Developmental perspective on size change and allometry in evolution

The evolution of changes in body size is one of the most important patterns in the history of life. Its importance arises from both the frequency of the pattern and the biological implications of size change itself, which affects myriad aspects of an organism's structure and function through well-known scaling relationships. Yet relatively little attention has been focused on the underlying genetic and developmental controls of size change or their implications with regard to other morphological changes. Here, I review the endocrine growth axis and show that variation in several key growth-control substances, particularly growth hormone (GH) and insulin-like growth factor I (IGF I), is clearly linked to intraspecific differences in postnatal growth rates and terminal body size. I intentionally review a considerable amount of literature on nonprimate mammals because this research is vital to an understanding of the general topic. Research on human pygmies, giant transgenic mice, and other models of growth disturbances indicates that shifts in GH and/or IGF I levels not only yield the expected changes in terminal body size, but also result in simple truncations or extensions of underlying allometric patterns. These data provide a possible developmental basis for the common finding of ontogenetic scaling and coordinated transformations in series of closely related fossil or living species that differ in body size. At present, however, this must be viewed as a hypothesis that requires testing through interspecific analyses. A consideration of previous interpretations of the morphological distinctions of human pygmies and some other organisms demonstrates the novel information that a developmental perspective brings to morphological comparisons. Clearly, knowledge of the genetic and developmental controls of morphogenesis will greatly enhance our understanding of a multitude of evolutionary patterns, processes and mechanisms, for it is perturbations in these these controls that ultimately produce the raw material for evolutionary transformations.     

Big cat,small cat: reconstructing body size evolution in living and extinct Felidae

The evolution of body mass is a fundamental topic in evolutionary biology, because it is closely linked to manifold life history and ecological traits and is readily estimable for many extinct taxa. In this study, we examine patterns of body mass evolution in Felidae (Placentalia, Carnivora) to assess the effects of phylogeny, mode of evolution, and the relationship between body mass and prey choice in this charismatic mammalian clade. Our data set includes 39 extant and 26 extinct taxa, with published body mass data supplemented by estimates based on condylobasal length. These data were run through ‘SURFACE’ and ‘bayou’ to test for patterns of body mass evolution and convergence between taxa. Body masses of felids are significantly different among prey choice groupings (small, mixed and large). We find that body mass evolution in cats is strongly influenced by phylogeny, but different patterns emerged depending on inclusion of extinct taxa and assumptions about branch lengths. A single Ornstein–Uhlenbeck optimum best explains the distribution of body masses when first‐occurrence data were used for the fossil taxa. However, when mean occurrence dates or last known occurrence dates were used, two selective optima for felid body mass were recovered in most analyses: a small optimum around 5 kg and a large one around 100 kg. Across living and extinct cats, we infer repeated evolutionary convergences towards both of these optima, but, likely due to biased extinction of large taxa, our results shift to supporting a Brownian motion model when only extant taxa are included in analyses.     

Fossils of parasites: what can the fossil record tell us about the evolution of parasitism?

Parasites are common in many ecosystems, yet because of their nature, they do not fossilise readily and are very rare in the geological record. This makes it challenging to study the evolutionary transition that led to the evolution of parasitism in different taxa. Most studies on the evolution of parasites are based on phylogenies of extant species that were constructed based on morphological and molecular data, but they give us an incomplete picture and offer little information on many important details of parasite–host interactions. The lack of fossil parasites also means we know very little about the roles that parasites played in ecosystems of the past even though it is known that parasites have significant influences on many ecosystems. The goal of this review is to bring attention to known fossils of parasites and parasitism, and provide a conceptual framework for how research on fossil parasites can develop in the future. Despite their rarity, there are some fossil parasites which have been described from different geological eras. These fossils include the free‐living stage of parasites, parasites which became fossilised with their hosts, parasite eggs and propagules in coprolites, and traces of pathology inflicted by parasites on the host's body. Judging from the fossil record, while there were some parasite–host relationships which no longer exist in the present day, many parasite taxa which are known from the fossil record seem to have remained relatively unchanged in their general morphology and their patterns of host association over tens or even hundreds of millions of years. It also appears that major evolutionary and ecological transitions throughout the history of life on Earth coincided with the appearance of certain parasite taxa, as the appearance of new host groups also provided new niches for potential parasites. As such, fossil parasites can provide additional data regarding the ecology of their extinct hosts, since many parasites have specific life cycles and transmission modes which reflect certain aspects of the host's ecology. The study of fossil parasites can be conducted using existing techniques in palaeontology and palaeoecology, and microscopic examination of potential material such as coprolites may uncover more fossil evidence of parasitism. However, I also urge caution when interpreting fossils as examples of parasites or parasitism‐induced traces. I point out a number of cases where parasitism has been spuriously attributed to some fossil specimens which, upon re‐examination, display traits which are just as (if not more) likely to be found in free‐living taxa. The study of parasite fossils can provide a more complete picture of the ecosystems and evolution of life throughout Earth's history.     

