'puelDu3lsaM qmos pue saleM U! oi 4~!4M U! MOJJnqe uado Du!daa)f pue DU!leAe~Xa DORSET TRACE FOSSILS asoui se qons 'S)f~OJ~!ozoaeled U! uaas aq uao U! sjeunue Jeln~!lJed JO suqeq aLil I\IPuo~as 'paAloAa asa4l se I\lapeA U! Du!seaJ~u! 'Slew!ue 'uorsseidun ue·aAealll!M juudiooj JOneJlDu!paaJ e Traces found in the Mesozoic rocks in Dorset are peipoq-prau JO Sa!l!A!l~e a4l JO sa~eJ1 'eu!48 Ja4la4M appsp IUMiusunpas a4l JO JaleM between 65 and 190 million years old. The most iusiuoo pue eneJlSnv U! ssuoispues pro Jeal\ uonnw pue aJnlxal a4l I\nSJL:I'uO!leAJasaJd J!a4l oi peal easily seen traces in Dorset rocks are in sand- OOL U! sl!eJl Du!zeJD JO puno, uaaq aAe4 ssoan II!MSUO!l!PUO~le!~ads I\JaAAIUO 'puel uo PU!Mpue stones, such as the 'doggers' in the Thorncombe jnq 'peipoq yos aJaM sjeunue lsa!pea I\JaA a41 u!eJ I\q JO'eas a4l U! uouoe lep!l pue'aAeM I\q aW!l Sands. On the beach under Thorncombe lJ04S e U!4l!Mpal\OJlSapaq II!Msl!eJl JOs)f~eJllsolI'J Beacon, west of Eype Mouth, these boulders can 'sa~eJl a4l aAJasaJd oi al!~le~ be seen from all sides, revealing trace fossils on 4l!M 1\1)f~!nbI\JaA peiueuiso aq .I\ew jueunpas 'I\e~ap SlOOJa4l aiojsq )f~OJ0lU! suapreq the bedding planes and down into the rock. The a4l 'SUO!l!PU03le~!wa4~ l4DP a4l uaA!8 'l! l~aloJd LI~!4M pnw 0lU! unJ Aew saaJl JO SlOOJ aLU fillings of the trails and burrows ·are shown by ol lUaw!pas aJOW I\q I\IP!deJ papnq aq Ol spaau their contrast with the texture or colour of the host UO!l!ppe UI 'aleJlsqns a4l.aAIOSS!P Ol·sle~!wa4~ 'luaw!pas a4l U!4l!M MOJJnqa4l JO'a~eJJns aLil uo Du!sn I\q JO 'sMel~ JO Lllaal se 4~ns 'slJed PJe4 rock. In the orange Thorncombe. Sand boulders l!eJl JO)f~eJl a4l 'I\nsel 'Jalel Dupoq JOMOJJnq84l Lll!M Du!dseJ I\q JaLll!a paleaJ~ aJe l\aLl1 'slew!ue the burrows tend ·to be deeper orange, perhaps II!JOl 'aJnlxal JOJnOIO~luaJaJJ!pe JOsde4Jad 'pnw JO Slueld I\q pa~npoJd aq I\ew S)f~OJpauapJe4 due to the former presence of pyrite produced by JO pues JOJalq!ssod ua4l S! II 'slepalew JapJeLi I\p~aJle JO 'pOOM 'sauoq 'sliaLis U! SDUP08 the decay of organic material which has later 0lU! aJoq JO 'uado l! daa)f Ol slallad JO sn~nw 'sal!IOJdo~ se UMOU)f aJe SDu!ddoJp lew!ue oxidised. 4l!M MOJJnq a4l aun slew!ue awos 'JaAaMOH pas!l!sso.::l 'sl!ssoJ 'I\POq, se UMOU)faJe '4leap uo 'uO!leqJnlO!q se UMOU)fS! pue 'pues aLil U!Lll!M sa!poq J!a4ll\q yal suo!ssaJdw! aLil pue 'SleW!Ue In the Jurassic Lower Lias shales near SJal\el I\ue dn sax!w S!41 'waLil pU!4aq lep9lew a4l JO su!ewaJ lenl~V 'aW!l JO popad DUOI e Charmouth the small Chondrites burrows show alseM DU!laJ~xa '00 l\a4l se Du!paaJ luaw!pas JaAO)f~OJpal~edwo~ U! paAJasaJd uaaq aAeLi 'l! up because they are filled with lighter grey a4l 4DnOJ4l aAOW I\ldw!s SJaMOJJnq l\uell'J U!4l!M SMOJJnqJO'pnw JOpues yos JOa~eJJnsaLH material. surrounded by dark grey shale. bivalves as a shelter from which to collect particles suspended in the water. Burrowing uo Slew!ue I\q apew sl!eJl pue s)f~eJl a41 'oDe 'a~eJl ou saAeal luaw!pas sJeal\ JOSUO!II!WpaAn 04M saJnleaJ~ JOSa!l!A!l~e In the yellow sandstones of the Bridport Sands, bivalves tend to have an elongated shape. Some a4l DU!