Wednesday, March 23, 2011

Intraspecific variation in Forresteria

Forresteria alluaudi (Boule, Lemoine & Thevenin) 1907, from depressed, evolute, spinose to compressed, involute, almost smooth, a nice range of intraspecific variation. Is it any wonder why these, and some like these, were placed in 4 separate species in 3 separate subgenera (Reeside, 1932), Barroisiceras (Forresteria) forresteri, B. (F.) stantoni, B. (Alstadenites) sevierense, and B. (Harleites) castellense. Kennedy, Wright and Klinger (1983) put them all in synonymy with F. alluaudi.

Many ammonoids show intraspecific variation, including, for example, Placenticeras from the Cretaceous and, at least in my mind and experience, Anasibirites and Wasatchites from the Early Triassic. Found in the same bed or group of beds worldwide, to see one would tell you precisely what age the rock they were in is. Unlike the Triassic example, where there are differences in ornamentation and shape from one part of the world to another (maybe some kind of latitudinal or provincial thing), F. alluaudi is basically the same, that is different, wherever found.

Forresteria was named after the late Robert Forrester of Salt Lake City, Utah, who found the type material in southern Castle Valley, eastern Sevier County, Utah.

7 comments:

  1. Can you tell me how to determine the evolute or involute of ammonoid base on U/D?

    Best regards,
    Ahmad Rosli

    ReplyDelete
  2. U/D is the umbilicus diameter divided by the shell diameter, this gives a ratio of the umbilicus to the diameter. In the photo above, the specimen upper left the umbilicus is 38% of the diameter, evolute, the specimen lower right the umbilicus is 11% of the diameter, involute.

    I hope this helps.

    A nice paper that may help is; Korn, D. (2010), A key for the description of Palaeozoic ammonoids. Fossil Record, 13: 5–12.

    http://onlinelibrary.wiley.com/doi/10.1002/mmng.200900008/abstract

    ReplyDelete
  3. What parameter did we use? Is U/D > 30% is considered evolute and U/D < 30% is considered involute?

    anyway thank for reply!

    Ahmad Rosli
    West Malaysia

    ReplyDelete
  4. From Korn 2010

    0.00-0.15 Involute
    0.15-0.30 Sub-Involute
    0.30-0.45 Sub-Evolute
    0.45-0.60 Evolute
    0.60-0.75 Very Evolute

    ReplyDelete
  5. Thank you for the explaination, very helpful for my study!

    AHMAD ROSLI
    West Malaysia

    ReplyDelete
  6. Did you know about Spiral Growth Rate (SGR%)?

    ReplyDelete
  7. No, I don't know about the SGR%. Would it be like the WER (Whorl Expansion Rate) of Korn 2010?

    ReplyDelete