Evolution of vertebrates was accompanied by a gradual increase in oxygen availability, from the irrespirable atmosphere of the Precambrian to a P at 7 mmHg with the first vertebrates (agnaths), to the present sea level P value of 160 mmHg with the first reptiles.

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  • vertebrate — /verr teuh brit, brayt /, adj. 1. having vertebrae; having a backbone or spinal column. 2. belonging or pertaining to the Vertebrata (or Craniata), a subphylum of chordate animals, comprising those having a brain enclosed in a skull or cranium… …   Universalium

  • agnathan — /ag neuh theuhn/, n. 1. any member of the vertebrate class Agnatha. adj. 2. agnathous. [AGNATH(A) + AN] * * * ▪ fish Introduction   any member of the group of primitive, jawless fishes (fish) that includes the lampreys (lamprey) (order… …   Universalium

  • agnathia — noun /æɡˈneɪ.θi.ə/ A birth defect in which the mandible is missing. When there is agnathia, instead of the inferior maxillary bone we find nothing but a kind of tubercle formed of skin, cellular tissue, fat, and some few muscular fibres. See Also …   Wiktionary

  • agnathic — adjective a) Jawless. […]the heads are inverted and agnathic (lacking a lower jaw)[…]. b) Afflicted by or characteristic of agnathia. Mouth is without jaws (agnathic) in lampreys and hagfishes and bounded by jaws (gnathic) in all other… …   Wiktionary

  • agnathous — adjective /ˈæɡnəθəs/ a) Jawless. The Onchidiidæ are described as agnathous, but I am confident of having observed the jaw figured. b) Afflicted by or characteristic of agnathia. The mouth is a transverse slit, which shows no teeth, nor any jaws… …   Wiktionary

  • agnathan — 1. adjective Belonging or pertaining to the superclass Agnatha, the jawless vertebrates. Haikouichthys is one of three species of jawless (or agnathan) fish to be found in the Early Cambrian period. 2. noun A member of the superclass Agnatha of… …   Wiktionary

  • agnathe — [ agnat ] adj. et n. m. • 1805; de 2. a et du gr. gnathos « mâchoire » ♦ Didact. Qui n a pas de mâchoire, de mandibule. N. m. pl. Zool. Classe de poissons vertébrés sans mâchoires. Les lamproies sont des agnathes. ● agnathe nom masculin Vertébré… …   Encyclopédie Universelle

  • caenagnathiformes — |sēˌnagˌnathəˈfȯrˌmēz, |kīˌn noun plural Usage: capitalized Etymology: New Latin, from caen (I) + agnath (from a a (II) + Greek gnathos jaw) + iformes more at gnath …   Useful english dictionary