Paleontological Samples Research Articles

Trace elements and their isotopes in bones and teeth: Diet, environments, diagenesis, and dating of archeological and paleontological samples

Fossil biogenic apatites display trace element (TE) compositions that can record environmental and biological signals, give insights into past water compositions, or be used for dating paleontological and archeological bones and teeth. Processes of TE incorporation into apatites of skeletal phosphatic tissues are described, ranging from those active in living organisms to those active during diagenesis. Abiotic incorporation mechanisms have been modeled theoretically and experimentally and include crystallographic controls on TE partitioning coefficients, inorganic surface adsorption and adsorption mediated through chelation and diffusion–adsorption processes, each leading to specific fractionation patterns. Predictions from models and experiments have been tested against TE concentration and isotopic composition data on remains of contemporary wild or raised animals and on fossils of various ages and environments. In living organisms, TEs incorporated in apatite are separated in two categories, non-essential elements whose concentration is compared to that of an essential element with similar behavior (e.g. Sr/Ca) to reconstruct trophic chains, and essential elements whose isotopic ratios are used to trace metabolic activity, diet, etc. In fossils, elements are incorporated during diagenesis, such as rare earth elements (REEs), and trace diagenetic processes ranging from very early quantitative adsorption likely mediated by chelators, protracted diagenesis through inorganic adsorption and fractionation through diffusion–adsorption, to low-grade metamorphism associated with intense redistribution under crystal-chemical control. These different steps are also identified as steps of radionuclide incorporation. Only samples where the earliest steps can be deciphered are useful for determining stratigraphic and archeological ages. Other samples will date late diagenetic events that have obliterated the paleoenvironmental signals, but may be of geological significance, e.g. tectonic events.

Anisian Dasycladales from Upper Silesia and adjacent regions

Anisian Dasycladales (calcareous algae) from the Diplopora Dolomite of the Upper Silesia and adjacent regions of S Poland are revised. All previously reported taxa are critically reviewed and illustrated. New paleontological samples were collected from 74 outcrops and from 45 boreholes. The abundant material includes both specimens visible on fractured rock surfaces and thin-sectioned ones; 24 species of Dasycladales are identified, including three new species: Oligoporella chrzanowensis n.sp., Physoporella polonoandalusica n.sp., and Salpingoporella krupkaensis n.sp. Best-preserved specimens are illustrated in 39 plates. The identified species were compared with Alpine and Carpathian forms of stratigraphic importance. Six Dasycladalean local horizons are defined. The Pelsonian-Illyrian boundary occurs in the middle part of the Diplopora Dolomite. Its uppermost part, despite the presence of Diplopora annulata, belongs to the Illyrian, not to the Fassanian, as also corroborated by conodont correlations. The peculiar state of preservation (internal moulds and double tubes) is discussed; it is due to early syngenetic dolomitisation. The palaeoenvironment of the algae is determined as sublittoral. Dasycladales flourished upon a peri-Tethyan carbonate platform, widely connected with the Alpine-Carpathian seas. Five palaeoecological assemblages are recognized, differing in their bathymetric and turbulence conditions, and living in marine shoals and shallow basins separating them. The Upper Silesian platform was separated from the hypersaline Germanic Basin of the Middle Muschelkalk by banks and oolite and bioclastic barriers, blocking dispersal of marine biota. A wide connection existed with the Alpine-Carpathian seas, allowing immigration of High-Tatric, Križna and even South Alpine flora to the Upper Silesian Carbonate Platform.

Biodiversity tracks temperature over time

The geographic distribution of life on Earth supports a general pattern of increase in biodiversity with increasing temperature. However, some previous analyses of the 540-million-year Phanerozoic fossil record found a contrary relationship, with paleodiversity declining when the planet warms. These contradictory findings are hard to reconcile theoretically. We analyze marine invertebrate biodiversity patterns for the Phanerozoic Eon while controlling for sampling effort. This control appears to reverse the temporal association between temperature and biodiversity, such that taxonomic richness increases, not decreases, with temperature. Increasing temperatures also predict extinction and origination rates, alongside other abiotic and biotic predictor variables. These results undermine previous reports of a negative biodiversity-temperature relationship through time, which we attribute to paleontological sampling biases. Our findings suggest a convergence of global scale macroevolutionary and macroecological patterns for the biodiversity-temperature relationship.