How species longevity, intraspecific morphological variation, and geographic range size are related: a comparison using late Cambrian trilobites

Phenotypic variation is fundamental to evolutionary change. Variation not only evinces the connectivity of populations but it is also associated with the adaptability and evolvability of taxa. Despite the potential importance of morphological variation in structuring evolutionary patterns, little is known about how relative differences in intraspecific morphological variation and its geographic structure are linked to differences in species longevity. This study offers a novel combination of analyses that reveal the quantitative relationships among intraspecific variation, geographic range size and duration in the fossil record using late Cambrian trilobites. Results show that geographic range size and duration are positively correlated. Surprisingly, longer lived species tend to have less intraspecific variation. Phylogenetic effects were also explored and found not to determine the association between these variables. However, the distribution of geographic range sizes shows strong phylogenetic signal. In light of previous work, one possible explanation for these results is that species with shorter durations have comparatively higher rates of morphological evolution, reflected in higher phenotypic variation overall.     

Large‐scale evolutionary trends of Acrochordiceratidae Arthaber, 1911 (Ammonoidea,Middle Triassic) and Cope’s rule

Abstract: Directed evolution of life through millions of years, such as increasing adult body size, is one of the most intriguing patterns displayed by fossil lineages. Processes and causes of such evolutionary trends are still poorly understood. Ammonoids (externally shelled marine cephalopods) are well known to have experienced repetitive morphological evolutionary trends of their adult size, shell geometry and ornamentation. This study analyses the evolutionary trends of the family Acrochordiceratidae Arthaber, 1911 from the Early to Middle Triassic (251–228 Ma). Exceptionally large and bed‐rock‐controlled collections of this ammonoid family were obtained from strata of Anisian age (Middle Triassic) in north‐west Nevada and north‐east British Columbia. They enable quantitative and statistical analyses of its morphological evolutionary trends. This study demonstrates that the monophyletic clade Acrochordiceratidae underwent the classical evolute to involute evolutionary trend (i.e. increasing coiling of the shell), an increase in its shell adult size (conch diameter) and an increase in the indentation of its shell suture shape. These evolutionary trends are statistically robust and seem more or less gradual. Furthermore, they are nonrandom with the sustained shift in the mean, the minimum and the maximum of studied shell characters. These results can be classically interpreted as being constrained by the persistence and common selection pressure on this mostly anagenetic lineage characterized by relatively moderate evolutionary rates. Increasing involution of ammonites is traditionally interpreted by increasing adaptation mostly in terms of improved hydrodynamics. However, this trend in ammonoid geometry can also be explained as a case of Cope’s rule (increasing adult body size) instead of functional explanation of coiling, because both shell diameter and shell involution are two possible paths for ammonoids to accommodate size increase.     

Combining fossil and molecular data to date the diversification of New World Primates

Recent methodological advances in molecular dating associated with the growing availability of sequence data have prompted the study of the evolution of New World Anthropoidea in recent years. Motivated by questions regarding historical biogeography or the mode of evolution, these works aimed to obtain a clearer scenario of Platyrrhini origins and diversification. Although some consensus was found, disputed issues, especially those relating to the evolutionary affinities of fossil taxa, remain. The use of fossil taxa for divergence time analysis is traditionally restricted to the provision of calibration priors. However, new analytical approaches have been developed that incorporate fossils as terminals and, thus, directly assign ages to the fossil tips. In this study, we conducted a combined analysis of molecular and morphological data, including fossils, to derive the timescale of New World anthropoids. Differently from previous studies that conducted total‐evidence analysis of molecules and morphology, our approach investigated the morphological clock alone. Our results corroborate the hypothesis that living platyrrhines diversified in the last 20 Ma and that Miocene Patagonian fossils compose an independent evolutionary radiation that diversified in the late Oligocene. When compared to the node ages inferred from the molecular timescale, the inclusion of fossils augmented the precision of the estimates for nodes constrained by the fossil tips. We show that morphological data can be analysed using the same methodological framework applied in relaxed molecular clock studies.     

