A group of paleontologists from the United States, the UK and China has discovered two new suspension-feeding species of the bony fish genus Rhinconichthys.
Artist’s reconstruction of Rhinconichthys sp. Image credit: Robert Nicholls.
The two new species — Rhinconichthys purgatoirensis and R. uyenoi — lived approximately 92 million years ago in the seas of Cretaceous North America and Japan.
“Rhinconichthys purgatorensis from the lowermost Carlile Shale (middle Turonian), southeastern Colorado, United States; and R. uyenoi from the Upper Cretaceous (Cenomanian) Mikasa Formation, Middle Yezo Group, Hokkaido, Japan,” the paleontologists said.
“Rhinconichthys are exceptionally rare, known previously by only one species from England,” added team member Dr. Kenshu Shimada, of DePaul University.
“But a new skull from North America, discovered in Colorado, along with the re-examination of another skull from Japan have tripled the number of species in the genus with a greatly expanded geographical range.”
“I was in a team that named Rhinconichthys in 2010,” Dr. Shimada said, “which was based on a single species from England, but we had no idea back then that the genus was so diverse and so globally distributed.”
Rhinconichthys belongs to an extinct bony fish group called pachycormids, which contains the largest bony fish ever to have lived.
According to the team, the total length of R. uyenoi is estimated to be between 11 and 15 feet (3.4 – 4.5 m). R. purgatoirensis is moderate in size with adults ranging between 6.5 and 9 feet (2 – 2.7 m) in total length.
The most peculiar aspect of Rhinconichthys is the pair of prominent, highly elongate bones called hyomandibulae.
“We consider this feature to be an important functional specialization that is key to the biology of Rhinconichthys,” the scientists said.
Newly-discovered Rhinconichthys purgatoirensis and R. uyenoi, and the previously known species R. taylori. Image credit: Kenshu Shimada.
According to Dr. Shimada and co-authors, Rhinconichthys fed on plankton.
“One pair of hyomandibulae formed a massive oar-shaped lever to protrude and swing the jaws open extra wide, like a parachute, in order to receive more plankton-rich water into its mouth,” Dr. Shimada explained.
“This specialized cranial construction was previously unknown among Cretaceous bony fish, and functionally parallels that of the modern paddlefish Polyodon and many sharks,” the researchers said.
“Based on our new study, we now have three different species of Rhinconichthys from three separate regions of the globe, each represented by a single skull,” Dr. Shimada noted.
“This tells just how little we still know about the biodiversity of organisms through the Earth’s history. It’s really mind-boggling.”
Research describing Rhinconichthys purgatoirensis and R. uyenoi is published online in the journal Cretaceous Research.
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Bruce A. Schumacher et al. 2016. Highly specialized suspension-feeding bony fish Rhinconichthys (Actinopterygii: Pachycormiformes) from the mid-Cretaceous of the United States, England, and Japan. Cretaceous Research, vol. 61, pp. 71-85; doi: 10.1016/j.cretres.2015.12.017
