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are plenty of hotels at West Yellowstone, just out-side the western boundary.

Geological visitors to Yellowstone should alsotake in some notable features outside the Park.The landslide and fault scarps of the 1959 earth-quake at Hebgen Lake are just north of WestYellowstone (Fig. 1). South of the Park, the faultblock of the Grand Tetons lies in another NationalPark with some fine granites, moraines and ter-races, and the 1925 Gros Ventre landslide is justoutside its south-eastern boundary. And every visi-tor is recommended to make one early morningstart while at Yellowstone � to walk or drive to OldFaithful, and then take a stroll around the UpperGeyser Basin; this can be truly magical when thesteam is at a maximum in cool dawn air, especiallyif there is a morning mist which clears in patchesover the warmer ground.

Suggestions for further readingBeus, S.S. 1987. Rocky Mountain Section.

Geological Society of America Centennial FieldGuide, v.2, 475pp.

Christiansen, R.L. and others. 1972�85. Geology ofYellowstone National Park. United StatesGeological Survey Professional Paper 729, parts A�H.

Good, J.M. & Pierce, K.L. 1996. Interpreting theLandscapes of Grand Teton and Yellowstone NationalParks. Grand Teton Natural History Association,58pp.

Waltham, T. 1997. Guidebook to the Geology ofCanyonlands, USA. Geologists� Association,London, 96pp.

Tony Waltham is Senior Lecturer in EngineeringGeology at Nottingham Trent University.

Fossils explained 23: Palaeozoic echinoids

Some years ago, Andrew Smith of the Natural His-tory Museum, London (NHM), discussed the seaurchins, or echinoids, in a �Fossils explained� article(Geology Today, v.1, pp.150�151, 1985). Fossilechinoids are best known from the post-Palaeozoic,such as the diverse faunas of the Upper CretaceousChalk of the British Isles and northern Europe.However, they first appeared in the Ordovician andformed a small, but highly distinctive, part of thePalaeozoic marine biota. Cambrian echinodermswere entirely suspension-feeding organisms, and itwas only in the Ordovician that deposit feeders,such as echinoids and starfishes, first appeared.Herein, we discuss the morphological diversity ofthe Palaeozoic echinoids, their evolutionary trendsand innovations, and their fate during the end-Permian extinction event, as well as illustratingsome particularly significant and/or well-preservedexamples from the collections of the NHM (Figs 1�4). We thank Phil Crabb of the NHM for taking thephotographs.

The Treatise on Invertebrate Paleontology recordssome 40 genera of Palaeozoic echinoids. It also in-cludes the Ordovician genus Bothriocidaris, which to-gether with another Ordovician genus Neobothriocidaris(Fig. 1A), are now regarded as echinoid-like echino-derms which differ from true echinoids in details ofcoronal plating and lantern structure.

All Palaeozoic echinoids are �regular� � that is, theperiproct, or opening for the anus, is within the api-cal system (Maccoya; Fig. 2B), a circlet of plates at thetop of the test, and the test itself is symmetricallyglobular or apico-orally compressed to a greater orlesser extent. The main evolutionary trends in thePalaeozoic echinoids included an expansion in thewidth of the ambulacra on the oral surface; flatteningof the test from its original globular form in groupssuch as Aulechinus, Ectinechinus, Echinocystites andPalaechinus (Figs 1B,C,E and 2D); the increase in thenumber and variability of plate columns in theambulacra and interambulacra (Melonechinus;

Figs 2G and 3), later followed by a decrease to themodern form of invariably two columns for eachambulacrum and interambulacrum (Miocidaris;Fig. 4C); an increase in the size of the test; differentia-tion of the spines and a related differentiation ofthe tubercles; and development of the lantern andincrease in its complexity.

