Graptolites Flashcards
Graptolites: (what about them)
Extinct, but can be used for very precise age dating, used to define stratigraphic boundaries
Why are graptolites thought to be related to vertebrate animals?
Their morphology, as seen in some exquisitely preserved chertified fossil specimens, seems to closely resemble that of modern pterobranchs, which are marine creatures that possess a small tube resembling a notocord. Since vertebrate animals possess a notocord in an early embryonic stage, it is infered that vertebrates and pterobranchs are distant cousins, although their common ancestor probably occurred way back in the Precambrian. Thus by extension, graptolites are commonly fit on an offshoot of a branch of the family tree that contains the true vertebrates. However, the arguments linking graptolites to vertebrates are so tenuous that their correct taxonomic affinities remain uncertain.
Phylum Hemichordata similar/different than Bryozoans?
Different phylum than Bryozoans, similar structures
Class Enteropneusta:
modern “acorn worms” - only fossil record is trace fossils
Class Pterobranchia:
modern pterobranchs - no fossil record at all
Class Graptolithina:
Paleozoic graptolites - good fossil record, mainly in black shales
Order Dendroidea Geologic Range:
Middle cambrian to Lower Pennsylvanian
Order Graptoloidea Geologic Range:
Lower Ordovician to Lower Devonian
Classes of Hemichordata:
Enteropneusta, Pterobranchia, Graptolithina
Orders of Graptolithina
Dendroidea, Graptoloidea
Rhabdosome:
Entire graptolite colony
Zooid:
Individual graptolite animal
Theca:
Chitinous shell of an individual zooid
Sicula:
The single, conical theca of the first zooid in the colony; attached at top end to the nema
Nema:
Hollow thread-like structure extending from the top of the sicula
