A new middle Permian microconchid from Chiapas, Mexico, and its palaeoecological implications

Abstract

Microconchids are a little-known group of vermiform invertebrates whose fossil record is limited to the preservation of their calcareous tubes. In the past, they were erroneously classified as polychaetes, specifically of the genus Spirorbis. Nonetheless, currently it is known that serpulid polychaetes appeared in the late Permian and earliest true Spirorbis appeared in the Miocene. Microconchids are characterized by an initial bulb, microlamellar microsctructure, and a punctate tube. In this work, Microconchus maya is described as a new species from the Monte Redondo locality (Roadian, middle Permian) of the Paso Hondo Formation in Chiapas, Mexico. The new species, along with hederelloids, bryozoans, and crinoid holdfast occur as sclerobionts on brachiopods. The occurrence of most microconchids on spire-bearing brachiopod shells suggests that inhalant currents were crucial to the settlement of M. maya. However, their settlement preference on the anterior region of brachiopods could imply that microconchids only colonised places far away from the sediment. The age of the microconchids was established by means of the host brachiopods which have been correlated to Roadian formations of Texas in the USA. Microconchids and brachiopods formed life associations, inhabiting an environment related to the open waters of a homoclinal carbonate ramp. The description of M. maya not only represents the first definite record of the genus Microconchus in the Permian but also is the first report of a possible syn-vivo relationship between brachiopods and microconchids in the late Paleozoic.