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Ichnological evidence for the arthropod invasion of land
Abstract
Palaeozoic terrestrial trace fossils (particularly arthropod trackways and trails) provide valuable data on the landfall, and the subsequent diversification of early arthropods on land. This ichnological evidence indicates that the earliest invasion of land, evident from trackways from the Late Cambrian of Ontario, occurred around 90 million years before the earliest reliable terrestrial body fossils. Terrestrial trace fossils are generally rare in the Ordovician. Eurypterid trackways from New York State indicate that this group was capable of amphibious excursions (via marine routes) from the Late Ordovician. Narrow myriapod trails from northwest England, and burrows from Pennsylvania, indicate that this group occupied the early bryophyte soils (via freshwater margin routes) in the Late Ordovician. Terrestrial trace fossil diversity and distribution increase in the Early Devonian, indicating the major phase of colonisation of coastal and fluvial settings. The widespread colonisation of land continued throughout the Devonian, until all non-marine habitats were colonised by the Carboniferous. Explanations for the arthropod invasion of land are traditionally linked to the exploitation of under-utilised ecospace, or aquatic predator evasion. The evolution of land plants in the Ordovician represents a major ecological shift and is probably associated with the terrestrialisation of the myriapods. Other groups probably had different reasons, possibly associated with their life cycles; a "mass-moult-mate" hypothesis for eurypterid reproductory behaviour is supported by abundant accumulations of their exuviae in marginal settings, the functional morphology of their reproduction and respiration, trackway occurrence, and modern analogues (e.g. Limulus).
... Additional original sections of trackway beds were also distributed. Although Owen's description was detailed given the material he had to work with, the genus seems to have become a catch-all to describe any arthropod trackway with a single continuous or discontinuous medial telson mark (e.g., Braddy, 2004). Due to the apparent lack of body fossils in the Potsdam Sandstone, it falls to these and other types of traces (e.g., Climactichnites (Yochelson and Fedonkin, 1993) to elucidate the paleoecology of the Cambro-Ordovician beachfront. ...
... Due to the apparent lack of body fossils in the Potsdam Sandstone, it falls to these and other types of traces (e.g., Climactichnites (Yochelson and Fedonkin, 1993) to elucidate the paleoecology of the Cambro-Ordovician beachfront. The presence of Protichnites in intertidal, low supratidal, and dune sand beds of the Potsdam seems to show some of the first terrestrialization efforts of arthropods Braddy 2004) in the Late Cambrian ( Figure 2). The Potsdam may also hold the first evidence of animal mating behavior, in the interaction implied by two Protichnites trackways (Erickson 2004). ...
... Since fully terrestrial forms have been found in the Middle Silurian of Scotland, the transition must have begun quite a bit earlier (Braddy 2004). The body fossil record is limited during this period, so behaviour and therefore functional morphology must be examined through trace fossils. ...
Late Cambrian arthropod trackways from the Potsdam Sandstone have been
known since the 1850’s. A site in northern New York is an outcrop of fine-grained,
quartz-rich, rippled, micro-laminated Potsdam Sandstone. A similar site near Kingston,
Ontario, has been described as the first evidence of land animals. Our study area includes
evidence of microbial mat growth on the original surface on which the trackways were
produced. Ripple marks presumably underlay and therefore were generated prior to the
microbial mat. Preservation of these trackways is variable over the outcrop and is
indicative of a high intertidal or low supratidal environment with microbial growth.
At least eleven distinctive trackways of multi-legged telson-bearing individuals
are present with a roughly bimodal size distribution (widths of 11.6 cm, 6.5 cm, 10 cm
and 7.2 cm). A disturbance at the intersection of trackways 1 and 2 has been interpreted to
show the earliest evidence of invertebrate mating activity (Erickson, 2004). Trackway 1
(11.6 cm wide) consists of repeated series of seven pairs of imprints arranged in a
chevron pattern. The organism was traveling in the direction of the convergence of the
chevron pattern.
The trackways are consistent in number of imprints per series (leg number?) and
stride lengths with members of the original descriptions of Protichnites Owen, 1852,
although a tridactyl condition cannot be recognized on any digit. Variable preservation
probably resulted from varying thickness of the microbial mat and/or varying water depth
or wind and wave action in an intertidal pool.