~elds!pI\ldw!s :luelJodw! S! paaJ pue aA!1 a4l JO s)f~OJ alH U! PJo~aJ a4l aJe SI!ssoJ a~eJl. both vertical and horizontal burrows are filled with could burrow down quickly using their shells, harder material of the same colour so that the others fixed themselves to the bottom of the rain washes away the surrounding rock, leaving a burrow using retractable threads with which to 611SS0:l3~"~! " SI !"HM honeycomb of burrows protruding from the cliffs pull themselves down quickly in case of danger. at Burton Cliff. The sand itself seems the same as the host rock, but the burrow fill has become harder because it has more calcite cementing the sand together. Trace fossils are usually not collectable, but provide excellent opportunities for close-up Sometimes a track is filled with softer sediment, photography, or sketching. Please do not be which washes out quickly when exposed to the tempted to hammer at the boulders - you will weather. A particularly good example of this is the not succeed in removing the traces intact, and dinosaur footprints in the Cretaceous Purbeck will in the process ruin them for the next visitor. TRACE FOSSILS limestones, where the tracks were made in a shell sand which has since hardened into limestone and were filled with a soft marl or clay. Illustrations by ANTONIA PHILLIPS The names given to trace fossils rarely have any connection with the animals that made them. Early· geologists were uncertain of the origin of DERC is a Registered Charity (No. 288674), the structures they saw, and even today we can concerned with the species, habitats and geology of only put forward theories based on the activities All the other traces were made by invertebrate Dorest. DERC is supported by the Dorset County of present day organisms, except in the rare case animals (without backbones) but vertebrate Council Environmental Services Department, the of a creature dying at the end of its track! animals, such as fish, reptiles and mammals also Dorset Natural History and Archaeological Society, the Dorset Wildlife Trust and English Nature. DIGS lived in the Mesozoic. Body fossils of these (Dorset's Important Geological Sites group) is a The·following are some examples from Dorset. creatures are found where the sediment was partnership between the Dorset Geologist's Simple movement trails are the most common suitable for their preservation, and trace fossils in Association Group and all these organisations, together with feeding and dwelling structures. the rare cases where footprints left a lasting supported by the District Councils. Systematic grazing trails are also common, like impression. Dinosaur footprints in the Purbeck rocks Dorset Environmental the marks made by modern limpets on rocks. show that there were many different species as Dorset Environmental Records Centre, Library many trackways' have been uncovered by the Headquarters, Colliton Park, Dorchester, Dorset. Records 'Centre The most common burrows seen in both quarrymen. They seem to be best preserved in DT1 1XJ I"~ siltstones and limestones .are the single vertical Y. limestones that consist of small shells cemented e-mail: [email protected] burrows. These were made by several different together with fine powdery calcium carbonate, that website: www.derc.org.uk hardened quickly in a sub-tropical climate. ENGLISH NATURE Thalassinoides is a quite large burrow system, with burrows 2 to 20cm diarneter. showing Y- Chondrites is a system of branching tunnels shaped branching. It is usually horizontal, following starting from a central vertical stem, and fanning bedding planes. The burrow fill sometimes contains out obliquely or horizontally. The radial burrows faecal pellets, or shows scratch marks on the walls. 