Species Longevity in North American Fossil Mammals 7 8 8 9 9 10 10 11 Donald R
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Integrative Zoology 2014; 9: 383–393 doi: 10.1111/1749-4877.12054 1 ORIGINAL ARTICLE 1 2 2 3 3 4 4 5 5 6 6 7 Species longevity in North American fossil mammals 7 8 8 9 9 10 10 11 Donald R. PROTHERO 11 12 12 13 Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, Los Angeles, CA, USA 13 14 14 15 15 16 16 17 17 18 Abstract 18 19 Species longevity in the fossil record is related to many paleoecological variables and is important to macroevo- 19 20 lutionary studies, yet there are very few reliable data on average species durations in Cenozoic fossil mammals. 20 21 Many of the online databases (such as the Paleobiology Database) use only genera of North American Cenozoic 21 22 mammals and there are severe problems because key groups (e.g. camels, oreodonts, pronghorns and probosci- 22 23 deans) have no reliable updated taxonomy, with many invalid genera and species and/or many undescribed gen- 23 24 era and species. Most of the published datasets yield species duration estimates of approximately 2.3–4.3 Myr 24 25 for larger mammals, with small mammals tending to have shorter species durations. My own compilation of 25 26 all the valid species durations in families with updated taxonomy (39 families, containing 431 genera and 998 26 27 species, averaging 2.3 species per genus) yields a mean duration of 3.21 Myr for larger mammals. This breaks 27 28 down to 4.10–4.39 Myr for artiodactyls, 3.14–3.31 Myr for perissodactyls and 2.63–2.95 Myr for carnivorous 28 29 mammals (carnivorans plus creodonts). These averages are based on a much larger, more robust dataset than 29 30 most previous estimates, so they should be more reliable for any studies that need species longevity to be accu- 30 31 rately estimated. 31 32 32 33 Key words: Cenozoic, genus, mammals, North America, species 33 34 34 35 35 36 36 37 INTRODUCTION lutionary and extinction-related characteristics. Stanley 37 38 (1975, 1979, 1981) uses species longevity in mammals 38 39 The topic of species longevity has long been of inter- versus bivalves as one of his tests of the ‘species selec- 39 40 est to paleontologists (see summary in Hopkins 2011). tion’ model of evolution. 40 41 The duration of species through geologic time has been 41 However, the comparison of species longevity to oth- 42 related to geographic range, body size, reproductive 42 er variables is often hampered by the problem of outdat- 43 rates, extinction probabilities, niche variation, intrapop- 43 ed or flawed species-level taxonomy in many groups. In 44 ulational variation, speciation rates and many other evo- 44 some cases, there is no recent reliable species-level tax- 45 45 onomy any other paleontologist can use, so comparison 46 46 to other ecological and evolutionary variables is impos- 47 47 sible. Nevertheless, paleontologists have tried to make 48 48 Correspondence: Donald R. Prothero, Department of Vertebrate such comparisons, using genera instead of species, in 49 49 Paleontology, Natural History Museum of Los Angeles County, databases such as FAUNMAP, MIOMAP, NEOMAP, as 50 50 900 Exposition Blvd., Los Angeles, CA 90007, USA. well as the Paleobiology Database (PBDB). Using gen- 51 Email: [email protected] 51 © 2013 International Society of Zoological Sciences, Institute of Zoology/ 383 Chinese Academy of Sciences and Wiley Publishing Asia Pty Ltd D. R. Prothero 1 era as a proxy for species is done as a matter of neces- 1998), oreodonts (see Stevens & Stevens 1996; Prothero 1 2 sity and by default, but there are many biologists and & Sanchez 2008), antilocaprids (see Janis & Manning 2 3 paleontologists who argue that the species is the only bi- 1998) and peccaries (Wright 1998; Prothero 2009; D. R. 3 4 ologically natural unit in the biota and regard genera as Prothero, unpubl. data). 4 5 relatively poor proxies for true species diversity (see re- These and many other important groups make up a 5 6 view of the topic in Cracraft 1989). significant part of many Cenozoic faunas and my own 6 7 It is well known that there are many higher-level taxa experience has taught me that their published taxono- 7 8 (e.g. the families of fossil mammals) that have not been my is so bad as to be positively misleading. I have per- 8 9 revised recently but potentially will undergo big chang- sonally revised other important Miocene groups, such 9 10 es in total species and generic diversity. Once they are as the Rhinocerotidae (Prothero 2005), the dromomery- 10 11 revised, the use of largely invalid generic data on these cine palaeomerycids (Prothero & Liter 2008), the blas- 11 12 poorly known groups is ‘garbage in, garbage out’. This tomerycine musk deer (Prothero 