... Trace fossils have previously been reported from Cambrian non-marine settings (Braddy, 2004), but the depositional environments have been subsequently reinterpreted as marginal marine or brackish (Braddy, 2004;Buatois et al., 1998Buatois et al., , 2005Maples and Archer, 1989). All Cambrian marginal marine examples, including those reported from the lowermost Cambrian Rusophycus avalonensis zone, contain arthropod trackways and other complex burrows that are typical of shallow-marine settings, and thus do not represent unique marginal marine or nonmarine assemblages (Braddy, 2004;Buatois et al., 1998Buatois et al., , 2005MacNaughton and Narbonne, 1999;Mángano and Buatois, 2004;Maples and Archer, 1989); they are distinctly different from the tiny Skolithos and Arenicolites described here and are interpreted to have resources largely marine in origin (Mángano and Buatois, 2004). ...
... Trace fossils have previously been reported from Cambrian non-marine settings (Braddy, 2004), but the depositional environments have been subsequently reinterpreted as marginal marine or brackish (Braddy, 2004;Buatois et al., 1998Buatois et al., , 2005Maples and Archer, 1989). All Cambrian marginal marine examples, including those reported from the lowermost Cambrian Rusophycus avalonensis zone, contain arthropod trackways and other complex burrows that are typical of shallow-marine settings, and thus do not represent unique marginal marine or nonmarine assemblages (Braddy, 2004;Buatois et al., 1998Buatois et al., , 2005MacNaughton and Narbonne, 1999;Mángano and Buatois, 2004;Maples and Archer, 1989); they are distinctly different from the tiny Skolithos and Arenicolites described here and are interpreted to have resources largely marine in origin (Mángano and Buatois, 2004). ...
... Trace fossils have previously been reported from Cambrian non-marine settings (Braddy, 2004), but the depositional environments have been subsequently reinterpreted as marginal marine or brackish (Braddy, 2004;Buatois et al., 1998Buatois et al., , 2005Maples and Archer, 1989). All Cambrian marginal marine examples, including those reported from the lowermost Cambrian Rusophycus avalonensis zone, contain arthropod trackways and other complex burrows that are typical of shallow-marine settings, and thus do not represent unique marginal marine or nonmarine assemblages (Braddy, 2004;Buatois et al., 1998Buatois et al., , 2005MacNaughton and Narbonne, 1999;Mángano and Buatois, 2004;Maples and Archer, 1989); they are distinctly different from the tiny Skolithos and Arenicolites described here and are interpreted to have resources largely marine in origin (Mángano and Buatois, 2004). Skolithos is typical in younger Phanerozoic active fl uvial channel facies (Fitzgerald and Barrett, 1986), where it commonly occurs with Arenicolites (Bromley and Asgaard, 1991) as an opportunistic assemblage in a manner similar to the occurrence described here. ...
Here we describe the oldest evidence of non-marine animals from the early Cambrian Wood Canyon Formation, California, evidence created by metazoans of a variety of sizes and behaviors. Millimeter-sized vertical trace fossils, including the U-shaped burrow Arenicolites and the vertical burrow Skolithos, as well as a centimeter-scale horizontal trace fossil, occur in conglomerate and gritty arkosic sandstone bed tops within fluvial channels. These fossils demonstrate that animals were dwelling in this habitat coincident with, or possibly predating, the first trilobites, and extend the freshwater record of animals back at least 80 m.y. The development of a functioning terrestrial ecosystem was concurrent with the early Cambrian marine radiation and suggests that freshwater environments were populated early by metazoans and that ecological opportunity likely played a determining role in metazoan exploitation of non-marine habitats versus commonly assumed influences from physiological or nonbiological barriers.
... Arthropod trackways characterise an epifauna of eurypterids and myriapods, with arthropods also producing resting and foraging traces (Morrissey et al., 2012). The ichnocoensis is typical of other Lower Palaeozoic marginal marine-brackish water settings (Braddy, 2005;Buatois et al., 2005;Minter et al., 2016). The wrinkle structures and millimetre ripples were probably microbially generated. ...
... The ichnology appears typical of other examples from Lower Palaeozoic transitional marine/coastal settings (e.g. Braddy, 2005;Hillier and Morrissey, 2010;Morrissey et al., 2012). ...