In post-Palaeozoic echinoids, the test consists of20 columns of plates, aligned from the apical systemto the margin of the peristome (opening for themouth), arranged as pairs of columns of fiveambulacra separated by five interambulacra. MostPalaeozoic echinoids differ in having the plates inmultiple columns. The ambulacra and interambu-lacra of Palaeozoic echinoids are more or less colum-nar. The interambulacra may be in one column, as incravenechinids (Cravenechinus; Fig. 2A), or manycolumns (Pholidechinus, Hyattechinus; Figs 1G,H and2C), and the ambulacra consist of two (Archaeo-cidaris; Fig. 4B) or many columns (Lepidesthes;Fig. 2E). For example, in Carboniferous echino-cystitids the ambulacral columns are six or more, andthe Upper Devonian to Upper Carboniferous genusLepidesthes (Fig. 2E) includes species with up to 20columns of plates in each ambulacrum. However,there are still five ambulacra and five interambulacra.The number of plates in echinoid tests increased fromthe Lower to the Upper Palaeozoic, so thatOrdovician genera had no more than around 1000plates in a single specimen, but by the Carboniferoussome taxa had well over 3000 plates.

The ambulacral pores gradually moved from aposition directly adjacent to the perradial suture(that is, the suture between columns of plates in thecentre of the ambulacrum), as in Aulechinus andEctinechinus (Fig. 1B,C), to a position closer to theadradial suture (between the ambulacrum andinterambulacrum), as in Pholidechinus and Archaeo-cidaris (Figs 2C and 4B). This shift decreased thestrain on parts of the test when the tube-feet wereattached to an object, by moving the forces from one

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Fig. 1. Some Ordovician to Devonian echinoids (B�H) and a bothriocidarid (A) of the British Isles. (A) Neobothriocidaris peculiaris Paul (UpperOrdovician, Ashgill). An external mould of part of this primitive, echinoid-like echinoderm. (B) Aulechinus grayae Bather & Spencer (UpperOrdovician, Ashgill). Note the highly globular test. (C) Ectinechinus sp. (Upper Ordovician, Ashgill). (D) Lepidocentrus eifelianus Müller (MiddleSilurian, Wenlock). Unusually, this specimen is an Aristotle�s lantern preserved separately from the test. (E) Echinocystites pomum Thomson (UpperSilurian, Ludlow). Note the slender spines. (F) Palaeodiscus ferox Salter (Upper Silurian, Ludlow). Latex cast of a specimen preserving a dense carpetof slender spines. (G,H) Hyattechinus sp. (Devonian). All scale bars represent 10 mm. Specimens (A�C, E, G, H) are natural moulds.

narrow area to a broader one. It also increased theflow area available for the extraction of oxygen fromthe water by the tube-feet.

The spines developed from short, fine spikes(for example, as in Palaeodiscus and Echinocystites;Fig. 1E,F), to long, robust ones more like those ofsome modern regular echinoids (such as those ofArchaeocidaris and Miocidaris? permica; Fig. 4A,D,F).

Tubercles were little more than several low pustuleson plates (see Fig. 2B,F, respectively, of Maccoyashowing tubercles of interambulacral plates andLovenechinus showing tubercles of ambulacral plates),but those of the archaeocidarids (Archaeocidaris;Fig. 4A,F) were modern in appearance and structure,with one primary tubercle occupying much of theplate, and secondary tubercles surrounding it. The

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Fig. 2. Lower Carboniferous, non-cidaroid echinoids; all specimens from the British Isles unless stated otherwise. (A) Cravenechinusuniserialis Hawkins. (B) Maccoya spherica (M�Coy). Apical view, showing the plates of the apical system. (C) Pholidechinus brauniJackson (USA). Note the multiple columns of interambulacral plates (see also D and F). (D) Palaechinus quadriserialis Wright.(E) Lepidesthes colletti White (USA). A species with multiple columns of ambulacral plates. (F) Lovenechinus lacazei (Julien).(G) Melonechinus multipora Owen & Norwood (USA). Apical view; note the multiple plate columns in the ambulacra and interambulacra.All scale bars represent 10 mm.

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Fig. 3. Melonechinusetheridgei Keeping (LowerCarboniferous,Lancashire). Thisspecimen shows a style ofpreservation typical ofmany specimens ofMelonechinus found in theLower Carboniferous ofthe British Isles (contrastwith Fig. 2G). The testcollapsed following deathand after the loss of thespines. The illustratedsurface was pressed intothe underlying substrate,which acted to stabilizethe plate columns,maintaining them in theiroriginal relative positions,more or less. The obversesurface is considerablymore disarticulated anddisrupted, having beenexposed to seafloorcurrents and the attentionsof the vagile benthos.Scale bar represents10 mm.

increasing importance of spines probably indicates agrowing need for defence against predators such asfishes.