Trait-dependent diversification and the impact of palaeontological data on evolutionary hypothesis testing in New World ratsnakes (tribe Lampropeltini)

For studies investigating trait evolution, there are at least two important questions. First, have traits under consideration influenced cladogenesis and extinction in the group? Second, how do fossil data alter inferences about trait evolution or diversification‐rate dynamics? However, relatively few studies have assessed these questions. Here, we use recently developed methods to test for trait‐dependent diversification in the New World colubrid snake tribe Lampropeltini. We also integrate data from fossil taxa into phylogenetic estimation of evolutionary parameters using a simple Monte Carlo randomization test. These analyses suggest that ecological conditions in temperate regions are tied to higher rates of cladogenesis, but that body size is not related to diversification in the group. We also find that the inclusion of fossil taxa alters absolute estimates of size and the rate of size evolution, but not the overall pattern of ecomorphological diversification, as well as estimates of evolutionary rates, particularly extinction.     

POTENTIAL PITFALLS OF RECONSTRUCTING DEEP TIME EVOLUTIONARY HISTORY WITH ONLY EXTANT DATA,A CASE STUDY USING THE CANIDAE (MAMMALIA,CARNIVORA)

Reconstructing evolutionary patterns and their underlying processes is a central goal in biology. Yet many analyses of deep evolutionary histories assume that data from the fossil record is too incomplete to include, and rely solely on databases of extant taxa. Excluding fossil taxa assumes that character state distributions across living taxa are faithful representations of a clade's entire evolutionary history. Many factors can make this assumption problematic. Fossil taxa do not simply lead‐up to extant taxa; they represent now‐extinct lineages that can substantially impact interpretations of character evolution for extant groups. Here, we analyze body mass data for extant and fossil canids (dogs, foxes, and relatives) for changes in mean and variance through time. AIC‐based model selection recovered distinct models for each of eight canid subgroups. We compared model fit of parameter estimates for (1) extant data alone and (2) extant and fossil data, demonstrating that the latter performs significantly better. Moreover, extant‐only analyses result in unrealistically low estimates of ancestral mass. Although fossil data are not always available, reconstructions of deep‐time organismal evolution in the absence of deep‐time data can be highly inaccurate, and we argue that every effort should be made to include fossil data in macroevolutionary studies.     

Correlates of body mass evolution in primates

Body mass is undoubtedly central to the overall adaptive profile of any organism. Despite this, very little is known of what forces drive evolutionary changes in body mass and, consequently, shape patterns of body mass distribution exhibited by animal radiations. The search for factors that may influence evolutionary processes in general frequently focuses on environmental parameters such as climate change or interspecific competition. With respect to body mass, there is also the suggestion that evolutionary lineages may follow an inherent trend toward increased body mass, known as Cope's rule. The present paper investigates whether overall directional trends of body mass change, or correlations between patterns of body mass evolution and environmental factors have influenced the evolution of body mass in plesiadapiforms and primates. Analyses of the global fossil record of plesiadapiforms and primates suggest that the former did indeed follow an overall trend toward increased body mass compatible with the predictions of Cope's rule. In contrast, neither primates as a whole, nor a number of individual primate radiations (Adapiformes, Omomyiformes, and Anthropoidea), show any indication of overall directional patterns of body mass change. No correlations of primate body mass change with either the latitudinal distribution of fossil species, or with estimates of global temperature trends, were found. There is evidence, however, that direct competition between omomyiforms and adapiforms (the two main primate radiations known from the Paleogene) influenced processes of body mass evolution in omomyiforms.     

Molecular clocks and explosive radiations

Molecular data are ideal for exploring evolutionary history because of its universality, stochasticity, and abundance. These features provide a means of exploring the evolutionary history of all organisms (including those that do not tend to leave fossils), potentially within a statistical framework that allows testing of evolutionary hypotheses. However, the discrepancy between molecular and paleontological dates for three key "explosive" radiations inferred from the fossil record--the Cambrian explosion of animal phyla and the post-KT radiations of modern orders of mammals and birds--have led to a reexamination of the assumptions on which molecular dates are based. Could variation in the rate of molecular evolution, perhaps associated with "explosive" radiations, cause overestimation of diversification dates? Here I examine four hypothetical causes of fast molecular rates in explosive radiations--body size, morphological rate, speciation rate, and ecological diversification--using available empirical evidence on patterns of variation in rate of molecular evolution.     

