''Entelognathus primordialis'' (“primordial complete jaw”) is a maxillate
placoderm from the late
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago (Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozoi ...
(
Ludlow epoch
In the geological timescale, the Ludlow Epoch (from 427.4 ± 0.5 million years ago to 423.0 ± 2.3 million years ago) occurred during the Silurian Period, after the end of the Homerian Age. It is named for the town of Ludlow in Shropshire, Engla ...
) of
Qujing
Qujing () is a prefecture-level city in the east of Yunnan province, China, bordering Guizhou province to the east and the Guangxi Zhuang Autonomous Region to the southeast; thus, it was called "Key between Yunnan and Guizhou" () and "Throat of Yu ...
,
Yunnan
Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the ...
, 419 million years ago.
A team led by Min Zhu of the Academy of Sciences'
Institute of Vertebrate Paleontology and Paleoanthropology The Institute of Vertebrate Paleontology and Paleoanthropology (IVPP; ) of China is a research institution and collections repository for fossils, including many dinosaur and pterosaur specimens (many from the Yixian Formation). As its name sugges ...
in Beijing discovered the intact, articulated fossil in rock formations at
Xiaoxiang
Xiaoxiang (), also transliterated ''XiaoXiang'', ''Hsiao Hsiang'', and ''Chiu Chiang'', in some older sources, refers to the "lakes and rivers" region in south-central China south of the middle-reaches of the Yangtze River and corresponding, more ...
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
arthrodire
Arthrodira (Greek for "jointed neck") is an order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine e ...
s, but the anatomy of the jaws strongly suggests the anatomies of
bony fish
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...
and
tetrapod
Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
s. Specifically, this is the first stem gnathostome with dermal marginal jaw bones. These bones are the
premaxilla
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
,
maxilla
The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...
, and
dentary
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
.
Most known placoderms had simple beak-like jaws made of bone plates.
[
The researchers' cladistic diagram suggests that ''E. primordialis'' forms a ]polytomy
An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
with arthrodires, ptyctodont
The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chi ...
s, and all advanced gnathostomes (namely bony fish, tetrapods, acanthodians
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...
, and chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissu ...
).[
]
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.
See also
* '' Bianchengichthys''
* '' Qilinyu''
* '' Silurolepis''
References
External links
National Geographic - Fish fossil has oldest known face, may influence evolution
Reuters - Fossil fish find in China fills in evolutionary picture
Discover Magazine - First jawed fish had one ugly face
Scientist hails 'jaw-dropping' fish fossil discovery
(BBC, 2013-09-27)
Placoderm genera
Placoderms of Asia
Silurian fish of Asia
Placodermi enigmatic taxa
Prehistoric animals of China
Transitional fossils
Fossil taxa described in 2013
2013 in China
{{Placoderm-stub