Qilinyu in Yunnan

Qilinyu rostrata is a “maxillate” placoderm from the late Ludlow epoch of Qujing, Yunnan, 419 million years ago. Qilinyu is a genus of early placoderm from the late Silurian (late Ludfordian stage, ~423 Ma) of China. It contains a single species, Qilinyu rostrata, from the Xiaoxiang fauna of the Kuanti Formation. Along with its contemporary Entelognathus, Qilinyu is an unusual placoderm showing some traits more similar to bony fish, such as dermal jaw bones and lobe-like fins. It can be characterized by adaptations for a benthic lifestyle, with the mouth and nostrils on the underside of the head, similar to the unrelated antiarch placoderms. The shape of the skull has been described as “dolphin-like”, with a domed cranium and a short projecting rostrum.

Specimen and taxonomy

The holotype and paratype of Q. rostrata are two exquisitely preserved specimens both featuring a domed cranium and a curved rostrum presenting a “dolphin-like profile.”

The researchers’ cladistic diagram show that Q. rostrata is the sister taxon of Entelognathus and all other crown gnathostomes (i.e., bony and cartilaginous fishes, and their descendants).

Evolutionary significance

Q. rostrata, together with Entelognathus, demonstrate additional irrefutable proof that modern gnathostomes evolved from arthrodire placoderms.

Discovery and Description:
Qilinyu is a genus of early placoderm discovered in the late Silurian (late Ludfordian stage, ~423 Ma) of China, specifically in the Xiaoxiang fauna of the Kuanti Formation. It is represented by a single species, Qilinyu rostrata. Alongside its contemporary, Entelognathus, Qilinyu exhibits unusual traits for a placoderm, resembling bony fish with dermal jaw bones and lobe-like fins. The skull shape is likened to a “dolphin-like” profile, featuring a domed cranium and a short, projecting rostrum. Adaptations for a benthic lifestyle include recessed mouth and nostrils on the underside of the head, reminiscent of unrelated antiarch placoderms.

Discovery Context:
Qilinyu rostrata fossils were unearthed in the Qilin district of Yunnan, China, part of the Xiaoxiang fauna deposited in the Kuanti Formation. Excavations spanning 1999 to 2016 yielded well-preserved specimens, now housed at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP).

Age and Naming:
Conodont biostratigraphy dates the Xiaoxiang fauna to the late Ludfordian stage of the Silurian, approximately 423 million years ago. The genus name Qilinyu derives from the Qilin district, named after the mythical Qilin creature, reflecting its hybrid traits. The species epithet “rostrata” refers to its distinctive rostrum.

Anatomy and Characteristics:
Qilinyu measures about 12.6 cm (5.0 inches) in length, with armor divided into head and trunk components adorned with oval-shaped tubercles. Its skull features a forward-projecting rostrum shielded above by a rostral plate and characterized by a set of dorsal plates. The jaws of Qilinyu, like Entelognathus, show external maxillate adaptations with three slender bones: premaxilla, maxilla, and dentary, lacking internal gnathal plates typical of other placoderms.

Head Armor and Skeletal Features:
The head armor includes paired central plates and two postorbital plates per side, distinguishing it from Entelognathus. Lacrimal, jugal, and opercular bones flank the upper jaw, hidden under the rostrum, with a distinct lateral line groove extending across the skull and trunk armor.

Trunk Armor and Appendages:
Trunk armor of Qilinyu, closely integrated with the skull, features three median dorsal plates and prominent dorsolateral and ventrolateral plates. Its unique interlocking neck joint resembles features found in antiarchs and certain arthrodires. Qilinyu is notable for preserving pectoral and pelvic fins, indicative of its evolutionary significance in early gnathostome development.

Classification and Phylogenetic Significance:
Phylogenetic analyses place Qilinyu rostrata as a key taxon linking placoderms to modern gnathostomes, sharing ancestry with Entelognathus, Janusiscus, and other crown-group gnathostomes. Its evolutionary context is further explored alongside related taxa like Silurolepis and Bianchengichthys, highlighting its pivotal role in vertebrate evolution.