Cercopithecoid cervical vertebral morphology and implications for the presence of Theropithecus in early Pleistocene Europe

While it is recognized that the overall configuration of the vertebral column, as well as the size and shape of individual vertebrae, differ within and between primate taxa, relatively little is known about the degree to which vertebral morphology reflects a phylogenetic signal or the degree to which vertebral elements can be used in accurate taxonomic classification. Isolated vertebrae are occasionally found in fossil assemblages, and proper taxonomic identification is necessary to make inferences about the animal's biology and place it in a broader phylogenetic and evolutionary context. Recently, three large primate cervical vertebrae (C3, C5, and C6) from Pirro Nord, Italy (early Pleistocene, late Villafranchian) were attributed to the genus Theropithecus based on size comparisons with extant cercopithecoid primates (Rook et al., 2004, J. Hum. Evol. 47, 267-277). These fossils were suggested to indicate an early dispersal of this genus out of Africa around 1.6-1.3 Ma possibly co-incident with early Pleistocene dispersals of Homo. Because of the potential importance of these fossils for interpreting Theropithecus evolution and the relatively few morphological data on primate cervical vertebral morphology, we examined the size and shape of cervical vertebrae in a large sample of extant cercopithecoid taxa (n=106). Specifically, we evaluated whether subfamily and genus level assignments can be made on the basis of isolated cervical elements. Discriminant analyses reveal that scaled shape variables are good discriminators of taxonomic affinity at the subfamily level but are poor discriminators at the genus level. Least-squares regressions show that raw linear dimensions of cervical vertebral morphology are good predictors of body mass in the extant sample. Our regression results produce a likely body mass estimate of 22-38 kg for the Pirro Nord cervical vertebrae. Based on these regression estimates, the poor ability to discriminate cervical vertebrae at the genus level, and paleoenvironmental reconstructions of Pirro Nord, it is unlikely that the Pirro Nord fossils can be confidently attributed to the genus Theropithecus. These findings have important implications for recent interpretations of the nature of Theropithecus dispersal out of Africa.     

中更新世晚期中国古人类化石的形态多样性及其演化意义

近年对许家窑、许昌、华龙洞、澎湖、夏河、哈尔滨等人类化石开展的系统研究,引发了学界对中更新世晚期人类演化及分类的不同认识。基于对相关中国人类化石形态特征的分析,作者提出这一时期中国人类化石形态特征表现为四种类型：1）以中更新世晚期人类共有特征为主;2）以原始特征为主;3）以现代特征为主;4）独特形态组合。多数化石形态特征表现为前三种类型,而许昌和许家窑这种以硕大的头骨和巨大颅容量构成的独特形态组合在其他同时期化石还没有发现。化石形态的多样性提示,不同类型的中更新世晚期中国古人类对现代人的形成贡献不同。作者认为在该时期的人类化石形态多样性规律还未阐明的情况下,将具有混合或镶嵌特征的相关人类化石归入分类地位不确定的人群较为合适。     

更新世中期中国古人类演化区域连续性与多样性的化石证据

以往,在东亚大陆发现的更新世中期人类化石被分别归入直立人和古老型智人。这种分类的主要依据是化石形态特征以及年代。魏敦瑞对周口店第一地点人类化石研究描述的一些头骨、下颌骨和牙齿特征通常被作为判定直立人的标准。根据这些化石的年代分布,一般将30万年前的中更新世晚期作为划分直立人与古老型智人的大致年代界限。近20年来,在非洲、欧洲和东亚新发现了一些更新世中期人类化石,目前古人类学界对中国更新世中期人类化石特征及演化有了与以往不同的认识。最近对大荔、许家窑、盘县大洞、许昌、华龙洞等人类化石的研究显示,近30万年以来东亚大陆人类演化呈现复杂的多样性,将这一时期人类全部归入古老型智人难以准确反映更新世中期中国古人类演化模式及规律。本文结合近年中国更新世中期人类演化研究进展,选择部分具有演化及分类价值的形态特征,分析这些特征在更新世中期中国古人类化石的表现特点。在此基础上,对更新世中期中国古人类演化模式做了尝试性探讨。本研究发现,周口店、和县、沂源、南京等中更新世早期人类化石呈现有较多的区域性特征,形态特征表现相对稳定;而大荔、金牛山、许家窑、许昌、华龙洞、马坝、盘县大洞等中更新世晚期人类在化石形态特征表现复杂多样,变异范围大。此外,在这一时期人类化石上发现较多与生存活动、健康、环境适应有关的证据。根据这些发现,作者认为中国中更新世早期组人类演化以形态连续性为主;进入中更新世晚期,中国古人类演化区域性特征减弱,演化模式以多样性为主。一系列新的化石发现和研究证据提示中更新世晚期东亚大陆可能生存有不同的古人类成员。根据目前掌握的化石形态和年代证据,大约30万年前是中国更新世中期演化变化关键时间节点。     

