Journal
PROCEEDINGS OF THE YORKSHIRE GEOLOGICAL SOCIETY
Volume 58, Issue -, Pages 227-246Publisher
GEOLOGICAL SOC PUBL HOUSE
DOI: 10.1144/pygs.58.4.300
Keywords
-
Categories
Funding
- National Science Foundation [EAR 0958372]
- Natural Sciences and Engineering Research Council of Canada
- Directorate For Geosciences
- Division Of Earth Sciences [0958308] Funding Source: National Science Foundation
- Division Of Earth Sciences
- Directorate For Geosciences [0958372] Funding Source: National Science Foundation
Ask authors/readers for more resources
Normalograptids constitute a distinctive but not easily identified group of derived axonophorans. A new study of Normalograptus morphology indicates that many Late Ordovician taxa previously identified as Normalograptus are actually referable to Styracograptus and related climacograptoids. These re-identifications have led to the recognition of a complex biogeographic history among normalograptids and their descendants, including an interesting pattern that had previously been overlooked - a late Katian disappearance of Normalograptus from the palaeotropics. Normalograptid palaeogeographic distribution in the Ordovician appears to comprise 5 main phases: (1) evolutionary origin probably in relatively high palaeolatitudes in the early to mid Darriwilian; (2) spread to global distribution in later Darriwilian time; (3) retreat from most low palaeolatitude regions during the early Katian, with perhaps a slightly delayed retreat from Laurentia; (4) a complete extirpation from the low palaeolatitudes by the mid Katian (Ea4); (5) reinvasion of the low palaeolatitudes that accompanies the remarkable ecological and evolutionary replacement of the Diplograptina by species of Normalograptus and their descendants during the latest Katian-early Hirnantian. We propose two sets of hypotheses to explain Late Ordovician Normalograptus distribution. First, given the low species diversity of Normalograptus, we suggest as a null hypothesis that the disappearance of normalograptids from low palaeolatitudes during the late Sandbian and early Katian as simply a stochastic outcome - a small clade simply went extinct by chance alone. The alternative is that the loss of Normalograptus species reflects some form of extinction for cause, either: (1) competitive exclusion; or (2) an inability to adapt to changing environmental conditions. In the case of the competitive exclusion hypothesis, the most likely competitors are species of the Diplograptina.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available