us to un.de:rstand how the very· first land animals pioneered the empty terrestrial h ,abitat ,400 m.il.lion yea.rs ago . It w·as thes ,e pio,neers which paved the way for the ·rich and varied . arthropod fauna which ab,ounds in 0 1ur countryside today.

REFER .ENCES

Bartram , K~M~, Jeram , A .J. & Selden , P.A. 1987~ A.rthropod cuticles in coal ~ 1~ Ge·ol. Soc ~ London 144: 513-517 ..

B,oudreaux, H·.B ~ 1979. Significance of intersegmental tend .on system in arthropod phyloge.ny an 1d a monophyletic classification o·f Arthropoda . In Gupta , A ~P 5 (ed) Arthropod Ph.ylo.geny, 551-586. Van Nostrand Reinhold, New York .

B1ristowe,, W.S .. 1958. The World of Spiders . Collins, London. Eskov , K ~ 1984. A new fossil spider family from the Jurassic of Transbaikalia ,(Araneae :

Chelicerata) ~ N. J. Geol . Paliio·nt Mh .. 11: 64~53. Grassh 10 1ff , M . 1978. A mo,d.el of the e;volution of the main chelicerate gro 1ups ,. Symp1

• Zool .. Soc. Lond. 42: 27~284 .

H irst ,, S. 1922 .. On so:me arachn id remains from the ,Qid ·Red Sandstone (Rhynie Chert Bed , Aberdeenshire) ~ An,n. Ma.g. Nat . . Hist. 12, 455-474 ..

Kjellesvig ·-Waering , E.N. 1986. A restudy of the fossil Scorpionida of the world. Palaeonto­,graphica Americana 55: 1-287 .

Rolfe , W.D.I. 1980. Early invertebrate terrestrial faun.as. In: Panchen, A ~L~ (ed.) The te.rrestrial environment and the 0 1ri,gin of land ·vertebrates. Syst. Assoc. Spee. ·vol. No. 15·, 117-157, A.cademic Press , London and Ne·w York.

Selden, P .,A. 1981. Functio ,nal morphology of the prosoma of .Baltoeurypterus letragonoph .. thalmus (Fischer) (Chelicerata : Eurypterida) ~ Trans . R . Soc . Edin ,b., Earth Sciences 72: 9- 48.

Selden, P.A. 1984. Autecology of Silurian eurypterids . Spee~ Paps . Palaeont. J,2: 39-54. , Selden , P .. A. 1986. Ricinulei,ds - · living fossils? Actas X Congr. lnt .Aracnol., Jaca, Espana 1:

425. Shear, W .A . 1986. A fossil fauna ,of early terrestrial arthrop 1ods from the Givetian (upper

Middle : Devonian) of Gilboa. , .New York ,, U .S.A . Actas X Congr. lnt. Aracnol ., Jaca, Espana 1: 387-392 .

Shear, W~A~, Bonamo, P.M ~, Grierson, J.D . ., Rolfe,. W.D .. I., Smith , E .L. &'. Norton, R~A ... 1984. Early land animals in North America: e·vidence from De·vonian age arthropods fr,om G·ilboa , N'ew York . Science 224= 492-494 .

Shear , W. A ~ & Selden , P .A . 1986. Phylogen .etic relationships 10.f the Trigonotarbida, an extinct order of arachnids . Acta s x· Congr . lnt. Aracnol., .Jaca, Espana 1·:· 39~397.

Shear, W.A. , Selden , P.A ~, Rolfe , W.D ~I ., Bonamo , P.M. & Grierson, . J .D . 1987~ New terrestrial arachnids from the Devonian . of Gilboa (Arachnida , Trigo .notarbida). New York . Novitates 2901 : 1- 74.

Siveter , D.J~ & Selden , P.A . 1987~ A new, g,iant, xtphosura.n from the lower Namurian of Weardale ,. County Durham. Proc. Yorks . geol. Soc . 46 : 15~168.

St01rmer , L. 1970, 1972, 1973, 1974, 1976. Arthr ,opods from th,e Lower Dev·onian (Lower :Emsia·n) of Alk ,en ... an .. der-Mos,el, Germany ~ Par ts 1-5 . Senckenberg .. Leth. 51: 335- 369; 53: 1-29; S4~ 119-205 , 359-451; 57: 87-183~

Waterston , C.D. 1'985. Chelicerata from the Dinan .tian of Fou lden , :Berwickshire , Scotland .. Trans . . R~ Soc ~ Edinb ., Earth Sciiences 76: 25-53 .

Weygoldt , P . 1980. Towards a cladistic classification of th.e Chelicerata. 8th lnt. Congr. Arachnol. , Vie·nna, 1980,. 331-334 .

Weygoldt, P . & P'aulus, H.F . 1979. Untersuchungen zu.r Morphologie, Taxonomie und Phylogenie der Chelicerata. z. Zoo[ Syst. Evolut. -forsch . 17: 177-200.