Department of Paleobiology, Smithsonian Institution, MRC 121, Washington, DC 20560; E-mail: email@example.com
Abstract.---Paleontologists long have argued that a major evolutionary radiation occurred during the early Cenozoic, but not that all mammals, or even all eutherians, originated from a single common ancestor at that time. Nonetheless, several recent molecular analyses claim to show that because several interordinal splits occurred during the Cretaceous, a radiation of therian mammals was then underway. These claims confuse basal splits with "radiations," employ exaggerated and unreliable molecular clock rates, and ignore the well-sampled late Cretaceous and Cenozoic North American fossil record. Evolutionary radiations only may involve changes through time in the number of species or the distribution of morphological (or other) attributes across these species. Statistical analyses of paleofaunal data confirm that the number of mammalian species was far lower throughout the late Cretaceous than during any interval of the Cenozoic, and that a massive diversification took place during the early Paleocene, immediately after a major mass extinction. Additional measurement data illustrate similar trends through time in the distribution of body mass, the most ecologically important morphological character of mammals. Cretaceous mammals were on average small and occupied a narrow range of body sizes; after the Cretaceous-Tertiary mass extinction, there was a rapid shift in the mean that overshadows the entire history of size increases during the rest of the Cenozoic. The fact that there was an early Cenozoic mammalian radiation is a firm statistical inference that is entirely compatible with the existence of a few modern mammal orders during the Cretaceous.
..........Over the past few years, there has been an explosion of interest in the early evolutionary radiation of mammals. Traditional scenarios based mostly on paleontological data have been challenged by inferences based on the calibration of molecular phylogenies to numerical time (Hedges et al., 1996; Cooper and Fortey, 1998; Janke et al., 1997; Kumar and Hedges, 1998; Springer, 1997). These new molecular studies purport not just to overthrow traditional higher-order phylogenetic groupings, but to show that a major diversification of therian mammals began much earlier than was previously thought, perhaps even in the early Cretaceous.
..........Several of these studies have been rebutted by molecular systematists on the grounds that the novel topologies are incorrect (e.g., D'Erchia et al., 1996; Sullivan and Swofford, 1997). Most others suffer from using just one or two "clock" calibration points (Cooper and Penny, 1997; Hedges et al., 1996; Janke et al., 1997; Kumar and Hedges, 1998). These investigators are willing to dismiss any number of conflicting, paleontologically-inferred dates of origin as being too young. Hedges et al. (1996) justify this approach with a non sequitur -- that there is a "long time span between the earliest [mammalian] fossils... and the first appearance of the modern orders." On the contrary, their data suggest that their single, Carboniferous calibration point is irrelevant because there was a speed-up in the clock between then and the Cretaceous.
..........Even the better studies have crucial flaws. Despite employing multiple calibration points and correcting for variation in the clock speed, Springer (1997) arrived at a clock rate with 95% confidence limits of +/- 13% ("XR adjusted") or +/- 15% ("MRR adjusted"). Furthermore, all but one of the points fell within the Cenozoic, forcing the interordinal divergence times to be based largely on extrapolation instead of interpolation. This one Cretaceous point was an estimate of 130 MYA for the marsupial-placental split. However, the data given by his source (Novacek, 1993) only demanded a split by 98 MYA (as confirmed by Cifelli et al., 1997). Changing to the 98 MYA estimate increases the clock rate, and therefore decreases all the estimated divergences, by 12%.
..........Despite all this, my purpose is not to challenge the inference that many therian orders diverged in the Cretaceous. Instead, I will make three simple points. First, the paleontological literature never has implied that all therian or even eutherian orders diverged after the Cretaceous. Any claim to the contrary is a misinterpretation that makes molecular results seem more novel than they really are. Second, molecular studies have failed to define the idea of "radiation" or "diversification" in a rigorous manner, leading to inferences from data that are not really relevant to the issue. Finally, clearer definitions imply that only two biological patterns are of interest in this discussion: changes through time in the overall number of species, and changes through time in the distribution of morphologies (or other attributes) across those species. The fossil record does provide clear-cut evidence regarding both of these patterns. It shows that Cretaceous mammals were taxonomically depauperate and morphologically uniform, and that the most important radiation of therian mammals in their history did occur in the earliest Paleocene. The bulk of this paper will be devoted to these latter, empirical issues.
