Entelognathus

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Entelognathus primordialis
Temporal range: Late Ludlow
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Placodermi
Order: incertae sedis
Genus: Entelognathus
Zhu et al., 2013
Species: E. primordialis
Binomial name
Entelognathus primordialis
Zhu et al., 2013

Entelognathus primordialis is a primitive placoderm from the late Ludlow epoch of Qujing, Yunnan, 419 million years ago.[1][2]

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.

This specimen is important because our knowledge of the base of this group is incomplete. To quote the abstract of the original paper:
"The gnathostome (jawed vertebrate) crown group comprises two extant clades with contrasting character complements. Notably, Chondrichthyes (cartilaginous fish) lack the large dermal bones that characterize Osteichthyes (bony fish and tetrapods). The polarities of these differences, and the morphology of the last common ancestor of crown gnathostomes, are the subject of continuing debate."[2]

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 11 centimetres (4.3 in) long, and the live animal is estimated to have been over 20 cm (8 in) long. In overall form, animal resembles primitive arthrodires, but the anatomy of the jaws strongly suggests the anatomies of bony fish and tetrapods. Specifically, this is the first stem gnathostome with dermal marginal jaw bones. These bones are the premaxilla, maxilla, and dentary.[2] Most known placoderms had simple beak-like jaws made of bone plates.[1]

The researchers' cladistic diagram suggests that E. primordialis forms a polytomy with arthrodires, ptyctodonts, and all advanced gnathostomes (namely bony fish, tetrapods, acanthodians, & chondrichthyes).[2]

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

Up until now, scientists assumed that the last common ancestor of jawed vertebrates was like a shark, 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.[1][3]

References

  1. ↑ 1.0 1.1 1.2 Barford, Eliot (25 September 2013). "Ancient fish face shows roots of modern jaw". Nature. doi:10.1038/nature.2013.13823. Retrieved 26 September 2013. 
  2. ↑ 2.0 2.1 2.2 2.3 Zhu, Min; Xiaobo Yu, Per Erik Ahlberg, Brian Choo, Jing Lu, Tuo Qiao, Qingming Qu, Wenjin Zhao, Liantao Jia, Henning Blom & You'an Zhu (2013). "A Silurian placoderm with osteichthyan-like marginal jaw bones". Nature (502): 188–193. doi:10.1038/nature12617. 
  3. ↑ Friedman, Matt; Martin D. Brazeau (2013). "News & Views: Palaeontology: A jaw-dropping fossil fish". Nature (502): 175–177. doi:10.1038/nature12690. 

External links

Placoderms
 
Major clades
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