Entelognathus primordialis (“primordial complete jaw”) is an early placoderm from the late Silurian (Ludlow epoch) of Qujing, Yunnan, 419 million years ago.
A team led by Min Zhu of the Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology in Beijing discovered the intact, articulated fossil in rock formations at Xiaoxiang reservoir.
Specimen and taxonomy
The holotype of
E. primordialis is the uncrushed and mostly intact anterior half of an individual with the articulating head and trunk armor preserved in three dimensions. The holotype is about long, and the live animal is estimated to have been over long. In overall form, the animal resembles primitive
, but the anatomy of the jaws strongly suggests the anatomies of
bony fish and
. Specifically, this is the first stem gnathostome with dermal marginal jaw bones. These bones are the
premaxilla,
maxilla, and
dentary.
Most known placoderms had simple beak-like jaws made of bone plates.
In 2023, newly described articulated specimen is shown, which have preserved large scales some of which are rhomboid, and fin spines on dorsal and anal fins (the latter of which was a trait that was previously only found in stem-group
chondrichthyes).
The researchers' cladistic diagram suggests that E. primordialis forms a polytomy with arthrodires, , and all advanced (namely bony fish, tetrapods, acanthodians, and chondrichthyes).
Etymology
The generic name translates as "complete jaw", referring to how the animal had a complete set of dermal marginal jaw bones. The specific name translates as "primordial".
Evolutionary significance
Prior to the discovery of
Entelognathus, scientists assumed that the last common ancestor of jawed vertebrates was a shark-like animal, with no distinct jawbones, and that modern jaws evolved in early bony fishes. This discovery shows that modern jaws evolved earlier. It is possible that Chondrichthyes started with distinct jaws and then dispensed with them. This has been called the earliest known animal with what looks like a face.
Gallery
File:Maxillate placoderms lateral view diagram.png|Diagram with other maxillate placoderms in lateral view
File:Maxillate placoderms dorsal view diagram.png|Diagram with other maxillate placoderms in dorsal view
File:Life restoration of Sparalepis tingi.tiff|Pair (middle background) among fauna from the Kuanti Formation including Sparalepis tingi (foreground), numerous , and Megamastax (background)
File:Video of the 3D reconstruction of Entelognathus primordialis.webm|Video of 3D reconstruction
See also
External links
