Shape analysis of two sympatric coral species: Implications for taxonomy and evolution

Fourier shape analysis of Pleurodictyum americanum and P. dividua , favositid tabulate corals, from the Middle Devonian Hamilton Group of New York State, indicates that colony growth form is variable within species between environments, but that the range of variability in growth form is static through time. Pleurpdictyum dividua displays a columnar morphotype; however, P. dividua and P. americanum cannot be discriminated from one another based on colony shape alone. A major prediction of the punctuated equilibrium model of evolution is morphological stasis throughout the duration of a species. Harmonic means of assemblages of P. americanum show no persistent temporal trends. The range of harmonic means present among the oldest assemblages (Centerfield Limestone) are comparable to those among the youngest assemblages (Windom Shale) and to those in the intervening intervals. Character stasis in colony growth form lasted throughout a 2.5-3.5 my interval. In tabulate corals, where reproductive modes such as fragmentation have not been demonstrated, growth form may be useful in interpreting ancient environments. In addition, the tempo of evolution of growth form can be analyzed when sufficient morphological data are available from a wide distribution of environments. Thus, Fourier shape analysis of growth form provides a powerful tool for paleontologists to interpret the ecology and evolution of colonial marine animals.     

Substrate specificity in the Devonian tabulate coral Pleurodictyum

The tabulate coral Pleurodictyum americanum Roemer has been cited as an example of a host-specific organism occurring exclusively on the shells of gastropods, particularly Palaeozygopieura hamiltoniae (Hall). Examination of over 1600 specimens of P. americanum, from the Middle Devonian Hamilton Group of western New York, reveals additional complexities which require reinterpretation. While substrate selectivity for Palaeozygopieura shells is evident in all 42 subsamples, a variety of other substrates were also utilized by Pleurodictyum including corals, brachiopods, other molluscs and pebbles. Recent scleractinian corals inhabiting soft bottoms show similar substrate preference, selecting for the tubes of live serpulids, or gastropod shells (invariably with a secondary sipunculid host), but also occasionally settling on unoccupied shells or pebbles. Shell surfaces of P. hamiltoniae, preserved as external molds on the Pleurodictyum epitheca, exhibit encrustation by worm tubes and bryozoans as well as borings and mechanical shell damage, suggesting that these were not the shells of live gastropods. However, the invariant aperture-downward orientation and the high degree of selectivity of P. americanum strongly suggest that the shells were occupied by secondary hosts. □ Substrate specificity, commen-salism, tabulate coral, gastropod, sipunculid, Devonian, Hamilton Group, New York.     

Quantifying coral morphology

Coral morphology has important implications across scales, from differences in physiology, to the environments they are found, through to their role as ecosystem engineers. However, quantifying morphology across taxa is difficult, and so morphological variation is typically captured via coarse growth form categories (e.g. arborescent and massive). In this study, we develop an approach for quantifying coral morphology by identifying continuous three-dimensional shape variables. To do so, we contrast six variables estimated from 152 laser scans of coral colonies that ranged across seven growth form categories and three orders of magnitude of size. We found that 88% of the variation in shape was captured by two principal components. The main component was variation in volume compactness (cf. convexity), and the second component was a trade-off between surface complexity and top-heaviness. Variation in volume compactness also limited variation along the second axis, where surface complexity and top-heaviness ranged more freely when compactness was low. Traditional growth form categories occupied distinct regions within this morphospace; however, these regions overlapped due to scaling of shape variables with colony size. Nonetheless, with four of the shape variables we were able to predict traditional growth form categories with 70 to 95% accuracy, suggesting that the continuous variables captured most of the qualitative variations implied by these growth forms. Distilling coral morphology into continuous variables that capture shape variation will allow for better tests of the mechanisms that govern coral biology, ecology and ecosystem services such as reef building and provision of habitat.

