Dominance of Miocene echinoderms in the equatorial Neogene marine platform of Brazil and their insights into the paleoenvironment

Abstract

Echinoderms are one of the most important invertebrates in the early to middle Miocene heterozoan carbonate deposits of the Pirabas Formation of the northern Brazil. The well-preserved echinoid tests were useful for accurate identification, whereas the sediment filling the tests provide significant information regarding the rock matrix. The dominance of disarticulated ophiuroid, crinoid, and asteroid plates, and damage on the echinoderm test provides a basis for taphonomic processes to elucidate a paleoenvironmental interpretation based on the echinoderm assemblages. Nineteen echinoderm taxa (ten valid species and nine species in open nomenclature) were recorded based on well-preserved and/or fragmented tests and/or spines, including Abertellidae †Abertella pirabensis (Santos, 1958), Placatenellidae †Placatenella complanata (Brito, 1981), Clypeasteridae †Clypeaster lamegoi Santos, 1958, †Clypeaster paraensis Brito, 1979, and †Clypeaster paulinoi Santos, 1958, Cassidulidae †Anisopetalus oliveirai (Santos, 1958) and †Rhyncholampas candidoi Fernandes and Morais, 1994, Echinolampadidae †Echinolampas paraense Santos, 1958, Schizasteridae †Agassizia eugeniae Brito and Ramires, 1974 and Schizaster sp., Cidaridae Cidaris sp., †Phyllacanthus priscus Brito and Ramires, 1974, and Prionocidaris sp. The smaller echinoderm fragments were dominated by disarticulated plates of Gorgonocephalidae ophiuroids, Comatulidae crinoid Sievertsella sp., Goniasteridae asteroids, as well as spines and tests fragments of Cidaridae echinoids. The echinoderm assemblages suggest an inshore and shallow water paleoenvironment over a soft sandy carbonate bottom. MicroCT analyses of sediments filling echinoid tests exhibited a microfossil assemblage, including foraminifera, sponges, ahermatypic corals, bryozoans, and mollusks. Broken and disarticulated tests and plates in chaotic orientations suggest high-energy paleoenvironment. Evidence of test damage indicates complex predator-prey interactions. Overall, the microCT analyses of echinoid tests proved to be a high accuracy non-destructive, analytical approach for investigating fossil echinoderms and their paleoenvironment.