Paleontology of Early Cretaceous deep-water samples dredged from the Wallaby Plateau: new perspectives of Gondwana break-up along the Western Australian margin

Sedimentary and paleontological samples from steep, deep-water escarpments of the Wallaby Plateau, 400 km west of Carnarvon, Western Australia, represent the first collected soft-rock geological data from this immense bathymetric high. The study of this vast marginal plateau of 100 000 km2 has been hampered by a paucity of rock data arising from difficulties in sampling in water depths of 2200–5700 m. Only modern carbonate, altered tholeiitic basalts and volcaniclastic rocks have been recorded previously. Variably fossiliferous to unfossiliferous claystone, siltstone and sandstone samples from 12 southern Wallaby Plateau stations (3015–5159 m water depths) range from interpreted paralic to shallow-water marine settings, and contain low to moderately diverse assemblages of Bivalvia, Gastropoda, Ostracoda, Foraminifera, palynomorphs, very rare nannofossils, and teleost fish fragments, which collectively point to an age range of latest Berriasian to Barremian–Aptian in the Early Cretaceous. This age range pre-dates, straddles and post-dates the breakup and opening of the Cuvier Abyssal Plain. Seismic imaging of the Wallaby Plateau exhibits a substantial thickness of both dipping and flat-lying, sub-parallel reflectors beneath parts of the Lower Cretaceous Gondwanan breakup unconformity. This information, taken together with the recent identification of Oxfordian–Kimmeridgian Foraminifera from the same location, indicates the presence of a pre-breakup sedimentary section beneath parts of the Wallaby Plateau.

Probabilistic correlation of single stratigraphic samples: A generalized approach for biostratigraphic data

Existing quantitative methods for biostratigraphic dating and correlation commonly ignore one of the key strengths of the microfossil record—relative abundance data. In this study, we present a maximum likelihood-based biostratigraphic method that demonstrates how microfossil abundance can be used in the stratigraphic placement of isolated samples. Precise correlation and dating of isolated paleontological samples is not possible with current methods, which are primarily intended for the alignment of longer stratigraphic sequences. In contrast, the probabilistic approach provided by likelihood analysis results in sample age estimates with defined confidence intervals. Therefore, all the uncertainties inherent in our age assessment (resulting from small sample sizes, incomplete sampling, imperfect knowledge of stratigraphic distributions, lack of taxonomic resolution of biostratigraphic data, and underlying environmental, paleogeographic, and sedimentologic processes) are explicit in our results. We conclude with a field test of the method, from data collected from an oil well from the Catatumbo Basin, Colombia, illustrating the use of our approach in a real-world case study and highlighting how our method could be generalized to a wide range of stratigraphic problems.

Problems with using rock outcrop area as a paleontological sampling proxy: rock outcrop and exposure area compared with coastal proximity, topography, land use, and lithology

Fossil specimens can be recovered easily only from exposed localities, so rock exposure area should represent a better proxy for rock availability than the frequently used outcrop (i.e., map) area. Data collected via remote sensing and GIS show that map area does not consistently correlate with exposure area in different regions. Proportional rock exposure is not geographically consistent and is influenced by a number of variables that are independent of outcrop area, including proximity to the coast, elevation, bedrock age, land use and lithology. These variables appear to be non-independent in their influence on rock exposure, and are not consistent in their effects across continents. The inconsistency in the correlation between outcrop and exposure area, and the variability in the influence of different factors on rock exposure, suggests that using outcrop area as a sampling proxy is poorly supported. The weaknesses in using outcrop area as a sampling proxy, highlighted by the lack of correlation with exposure area, suggest that a single accurate global sampling proxy may never be attained and it is premature to assume that paleodiversity curves can be corrected using such proxies. It is therefore preferable to work on a regional scale, comparing regional fossil collection data with a number of proxies representing all aspects of sampling. The lack of correlation between outcrop and exposure area suggests that the covariance detected between outcrop area and paleodiversity might be better explained by a common-cause model, and that geological megabiases may not have had as profound an effect on paleodiversity curves as previously thought.