     

Sedation of an original data matrix as applied to paleoecology

Seriation of an original data matrix provides the paleoecologist with a simple and powerful tool for data manipulation. The data matrix lists the presences and absences of taxa in samples. The strategy is to generate a unidimensional sequence by rearranging the taxa and samples so that the presences are blocked along the matrix diagonal. The method is illustrated with two case studies from the Middle Devonian Hamilton Group of New York. The first represents a regressive interval and the seriation sequence summarizes the changes in fauna and samples which can be explained by falling sea level. The second example embraces a much wider range of variation. Seriation mainly recovers a first-order pattern related to depth and correlated variables. This seriation does not reveal parameters such as the nature of the substrate, turbidity, habitat stability and distance offshore. These data cannot be reduced to one dimension. In all cases, seriation yields results similar to those of cluster analysis and the first axis of multivariate ordinations frequently used by ecologists and paleoecologists. Seriation, paleoecology, Devonian, Hamilton Group, multivariate analysis .     

A test of a new technique illustrating faunal dominance trends: Application to the 'Trilobite Beds' interval of the Middle Devonian Wanakah Shale in western New York

Batt, R.J. 1995 11 30 A test of a new technique illustrating faunal dominance trends: Application to the 'Trilobite Beds' interval of the Middle Devonian Wanakah Shale in western New York. Lethaia , Vol. 28, pp. 245–258. Oslo. ISSN 0024–1164.
A new graphic technique illustrating species relative abundance trends through stratigraphic sections facilitates recognition of subtle temporal and geographic changes in environment. This technique is illustrated by application to three closely spaced sections through the 'Trilobite Beds' interval of the Wanakah Shale of western New York. Graphic representation of faunal data from bed-by-bed sampling of these sections allows recognition of water depth changes that may be related to small-scale cyclic eustatic sealevel fluctuations. Comparison of graphs for neighboring sections, aided by excellent chronostratigraphic control, allows delineation of seafloor topography. During deposition of the study interval, the seafloor was consistently shallowest at the western section and deepest at the middle site. □ Wanakah Shale, Hamilton Group, Devonian, Darien Center Submember, 'Trilobite Beds', Expanded Bandygram .     

Species-specific growth responses of favositid corals to soft-bottom substrates

Gibson, Michael A. & Broadhead, Thomas W. 1989 07 15: Species-specific growth responses of favositid corals to soft-bottom substrates. Lethaia , Vol. 22, pp. 287–299. Oslo. ISSN 0024–1164.
Species of favositid corals from the Upper Silurian and Lower Devonian of Tennessee, USA, exhibit structural modifications related to corallum geometry, interfacial skeletal material, and biotic associations that enabled them to survive in terrigenous mud rich environments. Favosites conicus Hall (Lower Devonian) had a flat, holotheca-covered base and a radial pattern of colony growth, but apparently had a short life span and may not have survived beyond the first reproductive cycle (monocarpous). It was adapted for living between major episodes of terrigenous mud influx. F. foerstei (Lower Devonian) had a convex, pseudoholotheca-covered base and a modified axial pattern of colony growth. Its large size, in comparison to that of F. conicus , suggests a longer lived colony (polycarpous), in which continued upward and outward growth enabled it to survive episodic sediment influx. F. forbesi (Upper Silurian) exhibited radial growth to form either (1) a globose corallum that was symbiotic with the stalks of living crinoids permitting the colony to live entirely above the substrate, or (2) a Gorallum with a steeply convex, holotheca-covered base that represents a bottom-dwelling colony in which the rate of growth probably only slightly exceeded the rate of sediment accumulation. * Functional morphology, astogeny, paleoecology, Tabulata .     

