Luke C. Strotz

Assessing the role of cladogenesis in macroevolution by integrating fossil and molecular evidence

Assessing the extent to which population subdivision during cladogenesis is necessary for long-term phenotypic evolution is of fundamental importance in a broad range of biological disciplines. Differentiating cladogenesis from anagenesis, defined as evolution within a species, has generally been hampered by dating precision, insufficient fossil data, and difficulties in establishing a direct link between morphological changes detectable in the fossil record and biological species. Here we quantify the relative frequencies of cladogenesis and anagenesis for macroperforate planktic Foraminifera, which arguably have the most complete fossil record currently available, to address this question. Analyzing this record in light of molecular evidence, while taking into account the precision of fossil dating techniques, we estimate that the fraction of speciation events attributable to anagenesis is <19% during the Cenozoic era (last 65 Myr) and <10% during the Neogene period (last 23 Myr). Our central conclusion—that cladogenesis is the predominant mode by which new planktic Foraminifera taxa become established at macroevolutionary time scales—differs markedly from the conclusion reached in a recent study based solely on fossil data. These disparate findings demonstrate that interpretations of macroevolutionary dynamics in the fossil record can be fundamentally altered in light of genetic evidence.

Competition and mimicry: the curious case of chaetae in brachiopods from the middle Cambrian Burgess Shale

BackgroundOne of the first phyla to acquire biomineralized skeletal elements in the Cambrian, brachiopods represent a vital component in unraveling the early evolution and relationships of the Lophotrochozoa. Critical to improving our understanding of lophotrochozoans is the origin, evolution and function of unbiomineralized morphological features, in particular features such as chaetae that are shared between brachiopods and other lophotrochozoans but are poorly understood and rarely preserved. Micromitra burgessensis and Paterina zenobia from the middle Cambrian Burgess Shale are among the most remarkable examples of fossilized chaetae-bearing brachiopods. The form, functional morphology, evolutionary and ecological significance of their chaetae are studied herein.ResultsLike in Recent forms, the moveable but semi-rigid chaetae fringe both the dorsal and ventral mantle margins, but in terms of length, the chaetae of Burgess Shale taxa can exceed twice the maximum length of the shell from which it projects. This is unique amongst Recent and fossil brachiopod taxa and given their size, prominence and energy investment to the organism certainly had an important functional significance. Micromitra burgessensis individuals are preserved on hard skeletal elements, including conspecific shells, Tubulella and frequently on the spicules of the sponge Pirania muricata, providing direct evidence of an ecological association between two species. Morphological analysis and comparisons with fossil and extant brachiopod chaetae point to a number of potential functions, including sensory, defence, feeding, defouling, mimicry and spatial competition.ConclusionsOur study indicates that it is feasible to link chaetae length to the lack of suitable substrate in the Burgess Shale environment and the increased intraspecific competition associated with this. Our results however, also lend support to the elongated chaetae as an example of Batesian mimicry, of the unpalatable sponge Pirania muricata. We also cannot discount brachiopod chaetae acting as a sensory grille, extending the tactile sensitivity of the mantle into the environment, as an early warning system to approaching predators.

Effects of cyclone-generated disturbance on a tropical reef foraminifera assemblage.

The sedimentary record, and associated micropalaeontological proxies, is one tool that has been employed to quantify a region’s tropical cyclone history. Doing so has largely relied on the identification of allochthonous deposits (sediments and microfossils), sourced from deeper water and entrained by tropical cyclone waves and currents, in a shallow-water or terrestrial setting. In this study, we examine microfossil assemblages before and after a known tropical cyclone event (Cyclone Hamish) with the aim to better resolve the characteristics of this known signal. Our results identify no allochthonous material associated with Cyclone Hamish. Instead, using a swathe of statistical tools typical of ecological studies but rarely employed in the geosciences, we identify new, previously unidentified, signal types. These signals include a homogenising effect, with the level of differentiation between sample sites greatly reduced immediately following Cyclone Hamish, and discernible shifts in assemblage diversity. In the subsequent years following Hamish, the surface assemblage returns to its pre-cyclone form, but results imply that it is unlikely the community ever reaches steady state.

