Filling the gap in the evolution of the genus Echinochara Peck (Clavatoraceae, Charophyta)

Abstract

Echinochara is a poorly known charophyte genus of the extinct family Clavatoraceae that thrived in the Middle Jurassic–Early Cretaceous of North America, Europe, and North Africa. It represents the most ancient and plesiomorphic genus of this family. However, the fossil record of Echinochara is discontinuous, showing several gaps, one of them especially significant in evolutionary terms between the Kimmeridgian and the early Barremian. The new species Echinochara pontis sp. nov., described here from the late Berriasian of the Maestrat Basin (NE Spain), bridges, in part, this gap, shedding light on the evolution of the genus during the earliest Cretaceous. The fructification (utricle) has bilateral symmetry, formed by two superimposed series of bract cells that abaxially cover the gyrogonite. The inner series has three long cells that are born by a short basal cell and reach the oospore apex. The outer series shows a central fan of bract cells, composed of a small basal cell, triangular in shape, that bears three long cells growing upwards, which are in turn flanked by two long bract cells on each side. The central basal cell and these two lateral cells are directly attached to the base of the utricle. The thallus is corticated with cells twisting at an angle of 30°. The utricle of Echinochara pontis displays a combination of features present in Echinochara peckii (Bathonian?–Kimmeridgian) and Echinochara lazarii (Barremian–early Aptian). For instance, the bilateral symmetry of the utricle and the morphology of the inner bract-cell series allow comparison with E. lazarii, whilst the morphology of the outer bract-cell series is closer to E. peckii. Moreover, the thallus of the new species shows cortical cells with a twisting angle intermediate between that of Echinochara peckii (40°–45°) and Echinochara lazarii (not twisted). The unique combination of morphological characters in the utricle of Echinochara pontis fits well with its intermediate chronostratigraphic position between the two former species. Echinochara pontis formed monospecific meadows in brackish settings. A preference for brackish settings is also observed for Echinochara peckii and in the older populations of Echinochara lazarii, suggesting that this habitat is probably a basal character in the evolution of the genus Echinochara.