I like the idea of chronofaunas, this concept that the primary components of a terrestrial or marine community stay in some form of stasis for a period of time (millions of years) before being replaced by another community. One's ability to detect these seems to be based partially on what taxonomic sampling bin you use to dissect the community that is present at the time bins you establish.
For example, a previous study by Alroy (2004) looking for mammalian chronofaunas during the Cenozoic only found three chronofaunas. Although the Cenozoic is a long period of time (being the past 65 million years, or all the time since the non-avian dinosaurs went extinct), it seems tough to imagine that only three communities of mammals have been around. However, at the ordinal level, this does make some sense: the orders of mammals that were around during the Paleocene were almost all replaced during the Eocene (thus a Paleocene fauna); the Eocene saw the introduction of orders of mammals (artiodactyls and perissodactyls) which continued to be present until near- or true-modern times (thus a Modern fauna); while some of these introduced Eocene orders (primates) were only present during the Eocene (thus an Eocene fauna).
However, the chronofaunas established by North American mammalian palaeontologists (as were summarized by Webb and Opdyke [1995] and various authors of the Janis et al. (1998) volume) are usually based on a smaller taxonomic 'comb' than the Alroy (2004) analysis, so they usually find more chronofaunas. The Janis et al. (1998) authors for the carnivorous mammals (Carnivora + Creodonta) and Artiodactyla review chapters listed six chronofaunas which they were able to find. Their names, their first and last biochronological unit, and the time they represented according to the biochronology of Janis et al. (1998) are as follows:
- Paleocene: Puercan to Clarkforkian (65 to 55.5 Ma)
- Early to Middle Eocene: Wasatchian to Uintan (55.5 to 39.5 Ma)
- White River: Duchesnean to late early Arikareean (39.5 to 23 Ma)
Runningwater: early late Arikareean to early Hemingfordian (23 Ma to 17.5 Ma)
Clarendonian: late Hemingfordian to Clarendonian (17.5 to 8 Ma)
- Mio-Pliocene: Hemphillian to Blancan (8 to 1.8 Ma)
Most of these chronofaunas make sense. To focus on the Paleogene (I work on it more than the Neogene, sorry), the pre-PETM Paleocene terrestrial mammal assemblage is a fairly distinct ecological community from what replaces it after the PETM. The (probable) immigration of perissodactyls, primates, and artiodactyls makes the Early Eocene very faunally distinct from the Late Paleocene. An immigration event occurs near the Uintan-Duchesnean boundary, making that boundary (and the subsequent Duchesnean) one of the most complex NALMAs to interpret, but the communities found in the late Uintan and early Duchesnean are distinct from one another, as in the latter the stereotypical White River fauna begins to dominate North America's terrestrial ecosystems.
Figueirido et al. (2012) happen upon the same answer for the number of chronofaunas as the Janis volumes, but they do with a stricter statistical correlation of times and taxa then previous workers had done. Their chronofaunas are defined as such: communities of subfamilies which coexist together and are brought into North America's fossil record by immigration or climatic events and then 'motivated' to fade away by climatic events. These chronofaunas are similar, but not identical, to the six discussed above. Their manner of distinguishing the chronofaunas uses a factor loading performance of the subfamilies to determine when a subfamily (or group of subfamilies) is applying the most 'force' to the community: when the forcing is positive, the subfamilies are diverse and abundant, when the forcing is negative the subfamilies are (potentially) still present but in reduced in diversity, abundance, or both.
This lead me to a slight problem of trying to figure out how to compare their chronofaunas with those of Webb and Opdyke (1995) and the Janis et al. (1998) volume. One change is in biochronology: much work has been done since 1998 to better orientate North American Cenozoic terrestrial researchers in what time it is, so boundaries between NALMAs have moved around slightly, and NALMAs have been split more precisely into smaller bins (see Woodburne [2004] for more information). In addition, the previous work had a simultaneous stop point for one chronofauna and start point for the next. This is not actually how animals work in the fossil record: there is instead a lead-in time for taxa to increase in abundance (and for their possible competitors to decline in abundance), so we should expect a chronofauna's elements to be present before and after it fades from dominance.
