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 MiddleJurassic–Early Cretaceous of North America, Europe, and North Africa. It represents the most ancient and plesiomorphicgenus 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. Thenew 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. Thefructification (utricle) has bilateral symmetry, formed by two superimposed series of bract cells that abaxiallycover the gyrogonite. The inner series has three long cells that are born by a short basal cell and reach theoospore apex. The outer series shows a central fan of bract cells, composed of a small basal cell, triangular inshape, that bears three long cells growing upwards, which are in turn flanked by two long bract cells on eachside. The central basal cell and these two lateral cells are directly attached to the base of the utricle. The thallus iscorticated with cells twisting at an angle of 30◦. The utricle of Echinochara pontis displays a combination offeatures present in Echinochara peckii (Bathonian?–Kimmeridgian) and Echinochara lazarii (Barremian–earlyAptian). For instance, the bilateral symmetry of the utricle and the morphology of the inner bract-cell seriesallow 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 ofEchinochara peckii (40◦–45◦) and Echinochara lazarii (not twisted). The unique combination of morphologicalcharacters in the utricle of Echinochara pontis fits well with its intermediate chronostratigraphic position betweenthe two former species. Echinochara pontis formed monospecific meadows in brackish settings. A preference forbrackish 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.