Evolution of erect helical colony form in the Bryozoa: phylogenetic, functional, and ecological factors

Erect helical colony forms have evolved at least six separate times within the Bryozoa, but only among those in which branches are composed of a single layer of feeding zooids. The four best known genera with helical colony forms evolved independently, and each occupied different benthic marine environments, achieved different growth habits, and utilized different aspects of an array of functional potentials resulting from the radially symmetrical colonies. Examination of the distribution of these four genera ( Archimedes , Bugula , Crisidmonea , and Retiflustra ) within a theoretical morphospace of hypothetical helical colony form reveals that each occupies its own characteristic region of morphospace, broadly overlapping in some dimensions but separated in others. The genera differ markedly in the branching densities within their filtration-sheet whorls, reflecting their phylogenetic legacies rather than constructional or functional constraints associated with helical growth. In contrast, all tend toward helices in which the radiating whorls of the unilaminate branches are held at an average of 50–60° to the central axis of the colony, and this may reflect a common functional optimum associated with the cilia-driven, auto-generated currents within the helix. The region of morphospace characterized by high values of surface area – i.e. helical geometries with branches orientated at very low angles to the central axis, and with very closely spaced whorls along the axis – is entirely empty of bryozoans, and these geometries apparently represent functionally unrealistic colony forms.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 235–260.     

Microatoll microbialites of Lake Clifton,Western Australia: Morphological analogues ofCryptozoön proliferum Hall,the first formally-named stromatolite

Summary The Linnaean nameCryptozo?n proliferum Hall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozo?n proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms. Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake Clifton and the type exposures ofCryptozo?n proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the B?tsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological growth was adjusted to contemporary sea level.     

Endolith associations and their relation to facies distribution in the Middle Devonian of New York State, U.S.A.

Vogel, Klaus, Golubic, Stjepko & Brett, Carlton E. 1987 07 15: Endolith associations and their relation to facies distribution in the Middle Devonian of New York State, U.S.A.
Microborings of endolithic organisms (e.g. algae, fungi) provide valuable indicators of paleoenvironments. The present study documents the distribution of microborings in fossil skeletal substrates from a spectrum of shelf to basinal facies in the Middle Devonian of New York. Endolithic boring in brachiopod shells and corals were prepared using special embedding-casting method and were examined and photographed with SEM. About 13 distinct types of microborings were discovered, presumably produced by microorganisms (pro- and eukaryotic), sponges, bryozoans and worms. Brachiopod holdfast etchings are reported for the first time from Paleozoic rocks. Microborings are non-randomly distributed among Middle Devonian facies. The highest diversity and abundance of borings occurs in diverse brachiopod and coral-bearing calcareous mudstones; which appear to represent shallow, well-aerated and low turbidity environments. Microboring diversity and frequency decreases markedly into mudstones and siltstones recording higher turbidity as well as into dark gray to black basinal shale facies. 'Leiorhynchus' brachiopods from probable deeper water, dysaerobic environments display very low diversity of micro-boring assemblages. They include brachiopod pedicle borings and one endolith which shows similarities to a modem cyanobacterium. If the latter is correctly identified, it brackets all of the studied facies within the photic zone.     

Late Devonian marine ecology and zoogeography of the central Appalachians and New York

Ecological and statistical analyses of the Late Frasnian (Late Devonian) fauna of the Java Group of New York reveals the presence of three benthic marine communities. Using sedimentological and stratigraphic criteria, the Java Group may be divided into five physical-environmental zones: nearshore marine, platform, prodelta, open shelf, and slope and basin. The Ambocoelia- Cariniferella, Tylothyris-Schizophoria . and C yrtospirifer-Douvillina Communities inhabited open shelf, outer platform - prodelta, and inner platform - nearshore habitats. respectively. Java species gradient curves overlap each other continuously, and species optima replace each other in a regular fashion along the onshore-offshore environmental gradient. No evidence for competitive exclusion between species or sharp habitat discontinuities (ecotones) is seen for Java ecosystems. The same three-fold division of communities, in equivalent habitats, can be seen in central Appalachian marine environments located approximately 500 km away along the ancient shoreline. The distribution of communities in this region appears to be directly related to the position and relative size of fluviodeltaic systems along the 500 km of ancient coastline. Palacoecology. communities, Braschiopoda. palaegeography. Devonian. Frasnian, Appalachians, New. York .     