The echinoids evolved a feeding apparatus calledthe Aristotle�s Lantern, which enabled them to feedactively and thereby allowed them to exploit foodsources not available to the filter-feeding echino-derms. This was perhaps the most important develop-ment to influence the evolution of the echinoids.The earliest lanterns were simple structures consist-ing of five pairs of demipyramids and the equivalentsof the epiphyses, and five simple, rather flat teeth,but no rotulae or compasses, and were low-angled,more useful for scooping. By the early Silurian, thelantern, although still low-angled, had evolved into amuch more sophisticated structure, more like thatfound in the post-Palaeozoic regular echinoids. Itwas composed of five pairs of demipyramids and epi-physes, and five rotulae, teeth and compasses. Itprobably behaved more like a grab than a scoop andcould also be used for biting (Lepidocentrus; Fig. 1D).Carboniferous archaeocidarids had teeth whichwere more robust and complex, and so were able toutilize a wider diet. There were other active-feedingechinoderms present from the Ordovician � theophiuroids (brittle-stars) and asteroids (starfishes) �but the echinoids had more mobile jaw mechanismsthan either of these groups.

In Lower Palaeozoic echinoids there were no lan-tern supports, thereby limiting the movement of thelantern within the peristome. Lantern supports firstappeared in the Permian in Miocidaris as apophyses,which are thickened projections on the inside edge ofthe oral interambualcra. Post-Palaeozoic cidaroidshave apophyses, whilst non-cidaroids have auricles,which are structures developed on the inside edge ofthe oral ambulacra. Either of these structures formsthe perignathic girdle, which allows the lantern muchmore movement in and out of the peristome, and alsolateral and rotational movement, for more effec-tive feeding.

In modern echinoids, the lantern is much moreupright and is a strong apparatus with complex,curved, sometimes keeled teeth, which enables theechinoid to graze on algae and organic-bound sedi-ment as well as scavenging the organic remains ofdead and even moribund organisms. Moderncidarids are even able to chew through molluscshells. The compasses allow the internal volume tobe maintained when the lantern is moved in and out,so the presence of compasses in Silurian echinoids(for example, Lepidocentrus) indicates that this vol-ume maintenance had already become a problemand compensation was now necessary (compasses ofArchaeocidaris whatleyensis; Fig. 4E).

The Palaeozoic diversity of the group reached itsmaximum in the early Carboniferous, with echinoidsbeing known from a variety of carbonate palaeoenvi-ronments of that age. At the end of the Carbonifer-ous there was a marked decline in diversity of speciesin all groups, and the extinction of other groups,such as the palaechinoids. This decline continuedinto the Permian, from which we now only recognizesix species. By the end of the Permian, environmen-tal conditions were so harsh and unpredictable thatonly groups interpreted as extreme specialists (forexample, the proterocidarids) and opportunists(such as Miocidaris) were able to survive.

Probably only two closely related echinoid line-ages survived the end-Permian mass extinction eventand passed into the Triassic, thereby preventing theentire class from extinction; from these two lineagescame the great post-Palaeozoic echinoid radiation.One group had apophyses and gave rise to thecidaroids, and the other, as yet unidentified, had noapophyses, but had by the late Triassic developedauricles and was ancestral to all the non-cidaroidechinoids.

The modern cidaroids evolved from late PermianMiocidaris, such as Miocidaris keyserlingi (Fig. 4C), ora related species. Miocidaris has a �modern� appear-ance, with two columns of plates in eachinterambulacral column, rather than four as in theCarboniferous genus Archaeocidaris (Fig. 4B), whichat first appearance it may otherwise seem to resem-ble. The ambulacra are narrow relative to the inter-ambulacra and, in appearance, Miocidaris is notdissimilar to extant cidaroids. Andrew Smith andNeville Hollingworth have shown that the structureof the Aristotle�s lantern, particularly the teeth, ofMiocidaris was similar to that of modern cidaroids;however, it is different from that of Archaeocidaris andthe extant, non-cidaroid echinoids, the euechinoids.Thus the divergence of the cidaroid and euechinoidgroups, both of which evolved from an archaeo-cidaroid-like ancestor, must have occurred duringthe late Palaeozoic.

