REINTERPRETATION OF THE LOWER CAMBRIAN BRACHIOPOD HELIOMEDUSA ORIENTA SUN AND HOU, 1987A AS A DISCINID

Abstract

The Lower Cambrian brachiopod Heliomedusa orienta Sun and Hou, 1987a, first identified as a jellyfish, was later described as a craniopsoid brachiopod. A new sample of 1,150 individuals of this brachiopod, measuring 3.1–22.0 mm long, from two new sites of the Lower Cambrian Yuanshan Formation was examined. The sample includes several with exceptional preservation of nonmineralized skeletons that reveal previously unknown features and fill a gap in our knowledge of the detailed anatomy of fossil discinids. It is here reinterpreted as a discinid brachiopod on the basis of its many distinctive, discinid anatomical features, such as a longitudinally oval pedicle foramen in the anterior region of the posterior sector of the ventral valve; inferred, short, but large pedicle descending ventrally from the ventral valve; inferred, paired rectus muscles in the large pedicle canal; elongate scars of paired anterior adductors; and a median septum connected to a U-shaped ridge on the ventral interior. The two growing zones of its exceptionally preserved, unique lophophore extend posteriorly instead of anteriorly as in all Recent inarticulate brachiopods. Its recognition as the first and earliest discinoidean extends its stratigraphic range from the Ordovician to the Lower Cambrian. This reassignment implies that the organophosphatic, pediculate lingulids and discinids evolved before the carbonate, nonpediculate craniopsids and craniids. Its occurrence at chronological proximity to the Cambrian explosion of small metazoans sheds light on the early morphological diversification and the adaptive radiation of the Lower Cambrian brachiopods into the available niches.