Early Mississippian Crinoids and the Flowering of a Paleontologist
Forest Gahn (Department of Geology, Brigham Young University at Idaho, Rexburg, Idaho, USA)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
20 November 1998
Crinoids - there are ~615 living species today, and only ~100 of which are stalked. The rest are stalkless (the comatulids). Comatulid crinoids do have stalks as juveniles, but they are lost with ontogeny.
Wachsmuth & Springer - North American Crinoidea Camerata - a monumental work with 1000 pages and 87 plates.
A diverse set of crinoids exists in the Early Mississippian Burlington Limestone of Iowa - it is the immensest deposit for crinoid diversity in the whole geologic record.
Examples of crinoids from the Burlington Limestone: Eretmocrinus - has flared/flattened arm tips - these are older forms than those from the Ft. Payne Formation of Kentucky; Actinocrinites - with deeply lobed top of calyx; Scrotocrinus - with a flared, umbrella-like structure at top of calyx; Platycrinites - with a twisted stem and large radial plates; Eucladocrinus - also with a twisted stem, but columnals are very narrow; Agaricocrinus.
In the modern, fish like to nip off crinoid arms. In the past, this may also have happened with shell-crushing sharks eating off crowns. Crinoids started to evolve spines to thwart such predation (as the “fish-hook” crown of Dorycrinus). There has been a suggestion that there is a correlation through time between an increase in predation and the development in crinoids of anti-predator morphological structures.
We’ve been looking at camerate crinoids up to now.
Now the inadunates, which has 2 groups - the cladids (has 2 circlets of plates below the radials) and the disparids (has 1 circlet of plates below the radials).
The Burlington Limestone has species of the disparid Synbathrocrinus and Heliciocrinus (sp.?); the latter had a stem parallel to the seafloor, with arms bent upward into the water.
The Burlington also has species of the cladids Celiocrinus, Springercrinus, Hypsilocrinus, etc.
Springer (1920) - The Crinoidea Flexibilia.
The flexibles have tightly-plated, thin-plated calices and divets or grooves at the plate margins. Burlington flexibles include Taxocrinus, Wachsmuthocrinus, and Perithiocrinus.
Gahn has also looked at Burlington-aged rocks in northwestern Montana, just south of Glacier National Park, and collected nice crinoids and blastoids. Gahn also looked at Ireland, especially Hook Head, which has the same groups of crinoids.
Current research - looking at the Wassonville Formation, which occurs stratigraphically right below the Burlington Limestone in Iowa. The Burlington contains ~300 species of crinoids. Why was there such diversity then? Answering this question was the reason for looking below the Burlington at the Wassonville in southeastern Iowa. The Wassonville is the equivalent of a crinoid-rich horizon from central Iowa (the Legrande beds).
Collected Wassonville Formation crinoids from a quarry, and had sparse luck. Then, got to collect in a new quarry and found 150 crinoids in 1 day. Since, lots more crinoids have been found. There are two productive crinoid zones in the Wassonville - a lower zone near the base of the formation, and an upper zone near the middle of the formation.
The lower zone was deposited in a high-energy, near-shore environment. The upper zone was deposited in quieter water, near the lower limit of storm wave base. This deeper water environment has very finely laminated mudstones, with crinoids in it. These crinoids were deposited very rapidly, by storms. They preservation is so high that cirri are preserved intact on the stem. Also found echinoids with in-place spines. This deeper water fauna is dominated by cladids with delicate morphology. There are also some camerates and ophiuroids and asteroids.
The lower zone was a higher-energy environment, with cross-bedding and mud clasts - all storm sedimentation. There is a distinct crinoid fauna copmared with the deeper fauna. Still, the preservation is good. Crinoid fauna is dominated by cladids. The cladids from the deeper water zone are more delicate than in the shallow water environments. Also finding flexibles in the high-energy environment, but they are absent from the low-energy environment.
Southeastern Iowa - a middle shelf setting. Legrande, Iowa (central Iowa) - an inner shelf setting. The Legrande crinoids are truly phenomenal there. Species can be consistently identified based on color patterns alone (including varying coloration - dark stem & basal calyx and whitish upper calyx in one species). Legrande crinoids are on display at Beloit College, in Wisconsin.
In terms of species and generic diversity, the higher energy, middle shelf environment shows the highest diversity. The low energy middle shelf environment has less diversity, and the high energy inner shelf environment shows the lowest diversity, probably due to fluctuating temperature and salinity conditions. Can see onshore-offshore trends in abundance and diversity data for the different crinoid groups.
Crinoids with the most densely pinnulated arms are most abundant in high-energy inner shelf environments. Paired pores (interpreted as sensory) next to arm facets developed in many camerates in the Mississippian.
Belemnocrinus - previously only known from the Burlington Limestone, and was very rare there (only about 7 specimens). Now, this crinoid is known (rather commonly) in the older Wassonville Formation.
There are no encrusters on Wassonville Formation crinoids. There are a few on Burlington Limestone crinoids. One platycerid gastropod has been found on a crinoid by Gahn in all the years of collecting Burlington Limestone crinoids.
There is a specimen known of a crinoid stem wrapped around 2 blastoids, like a twisty tie.