Reassessment of sediments and trace fossils from old red sandstone (Lower Devonian) of Dunure, Scotland, described by John Smith (1909)

REASSESSMENT

OF SEDIMENTS AND TRACE FOSSILS FROM OLD

RED SANDSTONE

(LOWER DEVONIAN) OF DUNURE,

SCOTLAND,

D E S C R I B E D BY J O H N S M I T H (1909)

by JOHN E. POLLARD * & ELAINE F. WALKER *

ABSTRACT

RI~SUMI~

In 1909 John Smith, a Scottish naturalist and geologist, described 23 t~ genera ~ and 51 ~ species ~ of trace fossils from small patches of sediment associated with andesite lava flows at Dunure, Ayrshire. He interpreted the traces as evidence of a diverse invertebrate fauna which inhabited small pools and fissures in the lava surface between eruptions. Smith's collection (c 300 specimens) was presented to the British Geological Survey, Edinburg but has remained largely unstudied.

En 1909, John Smith, amateur 6cossais naturaliste et g~ologue d6crivit 23 t~ genres ~ et 51 ~ esp/~ces ~ de traces fossiles provenant de poches de s6diments associ6es ~ des coul6es and6sitiques ~t Dunure (Ayrshire). I1 interpr6ta ces traces comme t6moignage de l'existence d'une faune diversifi6e d'invert6br6s qui habitait entre les 6ruptions volcaniques des petites mares et des fissures situ6es dans les coul6es. La collection Smith (environ 300 6chantillons) fut donn~e au t~ British Geological Survey ~ d'Edimbourg, mais est rest~e, jusqu'/~ pr6sent mal &udi6e.

Re-examination of the Smith collection shows that it came from 3 separate localities which differ in composition of the ichnofaunas, associated sediments and sedimentary structures. Arthropod trackways dominate the ichnofauna occurring in laminated siltstone frequently with ripple marks and foam marks suggesting formation in shallow lacustrine conditions. Preservational and behavioural analysis of trace fossils reveals about 10 valid ichnogenera of locomotion and swimming trackways, resting traces, feeding trails but few burrows. Presence of early terrestrial arthropod traces is uncertain. Palaeoecological interpretation is of ephemeral lakes in distal braid-plain situation subject to subsurface invasion of andesite lava producing fluidization and deformation of wet sediment. The Dunure ichnocoenosis shows unique diversity of Devonian arthropod trace fossils.

Notre r6vision de la collection Smith montre que les ~chantillons proviennent de 3 localit~s distinctes, chacune avec sa propre ichnofaune, son propre s6diment et ses figures s~dimentaires associ6es. Les pistes d'arthropodes dominent l'ichnofaune et se trouvent dans des niveaux silteux lamin6s, souvent avec des rides et des figures d'6cume, semblant indiquer des conditions lacustres peu profondes. L'analyse 6thologique et taphonomique des traces fossiles montre qu'il existait environ 10 ichnogenres valables de pistes de locomotion et de natation, de traces de repos, de traces de nutrition, mais peu de terriers. La pr6sence de trace d'arthropodes terrestres est douteuse. L'interpr6tation pal6o~cologique suppose l'existence d'6tangs 6ph6m~res situ6s dans la partie distale d'une plaine ~t cours d'eau anastomos~s, avec des intrusions d'and6site en subsurface qui provoquaient la d~formation et la fluidisation du s6diment mouill~. L'ichnocoenose de Dunure montre une diversit6 unique de trace d'arthropodes d6voniens.

KEY-WORDS : ARTHROPOD TRACKS, TRACE FOSSILS, RIPPLE MARKS, FOAM MARKS, LACUSTRINE ENVIRONMENT, INTRUSIVE ANDESITE, LOWER DEVONIAN, AYRSHIRE, SCOTLAND. MOTS-CLI~S : PISTES D'ARTHROPODES, TRACES FOSSILES, RIDES, FIGURES D'I~CUMES, MILIEU LACUSTE, INTRUSION ANDI~SITIQUE, DI~VONIEN INFI~RIEUR, AYRSHIRE, I~COSSE.

* Department of Geology, University, Manchester, England, MI3 9PL.

Published in GEOBIOS, 17

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