Did shifting seawater sulfate concentrations drive the evolution of deep-sea methane-seep ecosystems?

The origin and evolution of the faunas inhabiting deep-sea hydrothermal vents and methane seeps have been debated for decades. These faunas rely on a local source of sulfide and other reduced chemicals for nutrition, which spawned the hypothesis that their evolutionary history is independent from that of photosynthesis-based food chains and instead driven by extinction events caused by deep-sea anoxia. Here I use the fossil record of seep molluscs to show that trends in body size, relative abundance and epifaunal/infaunal ratios track current estimates of seawater sulfate concentrations through the last 150 Myr. Furthermore, the two main faunal turnovers during this time interval coincide with major changes in seawater sulfate concentrations. Because sulfide at seeps originates mostly from seawater sulfate, variations in sulfate concentrations should directly affect the base of the food chain of this ecosystem and are thus the likely driver of the observed macroecologic and evolutionary patterns. The results imply that the methane-seep fauna evolved largely independently from developments and mass extinctions affecting the photosynthesis-based biosphere and add to the growing body of evidence that the chemical evolution of the oceans had a major impact on the evolution of marine life.     

Genomic signatures of evolution in Nautilus—An endangered living fossil

Living fossils are survivors of previously more diverse lineages that originated millions of years ago and persisted with little morphological change. Therefore, living fossils are model organisms to study both long‐term and ongoing adaptation and speciation processes. However, many aspects of living fossil evolution and their persistence in the modern world remain unclear. Here, we investigate three major aspects of the evolutionary history of living fossils: cryptic speciation, population genetics and effective population sizes, using members of the genera Nautilus and Allonautilus as classic examples of true living fossils. For this, we analysed genomewide ddRAD‐Seq data for all six currently recognized nautiloid species throughout their distribution range. Our analyses identified three major allopatric Nautilus clades: a South Pacific clade, subdivided into three subclades with no signs of admixture between them; a Coral Sea clade, consisting of two genetically distinct populations with significant admixture; and a widespread Indo‐Pacific clade, devoid of significant genetic substructure. Within these major clades, we detected five Nautilus groups, which likely correspond to five distinct species. With the exception of Nautilus macromphalus, all previously described species are at odds with genomewide data, testifying to the prevalence of cryptic species among living fossils. Detailed F_ST analyses further revealed significant genome‐wide and locus‐specific signatures of selection between species and differentiated populations, which is demonstrated here for the first time in a living fossil. Finally, approximate Bayesian computation (ABC) simulations suggest large effective population sizes, which may explain the low levels of population differentiation commonly observed in living fossils.     

The Evolution of Human Brain Development

The human brain is a large and complex organ, setting us apart from other primates. It allows us to exhibit highly sophisticated cognitive and behavioral abilities. Therefore, our brain??s size and morphology are defining features of our species and our fossil ancestors and relatives. Endocasts, i.e., internal casts of the bony braincase, provide evidence about brain size and morphology in fossils. Based on endocasts, we know that our ancestors?? brains increased overall in size and underwent several reorganizational changes. However, it is difficult to relate evolutionary changes of size and shape of endocasts to evolutionary changes of cognition and behavior. We argue here that an understanding of the tempo and mode of brain development can help to interpret the evolution of our brain and the associated cognitive and behavioral changes. To do so, we review structural brain development, cognitive development, and ontogenetic changes of endocranial size and shape in living individuals on the one hand, and ontogenetic patterns (size increase and shape change) in fossil hominins and their evolutionary change on the other hand. Tightly integrating our knowledge on these different levels will be the key of future work on the evolution of human brain development.     