The preservation of fossil echinoids depends onseveral factors, including the rigidity of their tests. Inmany post-Palaeozoic echinoids, interdigitating rodsand pegs of the stereom of adjacent plates lock themtogether, making a rigid and strong test. However,Palaeozoic echinoids relied on soft tissues to hold theplates together. Consequently, when the echinoid

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Fig. 4. Lower Carboniferous (A,B,E,F) and Permian cidaroids; all specimens from the British Isles unless otherwise stated.(A,F) Archaeocidaris whatleyensis Lewis & Ensom. Oral and apical views, respectively, of an unusually complete, but flattened, specimen.(B,E) Archaeocidaris whatleyensis Lewis & Ensom. (B) Apical surface. (E) Enlargement of the central area of (B), showing the madreporiteplate (1) of the apical system, but also the outer and inner portions of the compass of the Aristotle�s lantern (2, 3, respectively).(C) Miocidaris keyserlingi (Geinitz). Part of an interambulacrum, showing the reduction to only two columns of plates (contrast with thefour columns of Archaeocidaris; Fig. 4B). (D) Miocidaris? permica Wanner (Timor). Spine. All scale bars represent 10 mm.

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died and the tissues decomposed, the test collapsedand disintegrated (Fig. 3). Isolated plates were thusmore likely to be preserved than the entire test. How-ever, if the echinoid was rapidly buried (which mayalso have killed it), the test would only collapse, per-haps with minimum distortion, and the componentplates, together with the spines, would be preservedintact (Archaeocidaris whatleyensis; Fig. 4A,F).

A test, however complete, preserved withoutspines indicates that there was a period of time be-fore burial for the soft tissue holding the spines to thetest to decompose, allowing them to detach. The softtissues holding together the plates of the test tooklonger to decompose and gave a greater chance ofpreservation (Fig. 3), although there are specimenswhich are little more than random heaps of plates.Spines and pedicellariae of modern echinoids arealso the first components of the test to fall away, in amatter of hours, whilst the plates disarticulate moreslowly.

Suggestions for further readingDonovan, S.K., Paul, C.R.C. & Lewis, D.N. 1996.

Echinoderms, in Fossils of the Upper Ordovician(edited by D. A. T. Harper & A. W. Owen), pp.202�267. Palaeontological Association Field Guidesto Fossils 7. Palaeontological Association,London.

Jackson, R.T. 1912. Phylogeny of the Echini, with arevision of the Paleozoic species, Memoirs of theBoston Society for Natural History, v.7, 490pp.

Kier, P.M. 1965. Evolutionary trends in Palaeozoicechinoids, Journal of Palaeontology, v.39, pp.436�465.

Kier, P.M. 1966. Noncidaroid Palaeozoicechinoids, in Treatise on Invertebrate Palaeontology,Part U, Echinodermata 3 (in 2 volumes) (edited byR. C. Moore), pp.U298-U312. GeologicalSociety of America and University of KansasPress, New York and Lawrence.

Lewis, D.N. & Ensom, P.C. 1982. Archaeocidariswhatleyensis sp. nov. (Echinoidea) from theCarboniferous Limestone of Somerset, and noteson echinoid phylogeny, Bulletin of the BritishMuseum (Natural History), Geology, v.36, pp.77�104.

Paul, C.R.C. 1967. New OrdovicianBothriocidaridae from Girvan and areinterpretation of Bothriocidaris Eichwald,Palaeontology, v.10, pp.525�541.

Smith, A.B. 1984. Echinoid Palaeobiology. GeorgeAllen & Unwin, London, 190pp.

Smith, A.B. & Hollingworth, N.T.J. 1990. Toothstructure and phylogeny of the Upper Permianechinoid Miocidaris keyserlingi, Proceedings of theYorkshire Geological Society, v.48, pp.47�60.

DAVID N. LEWIS andSTEPHEN K. DONOVAN

Department of Palaeontology,Natural History Museum,

London SW7 5BD, UK

EDITOR: Peter J. Smith (Open University)ASSISTANT EDITOR: Eric Robinson (University College London)CHAIRMAN OF THE MANAGEMENT COMMITTEE: David B. Thompson

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