Problems with using rock outcrop area as a paleontological sampling proxy: rock outcrop and exposure area compared with coastal proximity, topography, land use, and lithology

Fossil specimens can be recovered easily only from exposed localities, so rock exposure area should represent a better proxy for rock availability than the frequently used outcrop (i.e., map) area. Data collected via remote sensing and GIS show that map area does not consistently correlate with exposure area in different regions. Proportional rock exposure is not geographically consistent and is influenced by a number of variables that are independent of outcrop area, including proximity to the coast, elevation, bedrock age, land use and lithology. These variables appear to be non-independent in their influence on rock exposure, and are not consistent in their effects across continents. The inconsistency in the correlation between outcrop and exposure area, and the variability in the influence of different factors on rock exposure, suggests that using outcrop area as a sampling proxy is poorly supported. The weaknesses in using outcrop area as a sampling proxy, highlighted by the lack of correlation with exposure area, suggest that a single accurate global sampling proxy may never be attained and it is premature to assume that paleodiversity curves can be corrected using such proxies. It is therefore preferable to work on a regional scale, comparing regional fossil collection data with a number of proxies representing all aspects of sampling. The lack of correlation between outcrop and exposure area suggests that the covariance detected between outcrop area and paleodiversity might be better explained by a common-cause model, and that geological megabiases may not have had as profound an effect on paleodiversity curves as previously thought.

Suppression of ring artefacts when tomographing anisotropically attenuating samples

There are many objects for which the attenuation varies significantly as they are rotated during computerized X-ray tomography, for example plate samples. This can lead to significant ring artefacts in the subsequent tomographic reconstructions. In this paper a new method is presented that can successfully suppress such ring artefacts and is applicable to both parallel and cone-beam geometries. Rapid correction is achieved via an analytical formula which involves only a matrix-vector multiplication, for which the matrix is known and depends on a regularization parameter. The efficacy of the method is demonstrated for a paleontological sample (calcified shark cartilage) and a carbon-carbon composite/Ti-SiC metal matrix composite test sample.

A Large Abelisauroid Theropod Dinosaur from the Early Cretaceous of Libya

Despite increasingly intensive paleontological sampling, Cretaceous terrestrial vertebrates from continental Africa remain relatively poorly known, frustrating efforts to characterize paleoecosystems in the region, as well as the paleobiogeography of the southern continents during this interval. Here we describe the partial skeleton of a large-bodied theropod dinosaur from the Early Cretaceous (early Aptian, ~125-120 Ma) of Libya. The specimen consists of associated elements (two incomplete dorsal vertebrae, a proximal caudal centrum, a partial proximal caudal neural arch, the distal right femur, and the mostly complete right tibia) and is referable to the widespread ceratosaurian clade Abelisauroidea. The discovery adds to the growing record of abelisauroids from mainland Africa, and firmly establishes the presence of the clade on the continent prior to its final separation from South America. Indeed, the age of the Libyan theropod predates or is penecontemporaneous with the accepted timing of fragmentation of most major Gondwanan landmasses, supporting the hypothesis that abelisauroids could have dispersed throughout the southern continents before land connections between these areas were severed. Moreover, the considerable size of the Libyan form challenges assertions that abelisauroids were ecologically subordinate to basal tetanuran theropods in Early and middle Cretaceous paleoenvironments of Gondwana.

