A primitive fossil fish sheds light on the origin of bony fishes

Living gnathostomes (jawed vertebrates) include chondrichthyans (sharks, rays and chimaeras) and osteichthyans or bony fishes. Living osteichthyans are divided into two lineages, namely actinopterygians (bichirs, sturgeons, gars, bowfins and teleosts) and sarcopterygians (coelacanths, lungfishes and tetrapods). It remains unclear how the two osteichthyan lineages acquired their respective characters and how their common osteichthyan ancestor arose from non-osteichthyan gnathostome groups1, 2. Here we present the first tentative reconstruction of a 400-million-year-old fossil fish from China (Fig. 1); this fossil fish combines features of sarcopterygians and actinopterygians and yet possesses large, paired fin spines previously found only in two extinct gnathostome groups (placoderms and acanthodians). This early bony fish provides a morphological link between osteichthyans and non-osteichthyan groups. It changes the polarity of many characters used at present in reconstructing osteichthyan inter-relationships and offers new insights into the origin and evolution of osteichthyans. Figure 1 Reconstruction of Psarolepis, a 400-million-year-old sarcopterygian-like fish with an unusual combination of osteichthyan and non-osteichthyan features. a, Head and anterior part of the fish with tentatively positioned median fin spine. b, Anterior view of the skull and lower jaws (from ref. 3). Scale bar, 5 mm. c, Median fin spine (from ref. 4). d, Shoulder girdle with pectoral spine, based on specimens shown in Fig. 2. e, Cheek plate with maxillary and preopercular, based on specimens shown in Fig. 3. Surface ornamentation of the cheek plate is omitted to show the pattern of sensory canals. Most Psarolepis specimens derive from four beds at the same locality in Qujing, Yunnan, China.