The developmental trend of labrum and median eyes of Orsten-type preserved Phosphatocopina (Crustacea)

Abstract

The Phosphatocopina (Crustacea, Arthropoda) is an important group of ‘Orsten-type’ preserved fossils. It is resolved as the sister-group of Eucrustacea. Here we study the labrum, the median eyes, and the inner lamella of Phosphatocopina based on the selected specimens respectively from western Hunan, South China, and Sweden. The labrum characters of different species exhibit interspecific difference. The ratio of labrum length/shield length is significant to the reconstruction of the ontogeny of phosphatocopines. The labrum characters should be introduced to the diagnosis of Phosphatocopina. In the earliest growth stage, the median eyes were inconspicuous, almost as flat as the other area of the hypstome. However, with the growth of the individuals they became conspicuous. There were two obviously concave structures on the ventral side of the hypostome in later stages, which were the positions where the median eyes should have been located. In addition, the two modes of preservation of the inner lamella, i.e., that of shrinking and that of bulging up, are interpreted as the preservational artifact herein.