2008) and the proto- 12 13 is particularly a problem with North American Cenozoic ceratids (Prothero 1998b), and documented the effects 13 14 mammals, where some groups are grossly oversplit and of the outdated taxonomy on the generic databases now 14 15 have no valid taxonomy available but were used in the available. For the example, the PBDB recognizes only 7 15 16 compilation of MIOMAP and PBDB nevertheless. Un- North American fossil genera of peccaries but Wright’s 16 17 like the database of Eurasian Cenozoic mammals, which (1998) revision suggests that there are a large number of 17 18 has been continuously revised for many years by a vari- undescribed and unnamed genera, and my revision (D. 18 19 ety of paleontologists, the North American mammalian R. Prothero, unpubl. data) recognizes at least 18 genera. 19 20 database is still full of invalid and outdated taxonomy In contrast, Prothero and Liter (2008) found that one- 20 21 that hampers efforts to look at species-level processes or third of the dromomerycid genera and half of the spe- 21 22 to conduct analyses on the species level. cies in the PBDB were invalid. Thus, the generic-lev- 22 23 23 This anomaly is due to several factors. First, the in- el databases can severely overestimate or underestimate 24 24 credible Frick Collection of fossil mammals in the the true number of taxa if the compilers have not kept 25 25 American Museum of Natural History remained virtu- up with these rapid taxonomic changes. Those who use 26 26 ally unstudied until the 1970s (and is only now being the PBDB for Neogene mammals may not fully appre- 27 27 slowly published piecemeal). That collection is so im- ciate the inherent flaws in calculating generic diversity 28 28 mense, with so many more complete and well-docu- based on bad taxonomy. 29 29 mented fossils, that it forced a complete revision of ev- Rather than accept generic longevity as a proxy for 30 30 ery group of Mid and Late Cenozoic mammals on the the evolutionary trends in species through time, it is 31 31 continent. In many cases, a major group remained virtu- worthwhile reviewing the published literature for areas 32 32 ally unstudied for decades until work on the Frick Col- where the taxonomy of a large group (such as Cenozoic 33 33 lection produced a monographic burst (e.g. Wang 1994; mammals) is well studied with widely accepted, up-to- 34 34 Wang et al. 1999; Tedford et al. 2009, on canids; my date species taxonomy and examining whether the true 35 35 own work on rhinocerotids in 2005). In addition, many longevity of species shows any consistent patterns with- 36 36 fossil mammalian families were burdened with the work in and between taxa and whether generic longevity data 37 37 of taxonomists who are widely considered incompetent, correlate well with species durations. 38 38 from Frick’s 1937 ruminant monograph, to the Schul- 39 39 tz and Falkenbach (1968) oreodont monograph, to Os- 40 MATERIALS AND METHODS 40 born’s brontothere monograph (1929) and proboscidean 41 41 monograph (1936). Although some of these works have Species longevities for Cenozoic fossil mammals 42 42 since been revised (e.g. Mihlbachler’s 2008 brontothere were compiled from the published literature and from 43 43 monograph replaces Osborn 1929), many others have large datasets from a number of sources. For the with- 44 44 not (e.g. there is still no updated taxonomy on probosci- in-family approach, I extracted longevities of species as 45 45 deans, oreodonts, pronghorns or many of Frick’s rumi- reported in the most recent revision of a group (cited be- 46 46 nants). In the Frick Collection alone, there is an entire low), modified as necessary when I was aware of prob- 47 47 floor of fossil camels and another floor of proboscideans lematic taxa. The time scale of the species ranges fol- 48 48 that are virtually unstudied. I have personally experi- lows the individual chapters in Woodburne (2004), as 49 49 enced this problem with the current outdated state of the modified in Janis et al. (1998, 2008). The time duration 50 50 taxonomy of camels (see Prothero 1996a; Honey et al. of each species was tabulated from their actual biostra- 51 51 384 © 2013 International Society of Zoological Sciences, Institute of Zoology/ Chinese Academy of Sciences and Wiley Publishing Asia Pty Ltd Species longevity in fossil mammals 1 tigraphic range using range-through methods. As much large mammal groups whose species-level taxonomy is 1 2 as possible, I used the chronostratigraphic dating of the still insufficiently studied: Camelidae, Antilocapridae, 2 3 actual documented stratigraphic first and last occur- oreodonts, primitive carnivorans (‘Miacidae’), Xenar- 3 4 rence data for each species, rather than lumping them thra and the Proboscidea.