..........FOSSILS VS. MOLECULES
..........Cretaceous Splits among Mammal Orders: Do Fossils and Molecules Really Disagree?
..........Apart from possible lipotyphlan insectivores (Novacek, 1993), no representatives of a modern therian order have ever been clearly shown to occur in the Cretaceous. Claims of Cretaceous xenarthrans were based on misidentified multituberculates (Krause, 1993). Claims of Cretaceous primates were based on earliest Paleocene finds of plesiadapiforms that occur together with reworked Cretaceous mammals (Lofgren, 1995); the affinity of plesiadapiforms and primates is contentious and the two are at best sister groups (Beard, 1993). Claims of Cretaceous marsupials in the broad sense are valid, the relevant fossils having been known since the 19th century; but the Cretaceous forms have no clear-cut affinities with the modern, ordinal-level marsupial groupings (Johanson, 1996). Claims of Cretaceous "ungulates" are irrelevant because even if ungulates are monophyletic, they are a clade of at least six orders, not a single order (Archibald, 1996).
..........Nonetheless, traditional, paleontologically-based phylogenies imply that some basal splits among therians did take place during the Cretaceous. The eutherian-metatherian split dates to at least 98 MYA (Cifelli et al., 1997). The Carnivora (Fox and Youzwyshyn, 1994) and Mesonychia (the probable paraphyletic sister group of the Cetacea: Thewissen, 1994) both appear in the earliest Paleocene. If "archonta" and "ungulata" are truly clades, they too definitely occur at this time (Novacek, 1993). The sister grouping of the Lagomorpha and Rodentia is well-established, and this superordinal grouping already existed during the late Cretaceous (Meng et al., 1994). Several depauperate, rarely fossilized orders (e.g., Tubulidentata, Pholidota, Macroscelida, Scandentia, and Dermoptera) might have originated in the Cretaceous. None of these conclusions are contentious among paleontologists, as indicated by the very liberal times of origin suggested by the phylogeny of Novacek (1993). Together, this suggests that at least seven or eight, and probably 10 or 20 therian lineages do date back to the Cretaceous.
..........In fact, with appropriate corrections for the previously-mentioned error in calibration, the most methodologically sound analysis (Springer, 1997) implies that only five eutherian orders had split from their sister groups by the K-T (Cretaceous-Tertiary) boundary (65 MYA): Xenarthra, Insectivora, Primates (the only member of "archonta" included in that study), Lagomorpha, and Rodentia. So far from overturning the traditional view, molecular studies confirm what the fossil record already has suggested: not just these five orders, but several others do date back to the Cretaceous.
..........This remarkable lack of conflict raises a key question: what exactly is the problem supposedly addressed by the latest molecular studies? Not only do all parties agree that several basal splits occurred during the Cretaceous, but mammalian paleontologists have shown little interest in the question of when exactly the modern mammal orders diverged. Instead, Simpson (1952), Lillegraven (1972), Savage and Russell (1983), and Stucky (1990) all focused on the pace of taxonomic diversification. These studies directly counted orders, families, and genera in different time intervals, living or not. Meanwhile, those paleontologists who work on the early evolution of mammal orders have tended to focus on documenting morphological transitions instead of exact dates of origin (e.g., Meng et al., 1994; Thewissen, 1994).
..........The real problem is that the traditional paleontological concern with taxonomic and morphological diversity simply has not been addressed by the molecular studies. The number of species that were present at different times can in theory be inferred from comprehensive molecular phylogenies (e.g., Nee et al., 1992, 1994). But this requires either sampling all living species, or at least all living lineages that are believed to extend beyond a certain point in time. None of the molecular analyses of mammalian diversification have made any effort to guarantee this kind of comprehensive sampling. Therefore, it is impossible to conclude from these studies whether the rate of diversification was the same or different during the Cretaceous and Cenozoic.