New Paleomagnetic and Stable‐Isotope Results from the Nanxiong Basin, China: Implications for the K/T Boundary and the Timing of Paleocene Mammalian Turnover

The Nanxiong Basin (Guangdong Province, China) preserves the most complete Asian stratigraphic record of the Cretaceous‐Paleogene (K/Pg) boundary extinction and the subsequent Paleocene mammalian radiation. Despite extensive study, the precise placement of the K/Pg boundary in the Nanxiong Basin sequence has been controversial, and the timing of subsequent mammalian turnover is poorly constrained. We present new paleomagnetic and geochemical data from the Late Cretaceous Pingling Formation (Nanxiong Group) and the overlying Paleocene Shanghu, Nongshan, and Guchengcun formations (Luofozhai Group). Our samples are directly correlated with previous geochemical and paleontological sampling localities, allowing for easy comparison with other local proxy records. Results indicate that the traditional placement of the K/Pg boundary at the base of a chaotic channel sandstone bed marking the highest stratigraphic appearance of dinosaur eggshell fragments and lowest stratigraphic appearance of Paleocene mammalian fossils lies about two‐thirds of the way up Chron C29R, consistent with the placement of the boundary in all other well‐documented sections. The average carbon isotope composition of paleosol carbonates decreases by >2‰ in the Early Paleocene, consistent with a major disruption to global carbon cycling after the K/Pg boundary. Constraints on the age of the first major Cenozoic mammalian turnover event in Asia (the Shanghuan‐Nongshanian Asian Land Mammal Age boundary) support its placement near the top of Chron C27N, which coincides with a similar turnover in North America and geochemical changes recorded in several deep sea cores.

The geological completeness of paleontological sampling in North America

A growing body of work has quantitatively linked many macroevolutionary patterns, including short- and long-term changes in biodiversity, rates of taxonomic extinction and origination, and patterns of extinction selectivity, to temporal variability in the sedimentary rock record. Here we establish a new framework for more rigorously testing alternative hypotheses for these and many other results by documenting the large-scale spatiotemporal intersection of the North American sedimentary rock and fossil records. To do this, we combined 30,387 fossil collections in the spatially explicit Paleobiology Database with a comprehensive macrostratigraphic database consisting of 18,815 sedimentary lithostratigraphic units compiled from 814 geographic regions distributed across the United States and Canada. The geological completeness of paleontological sampling, here defined as the proportion of the available sedimentary rock record that has been documented to have at least one fossil occurrence, irrespective of taxonomy or environment, is measured at four different levels of stratigraphic resolution: (1) lithostratigraphic rock units, (2) hiatus-bound rock packages, (3) regional stratigraphic columns, and (4) sediment coverage area (km2). Mean completeness estimates for 86 Phanerozoic time intervals (approximately stages; median duration 5.3 Myr) range from 0.18 per interval in the case of lithostratigraphic rock units to 0.23 per interval for stratigraphic columns and sediment coverage area. Completeness estimates at all four levels of stratigraphic resolution exhibit similar temporal variation, including a significant long-term increase during the Phanerozoic that is accentuated by an abrupt Campanian–Maastrichtian peak. This Late Cretaceous peak in completeness is approximately five times greater than the least complete Phanerozoic time intervals (Early Cambrian, Early Devonian, late Permian, and Early Cretaceous). Geological completeness in the Cenozoic is, on average, approximately 40% greater than in the Paleozoic. Temporal patterns of geological completeness do not appear to be controlled exclusively by variation in the frequency of subsurface rock units or an increase over time in the proportion of terrestrial rock, but instead may be general features of both the marine and terrestrial fossil records.

The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages

The nature of evolutionary changes recorded by the fossil record has long been controversial, with particular disagreement concerning the relative frequency of gradual change versus stasis within lineages. Here, I present a large-scale, statistical survey of evolutionary mode in fossil lineages. Over 250 sequences of evolving traits were fit by using maximum likelihood to three evolutionary models: directional change, random walk, and stasis. Evolution in these traits was rarely directional; in only 5% of fossil sequences was directional evolution the most strongly supported of the three modes of change. The remaining 95% of sequences were divided nearly equally between random walks and stasis. Variables related to body size were significantly less likely than shape traits to experience stasis. This finding is in accord with previous suggestions that size may be more evolutionarily labile than shape and is consistent with some but not all of the mechanisms proposed to explain evolutionary stasis. In general, similar evolutionary patterns are observed across other variables, such as clade membership and temporal resolution, but there is some evidence that directional change in planktonic organisms is more frequent than in benthic organisms. The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria and suggests that truly directional evolution is infrequent or, perhaps more importantly, of short enough duration so as to rarely register in paleontological sampling.

