Introduction and Definitions

Land mammal ages (abbreviated LMAs) are intervals of geologic time characterized by a distinctive set of mammals (usually genera) for a particular geographic region (Lindsay, 2003). The geographic regions for the most commonly used LMAs are entire continents, so there are different sets of LMAs for North America, South America, Australia, Asia, and Africa. A functionally similar system is used in Europe, but with a different style of terminology and names. The first set of LMAs to be established were proposed for North America by a seven-person committee of vertebrate paleontologists chaired by Horace E. Wood (Wood et al., 1941). They subdivided the Tertiary (Paleogene + Neogene) Period of the geologic time scale into 18 intervals they referred to as provincial ages. For each age, they provided a list of index taxa (genera or species known only from that age), taxa which first appeared during that age, taxa which became extinct during that age, and other taxa that were “characteristic” of that age but which were known from other ages. They also listed of all of the major fossil sites or faunas across North America that belonged to each age. The provincial ages of Wood et al. (1941) soon became known as North American Land Mammal Ages, or NALMAs. A decade later, Donald Savage (1951) named two NALMAs for the Pleistocene Epoch, a time interval not covered by the Wood Committee. Recently Barnosky et al. (2014) proposed two new NALMAs for the Holocene Epoch, but these have yet to be widely used. The NALMAs proved to be extremely useful and so the concept was implemented for other continents.

Although formally proposed in 1941, considerable work had been done in the previous decades discovering the chronologic ranges of vertebrate genera and the relative ages of major vertebrate fossil sites across North America. Among the most prominent of these studies were Osborn and Matthew (1909), Matthew (1924), Simpson (1933), and Stirton (1936). The Wood Committee synthesized the results of these and many other publications. But one of their novel ideas was in the method of naming the NALMAs. Each was given a name ending in the suffix “‑an” combined with the name of a well-known fossil fauna or a stratigraphic unit. For example, the Clarendonian NALMA was named for the Clarendon fauna from Texas, while the Barstovian NALMA was named for the Barstow Formation in California.

Conceptually, LMAs are independent of the geologic time scale, which is a world-wide, hierarchical system to subdivide time (Lindsay, 2003). However, LMAs are routinely correlated with the geological time scale; this was even done in the initial Wood et al. (1941) publication. Correlations with the geologic time scale and with LMAs from other continents are constantly being refined as new methods of numeric dating are developed and new fossil faunas are discovered.

The first complete revision of the NALMAs was published as a book in 1987 (Woodburne, 1987). Five committees composed of specialists worked on NALMAs from different parts of the Cenozoic. In addition to including many new fossil faunas discovered since 1941 and revising taxonomic names, the individual chapters of the 1987 Woodburne Volume added three significant improvements to the original Wood et al. (1941) and Savage (1951) contributions. First, they included all of the major advances in geochronology, such as K/Ar dating and magnetic polarity chronology, to provide numeric estimates for the durations of each NALMA in millions of years as well as numeric ages for the beginning and ending points for each age. This in turn allowed better correlation with the global geologic time scale. Second, they established standard subdivisions within each NALMA, so that individual fossil sites could be dated and correlated more accurately. And third, they more formally defined the beginning of each NAMLA by the first appearance of a genus in North America (or a small group of genera). Ideally, these were immigrants dispersing from Asia or South America, which then rapidly spread out over most of North America.

A second major overhaul of the NALMAs occurred in 2004 with the publication of the second Woodburne volume. It include the first formal recognition of LMAs for the late Cretaceous as well as incorporating all of the new faunas found since the first Woodburne volume and results from advances in dating methods. One very useful outcome of the second Woodburn volume was a standardize method to refer to the subintervals of NALMAs. Each NALMAs was provided with a two letter abbreviation. The subintervals are designated by a combination of this abbreviation plus a numeral. So, for example, the Hemingfordian is divided into just two subintervals, the older He1 and the younger He2. The Arikareean has four subintervals, Ar1, Ar2, Ar3, and Ar4. Another advancement in the second volume was greater coverage of North American faunas outside of the United States.