..........Molecular workers have granted that phylogenetic topologies may say little or nothing about the timing of the major morphological transitions that distinguish living orders (e.g., Cooper and Fortey, 1998). They even have used this argument to suggest that paleontologists have failed to recognize a greater diversity of surviving lineages in the Cretaceous because these Cretaceous mammals had not yet evolved the diagnostic morphological features of their living descendants. Indeed, all Cretaceous mammals were terrestrial and ecologically generalized, and as I will show all of them occupied a narrow range of the size spectrum. But the important point is not that this excuses the mismatch between paleontological and molecular data, because it doesn't. Instead, it shows that molecular workers agree that the fossil record is the best means of documenting morphological radiations.
..........If they are not directly addressing taxonomic or morphological diversity and if their data agree with the fossil record whenever they are analyzed properly, what is the evolutionary import of molecular clock studies? I would suggest that they have little to say about the theoretical problem of evolutionary radiations, and instead are of interest mostly to mammalogists who want to know when particular mammalian clades originated. The difficulty of finding relevance for evolutionary theory is demonstrated by some of the contradictions to be found in the molecular literature: Cooper and Penny (1997) claim that there were "incremental changes during a Cretaceous diversification of birds and mammals rather than an explosive radiation in the Early Tertiary," but Cooper and Fortey (1998) declare that "the explosive phases of evolution so amply demonstrated by the fossil record may, in many cases, have been preceded by an extended period of inconspicuous innovation." What is the meaning of an "innovation" that has no consequence for a group's observed taxonomic and morphological diversity?
..........The most important features of a large evolutionary radiation that could be inferred from a phylogeny would be: 1) the number and timing of evolutionary divergences and the tempo of taxonomic diversification they imply; and 2) the distribution of such attributes as morphology, physiology, behavior, and biogeography across a phylogeny, which might imply the tempo of ecological diversification. Although neither of these general issues have yet been addressed directly by molecular studies of the mammalian radiation, both of them may be addressed by the fossil record. Here I will reanalyze augmented versions of two data sets to show exactly what the differences were between Cretaceous and Cenozoic mammal faunas in North America. Most of the issues regarding data preparation have been discussed elsewhere (Alroy, 1992, 1994, 1996, 1998), so I will focus instead on new analyses and results. I first will treat the problem of taxonomic diversification, and then discuss morphological evolution in terms of body mass distributions.
..........The faunal data used here are an extension of a previously discussed compilation of 4015 North American mammalian fossil localities ranging in age from Campanian (about 84 MYA) to late Pleistocene (about 0.1 MYA). Because most of these localities pertain to a single quarry or a small outcrop, they serve as paleontological "snapshots" that each represent a short period of time in a restricted geographical area. Each locality is documented by a taxonomically standardized, species-level faunal list, and whenever possible is placed in a local stratigraphic section. Instead of relying upon a traditional time scale, the faunal lists are subjected to a multivariate ordination that is constrained by the stratigraphy and governed by a parsimony criterion, i.e., that of minimizing the number of temporal overlaps between species and/or genera. The arrangement of lists implies a relative "event sequence" of taxonomic first and last appearances that is numbered from oldest to youngest and calibrated to numerical time using geochronological age estimates (Alroy, 1992, 1994, 1996). The current version of the data set includes 186 stratigraphic sections, 1196 genera, 3181 species, and 152 geochronological calibration points. As previously, the statistical analyses focus only on the relatively well-sampled western region of the United States and Canada.
..........The calibrated event sequence can be used directly to infer counts of the species that existed at any of several arbitrary, evenly spaced moments in geological time. These counts would constitute a diversity curve for the entire 84 MY interval. Furthermore, counts of species appearing or disappearing between sampling points can be used to estimate speciation and extinction rates. Prior to doing this, however, a key problem must be solved: each interval is represented by a different number of fossils, as shown by variation in the number of faunal lists per MY (Alroy, 1996). These variations are exactly the "obvious deficiencies" that make a "literal reading of the fossil record" so dangerous (Cooper and Fortey, 1998).
..........Far from being intractable, however, this bias may be removed. The best method is to standardize sampling in each interval by drawing faunal lists at random until reaching a pre-defined limit. This limit is set by the total number of faunal lists that are drawn; the cutoff is made as high as possible given that all, or nearly all of the intervals should be able to make it. After drawing lists in each interval and recomputing the temporal durations of taxa based on these lists, the procedure is iterated 100 times to yield average diversity and turnover rate data.