DIFFERENTIATING MICROTUS XANTHOGNATHUS AND MICROTUS PENNSYLVANICUS LOWER FIRST MOLARS USING DISCRIMINANT ANALYSIS OF LANDMARK DATA

The distinct ecological requirements of Microtus xanthognathus (yellow-cheeked vole or taiga vole) and M. pennsylvanicus (meadow vole) warrant accurate discrimination of their remains in studies of paleoecology and past biogeographical shifts. An occlusal length of the lower 1st molars (m1) that is .3.2 mm for M. xanthognathus is the method most frequently used to separate these 2 taxa in archaeological and paleontological samples. However, these measurements alone are unreliable because some specimens of M. pennsylvanicus overlap smaller individuals of M. xanthognathus in size. Therefore, I created and tested a morphometric technique that discriminates Recent lower 1st molars (m1s) of M. pennsylvanicus from those of M. xanthognathus, and is applicable to other taxa (both modern and fossil). Despite overlapping occlusal length, my discriminant function based on landmark data correctly classified 100% (n ¼ 53) of Recent m1s from the 2 taxa and 97.7% (43 of 44) of (assumed) m1s of M. pennsylvanicus from an archaeological site from about AD 1200 in central Nebraska. This landmark scheme is applicable to fossil and modern Microtus worldwide.

Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens

Paleontologists are quite recent newcomers among the users of X-ray synchrotron imaging techniques at the European Synchrotron Radiation Facility (ESRF). Studies of the external morphological characteristics of a fossil organism are not sufficient to extract all the information for a paleontological study. Nowadays observations of internal structures become increasingly important, but these observations should be non-destructive in order to preserve the important specimens. Conventional microtomography allows performing part of these investigations. Nevertheless, the best microtomographic images are obtained using third-generation synchrotrons producing hard X-rays, such as the ESRF. Firstly, monochromatisation avoids beam hardening that is frequently strong for paleontological samples. Secondly, the high beam intensity available at synchrotron radiation sources allows rapid data acquisition at very high spatial resolutions, resulting in precise mapping of the internal structures of the sample. Thirdly, high coherence leads to additional imaging possibilities: phase contrast radiography, phase contrast microtomography and holotomography. These methods greatly improve the image contrast and therefore allow studying fossils that cannot be investigated by conventional microtomography due to a high degree of mineralisation or low absorption contrast. Thanks to these different properties and imaging techniques, a synchrotron radiation source and the ESRF in particular appears as an almost ideal investigation tool for paleontology.

Arm regeneration in Mississippian crinoids: evidence of intense predation pressure in the Paleozoic?

Abstract Although direct predator-prey interactions are unobservable in the fossil record, predation has been used to explain many evolutionary trends. Evidence of predation supporting such hypotheses is often presented as isolated instances of preserved sublethal damage, and less commonly, as the frequency of such injuries. For instance, numerous morphological and ecological trends and innovations observed in Phanerozoic crinoids have been causally linked to predation, and whereas the high frequency of arm regeneration in living crinoids is generally assumed to represent intense predation, attempts to assess regeneration frequency and patterns in paleontological samples are few. Can the frequency of fossil injuries be assessed to test hypothesized predation-driven trends, or are such data unavailable? To address this question, we analyzed regeneration in crinoids from the lower Mississippian (Kinderhookian) Maynes Creek Formation near Le Grand, Iowa, a locality renowned for the preservation of thousands ...