Coral encrusted concretions: a key to recognition of a 'shale on shale' erosion surface

Coral encrusted septarian concretions have been found along an obscure diastem within the Penn Yan Shale Member (Genesee Group, Upper Devonian) near Canandaigua Lake, New York. These hiatus-concretions ( sensu Voigt) are found associated with an erosional discontinuity surface in the shale. The restriction of coral growth to exposed concretion and wood found on the erosion surface, indicates a differential substrate preference exhibited by these animals. Diagenesis, both prior to and following this erosion event, produced various physical structures whose time of formation can be estimated from observed cross-cutting relationships with one another. An erosion-related feature establishing an early-diagenetic age for septarial fractures within concretions is described, and the origin and significance of the diastem is discussed.     

A comparison between coral colonies of the genus Madracis and simulated forms

In addition to experimental studies, computational models provide valuable information about colony development in scleractinian corals. Using our simulation model, we show how environmental factors such as nutrient distribution and light availability affect growth patterns of coral colonies. To compare the simulated coral growth forms with those of real coral colonies, we quantitatively compared our modelling results with coral colonies of the morphologically variable Caribbean coral genus Madracis. Madracis species encompass a relatively large morphological variation in colony morphology and hence represent a suitable genus to compare, for the first time, simulated and real coral growth forms in three dimensions using a quantitative approach. This quantitative analysis of three-dimensional growth forms is based on a number of morphometric parameters (such as branch thickness, branch spacing, etc.). Our results show that simulated coral morphologies share several morphological features with real coral colonies (M. mirabilis, M. decactis, M. formosa and M. carmabi). A significant correlation was found between branch thickness and branch spacing for both real and simulated growth forms. Our present model is able to partly capture the morphological variation in closely related and morphologically variable coral species of the genus Madracis.     

A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan, and an inferred secondary occupant

Morris, P. J., Linsley, R. M. & Cottrell. J. F. 1991 01 15: A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan. and an inferred sceondary occupant. Lethaia , Vol. 24. pp. 55–67. Oslo. ISSN 0024–1164.
The high-spired gastropod Palaeozygopleuru (Loxonematacea) of the Hamilton Group (Middle Devonian) in New York State is often entrusted by the trepostomatous bryozoan Leptotrypella (Hetero-trypidae). The form of this enerustation leads us to infer a secondary occupant that dwelled in empty shells of Palaeozygopleura while the bryozoan grew upon them. Encrusted specimens usually have an open aperture, thin or no enerustation on the apertural side of the gastropod shell. and thick encrustation on the abapertural side. While the aperture is invariably open. the columellar and parictal lips of the aperture, and the apertural face of the first whorl are usually encrusted. While the gastropod is alive, this area rests upon the dorsal surface of the foot and the remainder of the shell rests upon the substratc. These encrustation patterns suggest that an occupant of the shell. other than the gastropexl. prevented overgrowth of the aperture and oriented the shell aperture-down. This allowed ahapertural growth of the bryozoan. The presence of thin encrustation on the apertural side of the shell is problematical. It requires either the presence of a secondary occupant capable of holding a thinly encrusted shell off the substrate, or the ability of the juvenile bryozoan colony to extend onto portions of the shell that were in contact with the substrate. The presence of a secondary occupant. such as a sipunculan worm. capable of lifting the shell, but usually resting it aperture-down on the substrate seems the most suitable explanation for the nature of the encrustation. Gastropoda. bryozoa. symbiosis. Devoniun. hermit crab, Sipuncula     