..........In this paper the subsampling procedure keeps track of the number of faunal lists, instead of taxonomic records, that are drawn in each trial. This is because alpha (within-locality) diversity is higher during the Cenozoic than during the Cretaceous. Therefore, sampling the same number of fossils is likely to yield a larger number of distinct taxonomic records. Conversely, 10 records in the Cenozoic are likely to represent far fewer actual fossils than 10 records in the Cretaceous; but 10 lists are likely to represent about the same number of fossils.
..........In a previous analysis using an earlier version of the data set, I analyzed the Cenozoic data only and separated the diversity counts by 1.0 MY (Alroy, 1996). Because the calibration of the time scale is poorer in the late Cretaceous, this study uses a longer bin size 2.5 MY. The latest Cretaceous bin and all of the Cenozoic bins were able to meet a standardized sampling cutoff of 50 lists per bin (20 per MY). In the earlier study, I set a cutoff of 85 taxonomic records per 1.0 MY. Because each Cenozoic list averages about 6.8 taxonomic records, this was equivalent to only 12.5 lists per MY.
..........This study's relatively intense sampling is likely to highlight any differences between the Cretaceous and Cenozoic. Unfortunately, even though the 50-list sampling level is adequate for the last temporal bin of the Cretaceous, it is not for the preceding stretch of time going back to the 82.5--80 MYA bin. This, however, should have little effect on the results because: 1) my discussion will focus on contrasts between the last, fully sampled Cretaceous interval and the Cenozoic; 2) all available lists from all of the Cretaceous intervals before the last one will be sampled; and 3) computing full temporal ranges for the taxa ("ranging through") will extend ranges into this ultimate Cretaceous interval for some taxa that were present but not directly sampled.
..........The diversity data (Fig. 1) establish three key patterns. First, regional standing diversity was much lower during the Cretaceous than at any point during the Cenozoic (Fig. 1A). For example, standing diversity was about 27 species per 50 faunal lists across the time plane at 67.5 Ma, but this figure was never lower than 37 after 65 MYA, and averaged 73 across the 25 Cenozoic time planes.
..........Second, there was an abrupt transition between the two diversity levels. Diversity surged immediately after the K-T boundary, reached a plateau by about 55 MYA, and then fluctuated dynamically within relatively narrow limits. The lack of any true net diversification after this point can be shown in a simple way by analyzing the 22 data points from 55 MYA on, which show no significant correlation between time and standing diversity (Spearman's rank-order correlation r s = -0.259; t = 1.200; n.s.).
..........The S-shaped pattern seen in this semi-log plot is not consistent with either a simple exponential growth model, which predicts a linear curve, or a simple logistic model, which predicts an asymptotic curve. Nor is it an artifact of poor sampling during most of the Cretaceous: better sampling would only raise the first several data points relative to the fully-sampled, but very low 67.5--65 MYA data point, which would make the subsequent Paleocene diversification seem even more dramatic. Further evidence for a dynamic Cenozoic equilibrium has been presented previously (Alroy, 1996); this study's additional data suggest that distinct Cretaceous and Cenozoic equlibria were offset by the Paleocene radiation.
..........Finally, both origination and extinction rates increased dramatically around the K-T boundary: there were 0.85 extinctions/species/2.5 MY just before 65 MYA, and 2.19 originations/species/2.5 MY just afterwards. Both curves remained high during the Paleocene (roughly 65--55 MYA); the three relevant data points average 0.71 extinctions and 0.80 originations/species/2.5 MY. However, this does little to obscure the singular nature of the K-T event.