Ludlow to Přídolí age Old Red Sandstone alluvial to marginal marine deposits of the Anglo-Welsh Basin record a punctuated molasse shed from an inverted, transient catchment area. An erosional low-gradient sequence-bounding pediment surface was created basinward of a structurally controlled, transpressive piedmont junction. A thin mantle of debris flow dominated colluvium comprising the Abercyfor Formation was deposited proximal to a lowstand delta shoreline 90 km to the east and south. Temeside Mudstone Formation coastal plain deposits developed behind the delta passing westward into range front, dryland red bed alluvium of the Moor Cliffs Formation. This embryonic mudstone-dominated alluvial plain aggraded during an interval of enhanced accommodation space development. Muds were liberated from their Lower Palaeozoic bedrock provenance within and to the north of the piedmont junction. Dryland alluvial sandstone channels possibly represent stream capture events associated with hinterland drainage reorganisation. The latter promoted continued headward erosion and enlargement of the catchment area. Basinwide incision (to a depth of approximately 60 m)of the alluvial plain occurred in the mid Přídolí, with valley locations being structurally controlled. Subsequent fills preserve the middle reaches of a simple incised valley system. Lowstand to early transgressive streamflow conglomerates of the Freshwater East Formation were deposited, with sediment transport to the SE. Valley drainage was defeated by neotectonic uplift of the Orielton Anticlinorium in downstream valley reaches leading to drainage reorganisation. Accommodation space was developed at structural tip points where maximum rates of marine transgression ensued preserving backstepping parasequences of a wave dominated estuary. Localised debritic alluvial fans were shed below the interval of maximum flooding. Continued dryland sedimentation led to highstand filling and subsequent basinwide alluvial regression which continued into the late Přídolí.
... These estimates agree with the oldest known body fossils of arachnids (Silurian) 16 , but pre-date the oldest known terrestrial myriapods (middle Silurian) 16 and hexapods (Early Devonian) 18 . Trace fossils (known also as ichnofossils: the burrows, trails and trackways left behind by the activities of animals) provide a unique, but complementary, alternative record to molecular clock or body fossil data that further refines our understanding of the advent of motile life on land [19][20][21][22] . The oldest known non-marine trackways, from rocks of middlelate Cambrian age 20 , include those of a myriapod-like animal that are more congruent with molecular clock estimates of terrestrial myriapod origins. ...
... Another likely initial driver of terrestrialization was predator avoidance 37 . Reproduction, particularly for establishment of mating sites, has been suggested as an alternative hypothesis 22 . In addition to these potential causative mechanisms, the colonization of continental environments took place during an interval of Earth history when landscapes were being fundamentally and irreversibly altered by land plant evolution. ...
The colonization of land was one of the major events in Earth history, leading to the expansion of life and laying the foundations for the modern biosphere. We examined trace fossils, the record of the activities of past life, to understand how animals diversify both behaviourally and ecologically when colonizing new habitats. The faunal invasion of land was preceded by excursions of benthic animals into very shallow, marginal marine environments during the latest Ediacaran period and culminated in widespread colonization of non-marine niches by the end of the Carboniferous period. Trace fossil evidence for the colonization of new environments shows repeated early burst patterns of maximal ichnodisparity (the degree of difference among basic trace fossil architectural designs), ecospace occupation and level of ecosystem engineering prior to maximal ichnodiversity. Similarities across different environments in the types of behavioural programme employed (as represented by different trace fossils), modes of life present and the ways in which animals impacted their environments suggest constraints on behavioural and ecological diversification. The early burst patterns have the hallmark of novelty events. The underlying drivers of these events were probably the extrinsic limitation of available ecospace and intrinsic controls of genomic and developmental plasticity that enabled trace-maker morphological and behavioural novelty.
... These estimates agree with the oldest known body fossils of arachnids (Silurian) 16 , but pre-date the oldest known terrestrial myriapods (middle Silurian) 16 and hexapods (Early Devonian) 18 . Trace fossils (known also as ichnofossils: the burrows, trails and trackways left behind by the activities of animals) provide a unique, but complementary, alternative record to molecular clock or body fossil data that further refines our understanding of the advent of motile life on land [19][20][21][22] . The oldest known non-marine trackways, from rocks of middlelate Cambrian age 20 , include those of a myriapod-like animal that are more congruent with molecular clock estimates of terrestrial myriapod origins. ...
... Another likely initial driver of terrestrialization was predator avoidance 37 . Reproduction, particularly for establishment of mating sites, has been suggested as an alternative hypothesis 22 . In addition to these potential causative mechanisms, the colonization of continental environments took place during an interval of Earth history when landscapes were being fundamentally and irreversibly altered by land plant evolution. ...
