Wilson, Mark V. H. Department of Biological Sciences and Laboratory for Vertebrate Paleontology. University of Alberta, Alberta, Canada.
- Fossil record
- Related Primary Literature
- Additional Reading
Extinct, armored, jawless vertebrates with a single pair of external gill openings. Heterostraci first appeared in the Ordovician, according to some authorities, and became common in the Silurian and Devonian. Their bony dermal armor consists mainly of aspidine (bone with apparent growth layers, lacking cavities for bone cells) and is composed of three structural layers: a basal layer of lamellar bone, a middle layer usually with a honeycomb arrangement of cavities and partitions, and a superficial layer of dentine tubercles or ridges. See also: Jawless vertebrates; Ostracoderm
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information