..........There are many details here that could be discussed. For example, there is weak evidence that Cretaceous turnover rates were on average lower than Cenozoic turnover rates. There also is significant evidence that origination rates are negatively correlated with standing diversity levels. This mechanism creates the Cenozoic's dynamic equilibrium and partially explains why origination rates are much more variable than extinction rates. Finally, the major Cenozoic North American mammal orders had different diversity trajectories, suggesting that they were obeying different dynamic rules (Alroy, 1996).
|Fig. 1 North American mammalian diversity, origination (new appearance) rates, and extinction rates through the late Cretaceous and Cenozoic. Data are based on multivariate ordination and standardized sampling of faunal lists. (a) Standing diversity. Y-axis is logged to show the lack of either a log-linear (exponential) or asymptotic (simple logistic) pattern; instead, an offset between two logistic curves at about 65 MYA is indicated. (b) Origination rates. (c) Extinction rates.|
..........If taxonomic diversity was lower during the Cretaceous than during the Cenozoic, and if this transition was rapid and confined mostly to the early Paleocene, then the only remaining arena for a possible "Cretaceous radiation" would have to be ecology. The best way to capture ecological variation among fossil forms is to measure morphological disparity (sensu Foote, 1993), but this is hard to measure across the entire range of mammalian orders because of the great anatomical differences among these groups. For example, the only cheek tooth that is found in every toothed mammal is the first lower molar. Thus, it would be nearly impossible to construct a disparity measure for all groups of mammals based on homologous morphological features of cheek teeth (but see Jernvall et al., 1996, for "ungulates"). Worse, these very cheek teeth are the only easily preserved and identified parts of the mammalian skeleton; constructing morphospaces for, say, the post-cranium would not be feasible in a general analysis of all fossil mammals (but see Janis and Wilhelm, 1993, for large mammals).
..........Fortunately, there is one very easily quantified morphological feature of overwhelming ecological importance for mammals: body mass, which is correlated strongly not just with every linear measurement of the mammalian skeleton, but with dietary, locomotor, and life history variables. Despite important residual variation in these features, body mass distributions do capture a considerable amount of ecological information (Legendre, 1986). In this section I will outline late Cretaceous and Cenozoic trends in body mass distributions, showing again that a dramatic ecological transition did occur at the K-T boundary.
..........The raw data discussed here have been described previously (Alroy, 1998), so I will omit many details of data preparation. The latest version of the data set consists of body mass estimates for 1543 North American fossil mammal species that are based on a compilation of published measurements for 15,035 individual lower first molars (m1s). Separate regression equations are available for each of the major mammalian orders (e.g., Damuth, 1990; Legendre, 1986), and for the others I used a generic all-mammal equation (Legendre, 1986). All of these equations have very high r-squared values; most of them relate the log of m1 length times width to the log of body mass, although for ungulates the log of m1 length was used as the independent variable (see Damuth, 1990). Although no account is taken of such factors as sexual dimorphism, geographic variation, or within-species anagenetic change, these are all inconsequential in light of the study's shrew-to-mammoth size range.
..........The data were used to compute body mass distributions for each of 84 1.0 MY-long temporal bins. The age-range data that were used to compute presence and absence in bins were based on the same multivariate ordination of faunal lists described earlier. Species were considered to be present in a bin if they ranged anywhere into it, which does occasionally lump together species that never actually co-existed. The relatively short bin length largely avoids this problem, but the 66--65 MYA intervals' data average classic latest Cretaceous faunas and a few small, earliest Paleocene ("Pu0") faunas.
..........Trends in the size distribution were quantified in two ways. First, I computed the mean body mass across all species in each bin (Fig. 2A), which is important because there is a strong trend toward size increase (Alroy, 1998). Second, I computed the standard deviation of the same body mass values (Fig. 2B), which is important because it is a direct measure of disparity (this term is typically equated with such measures of morphological variation as the range, variance, or standard deviation: Foote, 1993).
..........Some caveats are in order. First, it is possible to compute all of these statistics separately for individual fossil localities, which would avoid the problem of lumping species together in a temporal bin. However, preliminary results indicate that after correcting these locality-specific data for sampling effects, one would arrive at almost exactly the same patterns that are seen in the lumped data. Second, additional features of the distribution also might be quantified, including the skewness and kurtosis. However, these statistics are noisy and add little to the key conclusions. Finally, these trends apparently are governed by a complex, non-random dynamic operating within evolutionary lineages (Alroy, 1998). However, I will constrain my discussion to the pattern itself, instead of the underlying evolutionary process, because this is sufficient to show that a Paleocene radiation occurred.