1.5cm branch again laterally, giving the impression that These clues have led to the conclusion that these Diplocraterion is a vertical U-shaped burrow, .... ~. the occupant was reaching out from a central probably occupied by a suspension feeding Gyrochorte looks like a fine plaited rope laid in a burrows were produced by decapod crustaeans, position. It is thought that a sipunculid worm animal, in which the reworking of the sediment curve over the rippled sand. The thin 'tiles' on similar to shrimps, and were used as a permanent could have lived in the centre, using a proboscis between the arms of the U is due to the upward, which it is found in the Forest Marble often show dwelling and for feeding and brooding. Large to feed on the particles in the sediment. The or, less commonly, downward migration of the U. matching grooves on the underside. There are Tha/assinoides burrows have been found in sands branching tunnels never cut each other, and were The sand may accumulate or be swept away several suggestions as to the animal that may which were deposited in environments affected by lined with mucus to keep them open as the fairly rapidly in shallow seas and the animal will have made these marks, including a spired wave activity, smaller ones in quiet environments. proboscis was retracted. Small Chondrites dig deeper, or move upwards, in order to maintain gastropod, a crustacean, or a worm. The thin In the Chalk many Tha/assinoides burrows appear as mottling within fine sediments, a constant depth in which to rest and reach out 'tiles' can be found on the shore of the Fleet, and became filled with flint, possibly due to the particularly light and dark grey in the Lower Lias for food suspended in the water above. Examples at Fault Corner east of Eype Mouth, where they presence of organic material. Strange shaped flints and Kimmeridge Clays. Large systems can also of Diplocraterion may be seen at the top of the weather out of the clays of the Forest Marble which have weathered out can be found over the be seen on theMiddle Lias siltstone boulders on Belemnite Marls near Charmouth. Formation. Chalk uplands, or redeposited in Tertiary gravels. the beach west.of Eype Mouth .

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Rhizocorallium is a large U-shaped burrow, up to 15 ern long, which is horizontal or oblique. The sediment between the arms of the U was reworked by the arthropod crustacean as the 2cm Imbrichnus has the appearance of tiles stacked burrow was extended. The crustacean may have on .one another along the length of the track. Arenicolites is a simple vertical U-shaped fed on material in the muddy sand during These may have been formed by a bivalve burrow without reworking between the arms of Phycosiphon is a small burrow system, where excavation of the burrow, and then have used it moving its foot forward, then contracting the the U. The burrow was probably the dwelling of a the animal was grazing systematically through the as a home while filtering food from the water muscle and pulling its body forward. These are detritus-feeding worm, which would shelter in the sediment. It consists of a series of tightly .packed above. Rhizocorallium is widespread in Jurassic also seen on the Forest Marble 'tiles', among burrow, and come out on to the surface to feed on U-shaped loops very similar to the grazing trails of rocks, particularly the Corallian and Portland many other traces perfectly preserved. The state organic matter which had settled on the sea floor. modern gastropods. They have been likened to Sand. It is one of the few trace fossils to be of preservation. suggests that the tracks and. trails They can be found in Corallian rocks between antlers, and may be seen on the bedd.ing planes preserved in the clays of the Fullers' Earth, hardened and were swiftly covered by new Weymouth and Ringstead. of siltstones and sands from the Middle Lias. Oxford and Kimmeridge Clays. sediment.

Millions CENOZOIC ME·SOlOIC PALAEOZOIC of years Tertiary Cretaceous Jurassic Triassic Permian Carboniferous Devonian Silu rian Ordivician __..._._.. " Precambrian ago o 50 100 150 200 250 300 350 400 450 500 550 600 4600