Phenotypic variance inflation in fossil samples: an empirical assessment

Evolutionary change during the interval in which a fossil sample accumulates will inflate the variance of that sample relative to the population-level standing variation. If this effect is widespread and severe, paleontological samples will not provide reliable estimates of population variation. Although the few published studies conducted to test this possibility have found similar levels of variation in samples differing greatly in temporal acuity, the paucity of case studies prevents assessing the generality of this pattern. In this paper, two independent, literature-based approaches are used to greatly expand the data available to address this issue. The first approach compares morphometric variability in Quaternary mammal samples with samples from related modern populations. The second approach artificially lumps separate samples from evolving lineages and calculates the variance effects of this analytical time-averaging. Both approaches yield consistent results indicating that variance observed in time-averaged samples is typically only slightly inflated (approximately 5%) relative to population-level values. This finding suggests that rates of evolution are typically slow when scaled to within-population variation, providing support for relative stasis as the dominant mode of within-lineage evolution. An important practical consequence of these findings is that time-averaged fossil samples generally show trait variances and covariances that are similar to population-level parameters, which has been an important but implicit assumption in many paleontological studies of phenotypic variation.

Prehistoric decline of genetic diversity in the nene.

The nene (or Hawaiian goose, Branta sandvicensis) once occurred on most of the main Hawaiian Islands (1), but by Captain Cook’s arrival in 1778, nene were found only on the island of Hawaii (2). A decline that began in the 1800s reduced the nene population to fewer than 30 individuals by the middle of the 20th century (2). Nene currently have extremely low levels of genetic variation (3). We questioned whether this low genetic variation resulted from the recent population decline or was a natural consequence of living on islands (theory and empirical evidence indicate that island taxa should and do have lower genetic variability than mainland ones) (4). Here we report an assessment of genetic variation in samples of nene on the island of Hawaii from four time periods (Fig. 1): 26 extant captive and wild birds, 14 historical museum specimens (collected between 1833 and 1928), 16 bones from archaeological middens [radiocarbon dated at 160 to 500 radiocarbon years before present (rybp)], and 14 bones from paleontological sites (500 to 2540 rybp). We analyzed DNA sequence variation in the mitochondrial control region (CR), following very strict methodologies to avoid and detect contamination in ancient samples (5). We found no variation across 847 nucleotide sites among extant nene (haplotype diversity, H 5 0) and designate this sequence as haplotype RH. For the ancient samples, we obtained sequences of 217 sites from the highly variable left domain of the CR and 92 sites from the less variable right domain (6). Unexpectedly, only one museum specimen sequence differed from the RH haplotype (E1, Fig. 1B), and all archaeological bones (sampled from eight different caves) possessed RH. Only the paleontological nene exhibited levels of CR variation typical of geese (5): six individuals had RH and eight had one of six additional haplotypes (Fig. 1B). H was 0.802 (multinomial lower 95% CI 5 0.67) in the paleontological sample, but only 0.067 (upper 95% CI 5 0.26) in the archaeological and museum specimen samples combined (6). Thus, our results indicate that the nene populations on Hawaii lost most of their mitochondrial DNA (mtDNA) variability long before their historic population decline. Monte Carlo simulations run for 150 generations (about 600 years) suggest that the most likely explanation is a prehistoric population bottleneck (6). A reduction of H from 0.80 to 0.26 in populations of varying size (500 to 10,000) can only occur if the populations decline to fewer than 270 females (for a rate of decline of r 5 20.01) or to fewer than 20 females (for r 5 20.05). In addition, in the absence of population decline, an improbably large selection coefficient of 0.10 would be required to change RH frequency from 0.43 to 0.97 over 150 generations. Our radiocarbon chronology (Fig. 1A) suggests that the nene’s loss of genetic variability took place during a period of prehistoric human population growth (900 to 350 years ago), when settlements expanded into marginal ecological zones (7). Radiocarbon dates (1, 5, 8) indicate that the extirpation of the nene on Kauai and the extinction of at least five of the nine large ground-dwelling Hawaiian birds (1) occurred during this time period. Ecological changes associated with human settlement are assumed to have caused the extinctions (1) and apparently caused a dramatic reduction in genetic diversity in the nene on Hawaii as well. Ultimately, we must ask why the nene population on Hawaii could escape prehistoric extinction while many other Hawaiian birds did not. Cultural changes may have created better conditions for nene, such as more open habitat, while at the same time warfare may have created noman’s-lands safer for wildlife. Prohibitions by ruling elites (kapu) may have protected the birds, and it is possible that they were even brought under domestication or semidomestication. Paleontological studies have shown how broadly devastating the impact of prehistoric humans was on insular organisms, as most vividly shown by the total extinction of hundreds of species (1). Our findings demonstrate that these prehistoric influences may still be reflected in the genetic makeup of insular species that survived until the present, many of which, like the nene, are endangered. In addition, we have confirmed that at least one islanddwelling species, the nene, did not always have the low genetic variability predicted by its history as an island taxon (4).