..........The body mass curves (Fig. 2) show four clear-cut patterns: 1) Cretaceous mammals were small and occupied a very narrow range of the size spectrum (67--66 MYA bin: n = 17, mean mass 4.61 ln g, standard deviation 1.95 ln g); 2) there was little change in the Cretaceous fauna during a period of > 10 MY (77--76 MYA bin: n = 15, mass 4.04 +/- 1.32 ln g, i.e., about the size of an elephant shrew); 3) there was an abrupt shift in the mean, but not the standard deviation, at the K-T boundary, which resulted from the sudden extinction of many small mammals and the addition of many medium-sized mammals (65--64 MYA bin: n = 34, mass 6.96 +/- 2.05 ln g); and 4) there was a steady expansion in the size range throughout the Cenozoic; mean body mass was forced to track this trend because the lower limit to size was static (see Fig. 1 in Alroy, 1998).
..........Was the K-T shift a true evolutionary event, or was it merely the side-effect of immigration (specifically from Eurasia) in the wake of a major mass extinction? For example, mean mass is 4.30 ln g at Flat Creek 5 (latest Cretaceous), but 5.57 ln g in the apparently earliest Paleocene component of the Bug Creek Anthills fauna (see Lofgren, 1995). A few of the larger Bug Creek species are clearly immigrants (e.g., Catopsalis joyneri, Stygimys kuszmauli , and at least one of three ungulates: Archibald, 1982, 1993; Archibald and Bryant, 1990). However, some in situ speciation may already have occurred at this point, and the mean mass was still 1.4 ln g short of the average for the whole 65--64 MYA interval. Given that this later shift was due to a rapid, in situ radiation, the overall transition clearly was less an effect of immigration than of trends within lineages and differential speciation of large forms. In any case, the earliest Paleocene shift certainly had a more important long-term effect than other other sudden transition during the Cenozoic: by the 1--0 MYA interval mean mass had increased by only another 1.17 ln g.
|Fig. 1 Trends through time in North American mammalian body mass distributions. All species falling into each 1.0 MY-long bin are considered. (a) Mean body mass. (b) Standard deviation of body mass.|
..........One possible criticism of the results is that they pertain to just a single continent. After all, only the North American record is relatively complete and well-studied through both the late Cretaceous and all of the Cenozoic. Thus, one could argue that molecular studies suggest that a significant Cretaceous radiation happened to take place elsewhere. But even apart from the fact that the best molecular clock data imply no such radiation, it is not true that the bulk of living eutherian diversity has its deep roots outside of North America. Basal members of the Carnivora, Insectivora, Primates, Artiodactyla, Perissodactyla all appear in North America during the early Cenozoic. Basal lagomorphs, rodents, and cetaceans do not, but on the other hand the fossil record demonstrates that these groups diverged in Asia during the Paleocene, not the Cretaceous, and that some major lineages within these orders (e.g., Geomyoidea; Sciuromorpha) did originate in North America. Of course there are many minor groups that do seem to have originated in times and places where the fossil record is not strong. But it simply is not reasonable to suggest that North America witnessed less than its fair share of mammalian evolution. Instead, North America not only can, but should serve as our baseline for understanding mammalian evolutionary radiations.
..........If we accept this point, there are few ways to avoid this study's major conclusions: both in terms of taxonomic diversity and body mass distributions, the single most important radiation of mammals occurred not in the Cretaceous, but in the earliest Cenozoic. Far from being a "literal reading of the fossil record," these are statistically robust results based on standardized sampling regimes. They show the folly of denying the Paleocene radiation on the basis of loosely calibrated molecular clocks. A more fruitful line of inquiry would be to take advantage of this obviously important event by exploring its impact on molecular evolution. For example, if the extraordinary early Paleocene burst of speciation and morphological evolution was correlated with intense selection at the molecular level, that might have created a simultaneous speed-up in the molecular clock across all early Paleocene lineages. In turn, such an effect might account for some of the much-touted, but virtually inconsequential discrepancies between molecular and paleontological data.
..........This research was supported by the Evolution of Terrestrial Ecosystems program at the Smithsonian. I thank G. Eble, J. Hunter, P. Wagner, and P. Wilf for comments on the manuscript, and J. Archibald and P. Waddell for helpful discussions.
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