''Entelognathus primordialis'' (“primordial complete jaw”) is an early

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

from the late

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

(

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, Engl ...

) of

Qujing Qujing ( zh, c= , p=Qūjìng) is a prefecture-level city in the east of Yunnan province, China, bordering Guizhou province to the north and east and the Guangxi Zhuang Autonomous Region to the south; thus, it was called "Key between Yunnan and Gui ...

Yunnan Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...

, 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 People's Republic of China, China is a research institution and collections repository for fossils, including many dinosaur and pterosaur specimens (many from the Yixian For ...

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 (biology), 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 penetratin ...

s, but the anatomy of the jaws strongly suggests the anatomies of

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

and

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

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 mammals h ...

maxilla In vertebrates, the maxilla (: maxillae ) 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 two maxil ...

, and

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

. 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 Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...

). 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 tree ...

with arthrodires, ptyctodonts, and all advanced gnathostomes (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

conodont Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...

s, and '' Megamastax'' (background) File:Video of the 3D reconstruction of Entelognathus primordialis.webm, Video of 3D reconstruction

References

External links