IMPORTANCE OF MACRO‐ VERSUS MICROSTRUCTURE IN MODULATING LIGHT LEVELS INSIDE CORAL COLONIES1

Adjusting the light exposure and capture of their symbiotic photosynthetic dinoflagellates (genus Symbiodinium Freud.) is central to the success of reef‐building corals (order Scleractinia) across high spatio‐temporal variation in the light environment of coral reefs. We tested the hypothesis that optical properties of tissues in some coral species can provide light management at the tissue scale comparable to light modulation by colony architecture in other species. We compared within‐tissue scalar irradiance in two coral species from the same light habitat but with contrasting colony growth forms: branching Stylophora pistillata and massive Lobophyllia corymbosa. Scalar irradiance at the level of the symbionts (2 mm into the coral tissues) were <10% of ambient irradiance and nearly identical for the two species, despite substantially different light environments at the tissue surface. In S. pistillata, light attenuation (90% relative to ambient) was observed predominantly at the colony level as a result of branch‐to‐branch self‐shading, while in L. corymbosa, near‐complete light attenuation (97% relative to ambient) was occurring due to tissue optical properties. The latter could be explained partly by differences in photosynthetic pigment content in the symbiont cells and pigmentation in the coral host tissue. Our results demonstrate that different strategies of light modulation at colony, polyp, and cellular levels by contrasting morphologies are equally effective in achieving favorable irradiances at the level of coral photosymbionts.     

Jurassic corals as emersion indicators

In Recent coral reefs, emersion has resulted in the development of special growth forms of colonial corals which have been named microatolls. These colonies grow almost exclusively in a horizontal direction. In the upper part of the colony, subaerial exposure leads to the decay of the living body. The growth then stops and the skeleton is later colonized by various boring and encrusting organisms. Here we show the first record of a massive Mesozoic coral colony displaying the main features of emersion. The colony has been collected in perireefal Oxfordian (Jurassic) limestones from the Jura Mountains (France). It is situated exactly at the expected place in a shallowing-upward sequence between infralittoral buildups and supralittoral limestones. We suggest that such growth structures could be more common than previously thought in ancient coralliferous sediments and add some new details to discriminate between these colonies.     

Turbid reefs moderate coral bleaching under climate‐related temperature stress

Thermal‐stress events that cause coral bleaching and mortality have recently increased in frequency and severity. Yet few studies have explored conditions that moderate coral bleaching. Given that high light and high ocean temperature together cause coral bleaching, we explore whether corals at turbid localities, with reduced light, are less likely to bleach during thermal‐stress events than corals at other localities. We analyzed coral bleaching, temperature, and turbidity data from 3,694 sites worldwide with a Bayesian model and found that K_d490, a measurement positively related to turbidity, between 0.080 and 0.127 reduced coral bleaching during thermal‐stress events. Approximately 12% of the world's reefs exist within this “moderating turbidity” range, and 30% of reefs that have moderating turbidity are in the Coral Triangle. We suggest that these turbid nearshore environments may provide some refuge through climate change, but these reefs will need high conservation status to sustain them close to dense human populations.     

CAMPANIAN CRUSTACEAN BURROW SYSTEM FROM ISRAEL WITH BROOD AND NURSERY CHAMBERS REPRESENTING COMMUNAL ORGANIZATION

Abstract:  Phosphorite-filled crustacean burrows associated with a Campanian-age omission surface in the north-western Negev are described. The phosphatic burrow casts weather out displaying scratches (bioglyphs) and two types of local swellings (chambers), which are flattened normal to the course of the burrow. The more abundant chamber type is a flattened spheroid (diameter 45–50 mm) or a flattened, highly prolate ellipsoid of larger dimensions, with bioglyphs. The other type is a flattened spheroid (diameter 45 mm), gently rounded on the upper side and flat on the base. Rings of elevations on the cast (representing moats) form interconnected circlets, each capped by about eight rounded hemispherical tubercles (4 × 4 mm) (pits on original), the whole forming a discrete network. The first type of chamber may have hosted the young (nursery chamber) and/or stored food. The second type of cast replicates a chamber with a pitted floor, which may have formed a brood chamber for 60–70 spherical eggs, each about 3 mm in diameter. Brood chambers in crustacean burrow systems were previously suspected, but only at burrow terminations. The interpreted K-type breeding strategy, brood care and associated functions require a high degree of social organization, none of which has been observed in extant crustaceans, but all occur within social insects.     