So it is possible to list the chronofaunas by when they start and stop to have a major effect on the biota of North America, and then place their time as being the midpoint of a biochron (obviously this second point is not the intent of the original authors):
- Paleocene: Puercan 1 to Wasatchian 7 (65 to 51.2 Ma)
- Early-Middle Eocene: Torrejonian 2 to Uintan 3 (61.9 to 41.4 Ma)
- Middle-Late Eocene: Wasatchian 7 to Whitneyan (51.2 to 31 Ma)
- Oligocene: Uintan 3 to Hemingfordian 2 (41.4 to 16.7 Ma)
- Miocene: Whitneyan to Blancan (31 to 3.4 Ma)
- Pliocene: Hemingfordian 2 to present (16.7 to 0 Ma)
This kind of listing seems to be not particularly useful, since all of the chronofauna show overlap. If we cut these down to when a particular chronofauna is the only dominant fauna, we get a better effect of cutting the Cenozoic into faunal communities (although once again cutting a biochron in half; not what the original authors intended):
- Paleocene: Puercan to Wasatchian 7 (65 to 51.2 Ma)
- Early-Middle Eocene: Wasatchian 7 to Uintan 3 (51.2 to 41.4 Ma)
- Middle-Late Eocene: Uintan 3 to Whitneyan (41.4 to 31 Ma)
- Oligocene: Whitneyan to Hemingfordian 2 (31 to 16.7 Ma)
- Miocene: Hemingfordian 2 to Blancan (16.7 to 3.4 Ma)
- Pliocene: Blancan to present (3.4 to 0 Ma)
These clear-cut divides more closely emulate the intention of the chronofaunas used by Webb and Opdyke (1995) to serve as bins with distinct boundaries between mammalian communities. However, I don't like em'. They're too clear cut and they ignore the concept discussed above wherein fauna take a while to establish themselves. Taking the first figure of Figueirido et al. (2012) and using a 0.5 factor loading as indicating when the community both begins and stops being the dominant fauna, and then assigning whole biochronological bins, I came up with the following estimates on when the chronofauna-identifying subfamilies were a major, if not dominant, component to the community.:
- Paleocene: Puercan 1 to Wasatchian 5 (65 to 53.0 Ma)
- Early-Middle Eocene: Wasatchian 3 to Uintan 2 (54.4 to ~44 Ma)
- Middle-Late Eocene: Bridgerian 3 to Orellan 2 (47 to 33 Ma)
Oligocene: (very late) Duchesnean to Arikareean 3 (37 to ~21 Ma)
Miocene: Arikareean 3 to Clarendonian 3 (23 to 9 Ma)
- Pliocene: Barstovian 2 to modern (13 to 0 Ma)
Is this division better than the previous division? I'm not sure, but I think it's worth looking at both of them to get a better understanding on what these chronofaunas mean for a biochronologically-concerned North American terrestrial mammalian palaeontologist.
Alroy J. 2004. Are Sepkoski's evolutionary faunas dynamically coherent? Evolutionary Ecology Research 6:1-32. Full text (pdf) available to EER subscribers here.
Figueirido B, Janis CM, P
érez-Claros JA, De Renzi M, Palmqvist P. 2012. Cenozoic climate change influences mammalian evolutionary dynamics. Proceedings of the National Academy of Sciences of the United States of America 109(3):722-727. Abstract and full text available to PNAS subscribers
here. DOI:
10.1073/pnas.1110246108
Janis CM, Baskin JA, Berta A, Flynn JJ, Gunnell GF, Hunt Jr. RM, Martin LD, Munthe K. 1998. Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Janis CM, Scott KM, Jacobs LL, editors. New York (NY): Cambridge University Press. 4, Carnivorous mammals, 73-90.
Janis CM, Effinger JA, Harrison JA, Honey JG, Kron DG, Lander EB, Manning E, Prothero DR, Stevens MS, Stucky RK, Webb SD, Wright DB. 1998. Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Janis CM, Scott KM, Jacobs LL, editors. New York (NY): Cambridge University Press. 22, Artiodactyla, 337-357.
Webb SD, Opdyke ND. 1995. Effects of Past Global Change on Life. Stanley SM, editor. Washington DC: The National Academies Press. 11, Global climatic influence on Cenozoic land mammal faunas, 184-208.
Woodburne MO, editor. 2004. Late Cretaceous and Cenozoic Mammals of North America: Biostratigraphy and Geochronology. New York (NY): Columbia University Press.