Sixty-one thousand recent planktonic foraminifera from the Atlantic Ocean

Marine microfossils record the environmental, ecological, and evolutionary dynamics of past oceans in temporally expanded sedimentary archives. Rapid imaging approaches provide a means of exploiting the primary advantage of this archive, the vast number of fossils, for evolution and ecology. Here we provide the first large scale image and 2D and 3D shape dataset of modern planktonic foraminifera, a major microfossil group, from 34 Atlantic Ocean sediment samples. Information on more than 124,000 objects is provided, including general object classification for 4/5ths of the dataset (~ 99,000 objects). Of the ~ 99,000 classifications provided, more than 61,000 are complete or damaged planktonic foraminifera. Objects also include benthic foraminifera, ostracods, pteropods, spicules, and planktonic foraminifera test fragments, among others. This dataset is the first major microfossil output of a new high-throughput imaging method (AutoMorph) developed to extract 2D and 3D data from photographic images of fossils. Our sample preparation and imaging techniques are described in detail. The data provided here comprises the most extensive publically available archive of planktonic foraminiferal morphology and morphological variation to date.

Getting somewhere with the Red Queen: chasing a biologically modern definition of the hypothesis

The Red Queen hypothesis (RQH) is both familiar and murky, with a scope and range that has broadened beyond its original focus. Although originally developed in the palaeontological arena, it now encompasses many evolutionary theories that champion biotic interactions as significant mechanisms for evolutionary change. As such it de-emphasizes the important role of abiotic drivers in evolution, even though such a role is frequently posited to be pivotal. Concomitant with this shift in focus, several studies challenged the validity of the RQH and downplayed its propriety. Herein, we examine in detail the assumptions that underpin the RQH in the hopes of furthering conceptual understanding and promoting appropriate application of the hypothesis. We identify issues and inconsistencies with the assumptions of the RQH, and propose a redefinition where the Red Queens reign is restricted to certain types of biotic interactions and evolutionary patterns occurring at the population level.

Reply to Aze et al.: Distinguishing speciation modes based on multiple lines of evidence

In a recent study, we present evidence that cladogenesis is the primary driver of long-term phenotypic evolution in planktic foraminifera (1). The phylogeny developed by Aze et al. (2) forms the basis of our study and represents an important contribution to the field. Aze et al. question our methodology (3), but the alternative they advocate, which relies entirely on a lineage-based approach (2), is by itself insufficient to distinguish speciation modes.

The Highest Southern Latitude Record of A Living Tridacna gigas

important and iconic component of coral reef communities (Rosewater, 1965; Yonge, 1981). Despite its iconic status, the species is under threat due to over-harvesting and habitat destruction, and is listed as “vulnerable” on the IUCN’s red list of threatened species (IUCN, 1983; Juinio et al., 1989; Hviding, 1993; Munro, 1993; Lucas, 1994). Like hermatypic scleractinians, T. gigas has a gellate algae, and it is this relationship that has allowed it to obtain its huge size (Schneider, 1998). Because of this similarity to corals, members of the Tridacninae, including T. gigas, have been used as a proxy to assess the health of coral reef settings (Aharon, 1991; Patzold et al., 1991) and for reconstructing paleoclimates at a high-resolution (Ahron, 1983, 1991; Aharon & Chappell, 1986; Watanabe & Oba, 1999). Previous paleoclimate studies have concentrated on shell chemistry, as T. gigas surrounding seawater (Watanabe et al., 2004). However, as temperature is a major control on distribution, with previous studies determining that shell growth is impossible below 19.2°C and optimum growth occurs in regions where the temperature is above 25°C for much of the year (Lucas et al., 1989), presence/absence of the taxon can also be an indicator of paleotemperature. The above factors make understanding the modern distribution of T. gigas vitally important, as it has implications for conservation of the species and both current and future studies of the effects of changing climate on tropical reef ecosystems. Given the strict ecological control on its distribution (temperature limited), T. gigas is an obvious candi-