Pleistocene reef environments, constituent grains, and coral community structure: Curaçao, Netherlands Antilles

We investigated the degree to which component grains vary with depositional environment in sediments from three reef habitats from the Pleistocene (125?ka) Hato Unit of the Lower Terrace, Curaçao, Netherlands Antilles: windward reef crest, windward back reef, and leeward reef crest. The windward reef crest sediment is the most distinctive, dominated by fragments of encrusting and branching coralline red algae, coral fragments and the encrusting foraminiferan Carpenteria sp. Windward back reef and leeward reef crest sediments are more similar compositionally, only showing significant differences in relative abundance of coral fragments and Homotrema rubrum. Although lacking high taxonomic resolution and subject to modification by transport, relative abundance of constituent grain types offers a way of assessing ancient skeletal reef community composition, and one which is not limited to a single taxonomic group. The strong correlation between grain type and environment we found in the Pleistocene of Curaçao suggests that constituent grain analysis may be an effective tool in delineating Pleistocene Caribbean reef environments. However, it will not be a sufficient indicator where communities vary significantly within reef environments or where evolutionary and/or biogeographical processes lead to different relationships between community composition and reef environment. Detailed interpretation of geological, biological, and physical characteristics of the Pleistocene reefs of Curaçao reveals that the abundance of the single coral species, Acropora palmata, is not a good predictor of the ecological structure of the ancient reef coral communities. This coral was the predominant species in two of the three reef habitats (windward and leeward reef crest), but the taxonomic composition (based on species relative abundance data) of the reef coral communities was substantially different in these two environments. We conclude that qualitative estimates of coral distribution patterns (presence of a key coral species or the use of a distinctive coral skeletal architecture), when used as a component in a multi-component analysis of ancient reef environments, probably introduces minimal circular reasoning into quantitative paleoecological studies of reef coral community structure.     

Morphological plasticity in scleractinian corals

When describing coral shape and form the term phenotypic plasticity, i.e. environment-induced changes in morphology, is often used synonymously with intraspecific variation. Variation, however, may simply be due to genetic differentiation (polymorphism). Of the 1314 extant scleractinian coral species, less than 20 have been tested for plastic responses. Morphological plasticity has important implications for coral identification, as skeletal features used in coral systematics are directly affected by environment. Furthermore, plastic changes can indicate how corals acclimatise to environmental change. The studies that have examined phenotypic plasticity in corals experimentally can be divided into two groups, i.e. 'non-clonal'—those that have transplanted whole colonies or fragments of colonies (but not treated the fragments as clones) to new environments, and 'clonal'—those that have transplanted colony fragments and used them as clone-mates. The use of clone-mates is preferable as it facilitates the identification of among-genotype variation for plasticity. The heterogeneous nature of the reef environment makes identifying the parameters that affect coral morphology difficult in the field, but there are also many problems conducting suitable aquarium experiments. Nevertheless, evidence to date suggests light and water movement are the most important variables inducing change. As these factors are known to be axiomatic to coral growth, it is possible that associated plastic changes in corals are adaptive; however, this hypothesis is yet to be tested rigorously.     

The effect of habitat on modern shark diversification

Sharks occupy marine habitats ranging from shallow, inshore environments to pelagic, and deepwaters, and thus provide a model system for testing how gross habitat differences have shaped vertebrate macroevolution. Palaeontological studies have shown that onshore lineages diversify more quickly than offshore taxa. Among onshore habitats, coral reef‐association has been shown to increase speciation rates in several groups of fishes and invertebrates. In this study, we investigated whether speciation rates are habitat dependent by generating the first comprehensive molecular timescale for shark divergence. Using phylogenetic comparative methods, we rejected the hypothesis that shelf (i.e. onshore) lineages have higher speciation rates compared to those occupying deepwater and oceanic (i.e. offshore) habitats. Our results, however, support the hypothesis of increased speciation rates in coral reef‐associated lineages within the Carcharhinidae. Our new timetree suggests that the two major shark lineages leading to the extant shark diversity began diversifying mostly after the end‐Permian mass extinction: the squalimorphs into deepwater and the galeomorphs into shelf habitats. We suggest that the breakdown of the onshore–offshore speciation rate pattern in sharks is mediated by success in deepwater environments through ecological partitioning, and in some cases, the evolution of morphological novelty.