Applications of NALMAs

Despite tremendous advances in the field of geochronology over the past century, they cannot be applied to many terrestrial/freshwater fossil sites, especially in Florida. Then, in practice, LMAs are typical used to determine the relative age of a particular fossil site and to find its closest match among all other fossil sites in terms of age across the continent. Because the boundaries (beginning and ending dates) of all LMAs and most of their subintervals are now well calibrated, the method also provides the best way to determine the numeric age of a fossil site, within a range. The degree of precision generally depends on the number of well identified species found at the fossil site in question, with more species generally leading to a more precise date. Obviously finding one or more index species for a subinterval of a NALMA is ideal. But even an assemblage consisting just of long-ranging species can provide a precise date if their known interval of chronologic overlap is relatively short. For example, a fossil site producing a species whose known chronologic range is He1, He2, and Ba1 and a second species whose range is Ba1, Ba2, Cl1, and Cl2 is constrained to the Ba1 interval.

Florida, Intracontinental Correlation, and NALMAs

Because of the great distance between Florida and the closest other Tertiary fossil faunas (typically in Texas), correlation was historically not always easy. Charts in Osborn (1910) and Merriam (1917) are among the first to correlate Florida vertebrate fossil faunas with those from the rest of North America. Osborn (1910) placed what he called the “Archer” fauna (i.e., Mixson’s Bone Bed) as contemporaneous with those that would later be placed in the Hemphillian or the Blancan NALMA. Merriam (1917) regarded Mixson’s Bone Bed and the Palmetto Fauna of current usage as contemporaneous with or slightly younger than those that would later be regarded as Clarendonian. Simpson (1933) and Stirton (1936) both correlated Mixson’s Bone Bed and the Palmetto Fauna (“Bone Valley”) with Hemphillian-equivalent sites in Nebraska and Kansas, while Simpson (1933) also showed the Thomas Farm and Midway sites as contemporaneous with Nebraska faunas later placed in the Hemingfordian NALMA.

Wood et al. (1941) followed Simpson (1933) and included Mixson’s Bone Bed (“Alachua Formation”) and the Bone Valley in the Hemphillian NALMA and Thomas Farm and Midway in the Hemingfordian. No other NALMAs were recognized from Florida at this time. The first Whitneyan, Arikareean, Barstovian, Blancan, and Irvingtonian sites in Florida were recognized in the 1960s, and the first Clarendonian sites in the 1970s. Thus all NALMAs from the Whitneyan to the Rancholabrean are known from Florida, and from multiple sites for all except the Whitneyan.

The last several decades have seen an increase in the number of Tertiary vertebrate fossils sites from the eastern United States outside of Florida. For example, the Clarkforkian Williamsburg Formation site in South Carolina (Sanders, 1998); the Wasatchian Red Hot local fauna of Mississippi (Beard and Dawson, 2009); the Hemingfordian Pollack Farm fauna of Delaware (Emry and Eshelman, 1998); the Hemphillian Gray Fossil Site of Tennessee (Wallace and Wang, 2004; Hulbert et al., 2009); and the late Hemphillian or earliest Blancan Pipe Creek Sinkhole site in Indiana (Farlow et al., 2001). However, Florida still provides the most detailed and voluminous record for Neogene and Pleistocene terrestrial vertebrates in eastern North America.

Investigate the nine NALMAs found in Florida. For each NALMA, there will be a list of major faunas or localities of that age, along with lists and images of index and characteristic species.

Pliocene and Pleistocene Epochs (4.75 million years ago to 12 thousand years ago)

Rancholabrean (Ra)
Irvingtonian (Ir)
Blancan (Bl)

early Miocene- very early Pliocene Epochs (19 to 4.75 million years ago)

Hemphillian (Hh)
Clarendonian (Cl)
Barstovian (Ba)
Hemingfordian (He)

Oligocene-very early Miocene Epochs (32 to 19 million years ago)

Arikareean (Ar)
Whitneyan (Wh)

Author: Richard C. Hulbert Jr.
Orignal Publication Date: June 18, 2015
Last Updated on: August 2, 2015

References

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Beard, K. C., and M. R. Dawson. 2009. Early Wasatchian mammals of the Red Hot local fauna, uppermost Tuscahoma Formation, Lauderdale County, Mississippi. Annals of Carnegie Museum 78(3):193-243.

Emry, R. J., and R. E. Eshelman. 1998. The early Hemingfordian (early Miocene) Pollack Farm Local Fauna: First Tertiary land mammals described from Delaware. Pp. 153-174 in R. N. Benson, ed., Geology and Paleontology of the Lower Miocene Pollack Farm Fossil Site, Delaware. Delaware Geological Society, Special Publication 21.

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