Ammonoids
300 m.y. history
marginal siphuncles
fluted septa (sutures best seen in internal molds)
Origins
- derived from early Devonian nautiloids
Septal fluting function
- butressing
- many had thin walls so fluting strengthened
- e.g. Nautilus diving depths
- 345 m collapse depth
- but found at 549m
- internal pressure must be > 1 atmosphere
- greater rate of depth change
- greater ease of changing specific gravity
Variations in Shells
A. Sutures
- Goniatite
- simple undivided sutures
- Paleozoic
- One Triassic genus
- One Cretaceous genus
- Ceratite
- arose in the Mississippian
- disappeared and then reappeared in the Creataceous
- lobes divided into small second order lobes and saddles
- Ammonitic
- range: Permian-Cretaceous
- lobes and saddles subdivided into smaller second order lobes and saddles
- some have third order serrations
B. Ornamentation
- Paleozoic: unornamented
- Mesozoic: many highly ornate
C. Size
- Paleozoic: golf ball size
- Triassic: many saucer size
- Jurassic-Cretaceous: up to 2m
Geologic History
- Devonian
- all goniatite sutures
- Mississippian
- most goniatite
- first ceratite
- Pennsylvanian
- goniatite most common
- some ceratite
- Late Penn. transitional to ammonitic type
- broader rounded venters
- Permian
- goniatite and ceratite present
- best index fossil were ammonitic type
- Triassic
- goniatie all but vanishes
- Lower and Middle dominanted by ceratite
- ornamentation incresed in the Middle and Upper portion of the period
(ribs, nodes, and keels)
- Jurassic
- only ammonitic type
- many highly ornamented
- Cretaceous
- Lower: all ammonitic
- Middle and Upper: increase in goniatite and ceratite
- some loosely spiraled and u-shaped