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Depositional palaeoenvironments in a tide-influenced delta plain with amphibian and Cycadophyta remains – the Triassic Zarzaitine Formation (Algerian eastern Sahara)

Mazrou S., Lasnami Y., Amer J., Boutaleb A.

Geologos

2024

Vol. 30, No. 3

209--229

In the region of In Amenas (south-eastern Algeria) Triassic deposits crop out, on top of Palaeozoic rocks under an angular unconformity. A study of their sedimentological and palaeoenvironmental characteristics has revealed a sandy-clay unit with vertebrate remains, tree trunks and ichnofossils of Middle-Late Triassic age. This so-called Zarzaitine Formation, which reflects deposition in a deltaic environment, shows three facies associations. The first defines an upper and shallow intertidal mixed flat zone with in-situ temnospondyl capitosauroid remains, associated with Skolithos ichnofacies, while the second one defines a lower intertidal delta plain zone, composed of sandstone, distributary channels and muddy interdistributary areas,which record daily tidal rhythmites and monthly tidal bundles. The third, and last, facies association represents an upper delta plain of the supratidal zone, characterised by coarse sandstone deposits of braided rivers and a Cycadophyta palaeosol. Previous work, which led to considering these Triassic deposits as a series composed of four formations, deposited in a braided rivers environment, whose lower floodplains constituted ecological niche for a temnospondyl fauna, under hot and dry climatic conditions and plants belonging to this same formation, for mushrooms and algae, will also be considered.

Myzostomid and copepod infestation of Jurassic echinoderms: A general approach, some new occurrences, and/or re-interpretation of previous reports

Radwańska U., Radwański A.

Acta Geologica Polonica

2005

Vol. 55, no. 2

109-130

A study of diverse cysts developed on fossil echinoderms from Poland results in the recognition that these on Late Jurassic crinoid stems are attributable to the life activity of myzostomidan polychaetes, and those on Middle and Late Jurassic echinoids, to the activity of copepod arthropods. A review of formerly reported cysts of coeval age from Europe and western Asia permits the distinguishing of several types that differ in shape and/or location on the echinoderm skeleton. Although the studied cysts qualify as trace fossils (which require a separate ichnotaxonomy), their ethological and ecological characteristics are presented in terms of interspecific parasite-host relationships. The classical interpretation of VON GRAFF (1885) is affirmed for myzostomidan endocysts in crinoid stems, whereas for echinoid tests a new interpretation is offered for large exocysts ("Halloween pumpkin-mask" type) as having been induced by copepods,comparable in their ethology to those on present-day biota (hydrocorals) other than echinoderms. A copepod attribution by MERCIER (1936) of cysts (Castexia type) on some Middle Jurassic collyritid echinoids from France is fully accepted. This is supplemented by some new finds in Poland, a re-study of the Castexia cysts from France, and a re-interpretation of former reports from the literature. Eco-ethological consequences of the location of copepods in the ambulacral and peristomial parts of cidaroid and hemicidaroid echinoids are discussed; larval settling apparently took place at the tubefeet pores and gonopores, through which the copepod larvae reached the echinoid.s interior and began to parasitize it. Attribution of the discussed cysts to copepods yields, consequently, an extension of the stratigraphical range of the class Copepoda H. MILNE-EDWARDS, 1840, to the Early Jurassic.In POSTSCRIPT, suggested is the bald-sea-urchin disease to have caused some lesions in the collyritid echinoids (Middle Jurassic: Callovian) from France.