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Cranial Bones: Lateral View01:27

Cranial Bones: Lateral View

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The lateral view of the cranium is dominated by temporal, sphenoid, and ethmoid bones.
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The fossil record documents only a small fraction of all organisms that have ever inhabited Earth. Fossilization is a rare process, and most organisms never become fossils. Moreover, the fossil record only exhibits fossils that have been discovered. Nevertheless, sedimentary rock fossils of long-lived, abundant, hard-bodied organisms dominate the fossil record. These fossils offer valuable information, such as an organism's physical form, behavior, and age. Studying the fossil record helps...
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The superior view of the cranium shows the frontal and paired parietal bones.
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Neurulation01:30

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Neurulation is the embryological process which forms the precursors of the central nervous system and occurs after gastrulation has established the three primary cell layers of the embryo: ectoderm, mesoderm, and endoderm. In humans, the majority of this system is formed via primary neurulation, in which the central portion of the ectoderm—originally appearing as a flat sheet of cells—folds upwards and inwards, sealing off to form a hollow neural tube. As development proceeds, the...
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Related Experiment Video

Updated: Mar 12, 2026

Dissection and Flat-mounting of the Threespine Stickleback Branchial Skeleton
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A Silurian maxillate placoderm illuminates jaw evolution.

Min Zhu1,2, Per E Ahlberg3, Zhaohui Pan4,2

  • 1Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, PO Box 643, Beijing 100044, China. zhumin@ivpp.ac.cn per.ahlberg@ebc.uu.se.

Science (New York, N.Y.)
|November 16, 2016
PubMed
Summary
This summary is machine-generated.

A new Silurian placoderm discovery clarifies the evolution of jaw bones. Maxilla, premaxilla, and dentary bones in placoderms are homologous to gnathal plates, revealing a shared dental arcade origin.

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Area of Science:

  • Paleontology
  • Vertebrate Evolution
  • Comparative Anatomy

Background:

  • The discovery of Entelognathus in Silurian strata presented maxilla, premaxilla, and dentary bones in a placoderm, challenging previous classifications.
  • The homology between these marginal jaw bones and the gnathal plates of other placoderms, traditionally considered the inner dental arcade, remained unresolved.
  • Understanding this relationship is crucial for tracing the evolutionary origins of jaw structures in vertebrates.

Purpose of the Study:

  • To investigate the evolutionary link between the gnathal plates of placoderms and the marginal jaw bones (maxilla, premaxilla, dentary) found in some Silurian placoderms.
  • To clarify the homology of these jaw elements and their contribution to the early dental arcade.
  • To reconstruct the transformation process of jaw structures during vertebrate evolution.

Main Methods:

  • Analysis of a newly discovered Silurian maxillate placoderm specimen.
  • Comparative morphological analysis of jaw elements between different placoderm groups and early osteichthyans.
  • Phylogenetic bracketing to infer homology and evolutionary relationships.

Main Results:

  • The newly described placoderm exhibits intermediate jaw characteristics, bridging the gap between typical placoderm gnathal plates and maxillate jaw structures.
  • Evidence suggests that the maxilla, premaxilla, and dentary in these Silurian placoderms are homologous to the gnathal plates of other placoderms.
  • The study indicates that the transformation from gnathal plates to maxillate bones occurred simultaneously in both upper and lower jaws.

Conclusions:

  • The maxilla, premaxilla, and dentary are integrated into the placoderm dental arcade, suggesting a shared evolutionary origin with gnathal plates.
  • This gnathal-maxillate transformation predates the evolution of infradentary bones in the lower jaw.
  • The findings provide critical insights into the early evolution of jaw apparatus in vertebrates, particularly the transition towards osteichthyan-like jaw structures.