Advanced Cambrian hydroid fossils (Cnidaria: Hydrozoa) extend the medusozoan evolutionary history

Primitive cnidarians are crucial for elucidating the early evolution of metazoan body plans and life histories in the late Neoproterozoic and Palaeozoic. The highest complexity of both evolutionary aspects within cnidarians is found in extant hydrozoans. Many colonial hydrozoans coated with chitinous exoskeletons have the potential to form fossils; however, only a few fossils possibly representing hydroids have been reported, which still require scrutiny. Here, we present an exceptionally well-preserved hydroid found in the Upper Cambrian Fengshan Formation in northern China. It was originally interpreted as a problematic graptolite with an uncertain systematic position. Based on three characteristic morphological traits shared with extant hydroids (with paired hydrothecae, regular hydrocaulus internodes and special intrathecal origin pattern of hydrocladium), we propose this fossil hydroid as a new genus, Palaeodiphasia gen. nov., affiliated with the advanced monophyletic hydrozoan clade Macrocolonia typically showing loss of the medusa stage. More Macrocolonia fossils reviewed here indicate that this life strategy of medusa loss has been achieved already as early as the Middle Devonian. The early stratigraphical appearance of such advanced hydroid contrasts with previous molecular hypotheses regarding the timing of medusozoan evolution, and may be indicative for understanding the Ediacaran cnidarian radiation.     

Experimental taphonomy of Artemia reveals the role of endogenous microbes in mediating decay and fossilization

Exceptionally preserved fossils provide major insights into the evolutionary history of life. Microbial activity is thought to play a pivotal role in both the decay of organisms and the preservation of soft tissue in the fossil record, though this has been the subject of very little experimental investigation. To remedy this, we undertook an experimental study of the decay of the brine shrimp Artemia, examining the roles of autolysis, microbial activity, oxygen diffusion and reducing conditions. Our findings indicate that endogenous gut bacteria are the main factor controlling decay. Following gut wall rupture, but prior to cuticle failure, gut-derived microbes spread into the body cavity, consuming tissues and forming biofilms capable of mediating authigenic mineralization, that pseudomorph tissues and structures such as limbs and the haemocoel. These observations explain patterns observed in exceptionally preserved fossil arthropods. For example, guts are preserved relatively frequently, while preservation of other internal anatomy is rare. They also suggest that gut-derived microbes play a key role in the preservation of internal anatomy and that differential preservation between exceptional deposits might be because of factors that control autolysis and microbial activity. The findings also suggest that the evolution of a through gut and its bacterial microflora increased the potential for exceptional fossil preservation in bilaterians, providing one explanation for the extreme rarity of internal preservation in those animals that lack a through gut.     

Ecomorphological adaptation of acorn weevils to their oviposition site

Comparisons between closely related species in different habitats provide a window into understanding how biotic factors shape evolutionary pathways. Weevils in the genus Curculio have radiated extensively across the Palearctic, where similar ecomorphs have evolved independently on different hosts. We examined ecological and morphological data for 31 Curculio species using multivariate statistics to determine which morphological traits covary and which correlate with the host seed size. A subset of 15 taxa for which phylogenetic relationships were known were used for comparative analyses and inferring historical patterns of trait evolution. The morphological analyses suggest rostrum size increased proportionately to body size in both males and females and that both rostrum and body size correlate with host seed size but that rostrum shape does not correlate with any of the seed traits used in the analyses. Host shifts from small seeds to considerably larger seeds or vice versa have occurred several times independently and historical trait evolution indicates that these host shifts were accompanied by morphological changes in rostrum size. These patterns suggest that seed size is an important selective agent for changes in rostrum length along with body size and thus may be a key factor promoting morphological differentiation in the genus Curculio.     

THE FIDELITY OF THE FOSSIL RECORD: THE IMPROBABILITY OF PRESERVATION

Abstract:  The fidelity of the fossil record reflects how accurately it preserves the history of life. Since Darwin's time any mismatch between our theories and the fossil record has been attributed to the imperfections of the record. For over a century scarcity of gradual evolutionary trends was explained in this way until the punctuated equilibrium model was proposed. A null hypothesis that all morphological patterns in the fossil record are unbiased random walks can be rejected because it predicts far more apparent trends than exist. Current best estimates suggest that trends occur in at most 5% of characters. When an organism dies either it becomes fossilized or it doesn't. To be confident a species has not been preserved the probability against preservation must be significantly larger than the total number of individuals of that species that ever existed. For skeletized species preservation was the norm not the exception. Nevertheless, fossils must then avoid subsequent destruction and be discovered to be useful.