Detection of bone glue treatment as a major source of contamination in ancient DNA analyses.

Paleogenetic investigations of ancient DNA extracted from fossil material is for many reasons susceptible to falsification by the presence of more recent contamination from several sources. Gelatine-based bone glue that has been used extensively for nearly two centuries by curators to preserve hard tissues contributes nonauthentic DNA to paleontological material. This fact has been frequently neglected and is barely mentioned in the literature. Now paleogeneticists, curators, and conservators are faced with the problem that treatment of samples with adhesives and consolidants for conservatory purposes has seldom been recorded. Here, we show that racemization of amino acids, and in particular serine, is an excellent indicator for the treatment of paleontological samples with glue.

Dating the Time of Origin of Major Clades: Molecular Clocks and the Fossil Record

▪ Abstract Molecular and paleontological data provide independent means of estimating when groups of organisms evolved in the geological past, but neither approach can be considered straightforward. The single most fundamental obstacle to developing an accurate estimate of times of origination from gene sequence data is variation in rates of molecular evolution, both through time and among lineages. Although various techniques have been proposed to circumvent this problem, none unambiguously allow the components of time and rate to be separated. Furthermore, problems of establishing accurate calibration points, correctly rooted phylogenies, and accurate estimates of branch length remain formidable. Conversely, paleontological dates fix only the latest possible time of divergence, and so probabilistic methods are required to set a lower boundary on origination dates. Realistic confidence intervals that take preservational biases into account are only just becoming available.Although molecular and paleontological approaches to dating often agree reasonably well, there are two notable areas of disagreement; when mammal and bird orders originated and when the major phyla originated. The discrepancy in dating bird/mammal ordinal origins probably reflects a global rock-record bias. Paleontological sampling in the Late Cretaceous is still too restricted geographically to draw any firm conclusions about the existence of a pre-Tertiary record for modern orders of bird or mammal from anywhere other than North America. Dating the time of origin of phyla is more complicated, and is confounded by both preservational biases and problems of molecular clock estimation.

Taxonomy and temporal diversity patterns

Temporal diversity patterns have traditionally been analyzed by counting the number of families or genera present over a series of time periods. This approach has been criticized on the grounds that paraphyletic taxa might introduce artifacts. Sepkoski and Kendrick (1993) simulated phylogenetic trees and different classifications of those trees and concluded that paraphyletic taxa need not be rejected. We have reimplemented their model, extended it, and carried out statistical analyses under a variety of experimental conditions. Our results show that the focus on monophyly vs. paraphyly is misplaced. Instead, it appears that the number of groups in the classification and the distribution of the sizes of those groups have dramatic effects on the recovery of diversity information. Furthermore, the influence of these factors depends on whether the fossil record represents a low- or high-frequency sampling of lineages. When sampling is good, the best results are achieved by classifications with large numbers of small taxa. When sampling is poor, however, the best results are achieved by classifications that include some large and medium-sized groups as well as many smaller groups. This suggests that the best estimates of underlying diversity will be achieved by counting (in the same study) taxa assigned to different ranks, so as to best match the inferred quality of the paleontological sample. In practice this will mean abandoning the commitment to counting taxa at a single rank.