The colonization of land was one of the major events in Earth history, leading to the expansion of life and laying the foundations for the modern biosphere. We examined trace fossils, the record of the activities of past life, to understand how animals diversify both behaviourally and ecologically when colonizing new habitats. The faunal invasion of land was preceded by excursions of benthic animals into very shallow, marginal-marine environments during the latest Ediacaran Period and culminated in widespread colonization of non-marine niches by the end of the Carboniferous Period. Trace-fossil evidence for the colonization of new environments shows repeated early-burst patterns of maximal ichnodisparity (the degree of difference among basic trace-fossil architectural designs), ecospace occupation, and level of ecosystem engineering prior to maximal ichnodiversity. Similarities across different environments in the types of behavioural programmes employed (as represented by different trace fossils), modes of life present, and the ways in which animals impacted their environments, suggest constraints on behavioural and ecological diversification. The early-burst patterns have the hallmark of novelty events. The underlying drivers of these events likely were the extrinsic limitation of available ecospace and intrinsic controls of genomic and developmental plasticity that enabled trace-maker morphological and behavioural novelty.
... Note absence of breaching of adjacent portion of ripple crest. diversity is low within Cambrian intertidal-supratidal, eolian, and continental environments (see the reviews in MacNaughton et al., 2002;Braddy, 2004;Mángano and Buatois, 2004) and, thus, bedding-plane bioturbation indices are also likely to be low in these settings. Preliminary ichnofabric data from Furongian sandstones of Wisconsin and New York suggests that minimal vertical or horizontal bed disruption occurred in these facies (Bjerstedt and Erickson, 1989;Hagadorn et al., 2003; Table 1). ...
... Preliminary ichnofabric data from Furongian sandstones of Wisconsin and New York suggests that minimal vertical or horizontal bed disruption occurred in these facies (Bjerstedt and Erickson, 1989;Hagadorn et al., 2003; Table 1). Marginal-marine ichnofossil diversity increases in the Ordovician, and by the Early Devonian (Braddy, 2004;Mángano and Buatois, 2004) reaches levels as high as the Cambrian subtidal trace-fossil record STRANDED IN UPSTATE NEW YORK (Crimes et al., 1977;Crimes, 1992). By the time ichnodiversity is high in marginal-marine settings, soft-bodied preservation and microbial binding should be absent. ...
The Cambrian portion of the Potsdam Sandstone contains a suite of scyphomedusae impressions in fine-grained to medium-grained quartz arenites that outcrop on the periphery of the Adirondack Mountains, New York. The fossils are similar taphonomically and morphologically to coeval scyphomedusae from the Elk Mound Group of Wisconsin and were likely stranded on a sand flat. Soft-tissue preservation in such sandstones is rare, except in Ediacaran Konservat-Lagerstatten. Although subtidal facies are abundant and continental facies are present in the Potsdam, soft-bodied fossils are found only in emergent coastal facies. These units are characterized by microbial structures including domal sand buildups, sand shadows, and breached ripples and by such horizontal trace fossils as Climactichnites and Protichnites. Domal sand buildups mantle some medusa carcasses and suggest that carcasses were exposed at the sediment-water or sediment-air interface for significant intervals of time prior to burial. It is unknown if microbial binding mediated preservation of these carcasses, but evidence for rapid flow regime changes in the section suggest stranded medusae were resistant to the upper-flow regime deposition that buried them. In many Laurentian Cambrian sandstones, microbial binding is common, and metazoan bioturbation is minimal in intertidal and emergent facies. The Potsdam Sandstone, thus, exemplifies how the Ediacara-style taphonomic window persisted in emergent Cambrian settings. This preservational regime may persist because bioturbating metazoans did not fully colonize tidal flats until the Middle Ordovician, which allowed soft-bodied animal tracks and carcasses to be preserved without scavenging or disturbance.
... The landward route via marginal-marine, brackishwater environments may have been employed more extensively all through the Paleozoic, with the search for food, reduced competition, predator avoidance, and reproduction all being possible drivers (Shear and Kukalová-Peck 1990;Braddy 2001;Minter et al. 2016aMinter et al. , 2017. Ichnologic evidence shows a protracted colonization of marginal-marine environments, involving a sustained increase in degree of bioturbation and trace-fossil diversity, as well as expansion in the number of organisms capable of dealing with the stresses of brackish-water settings (Buatois et al. , 2005Minter et al. 2016aMinter et al. , 2017Shillito and Davies 2020;Mángano et al. 2021). ...
Synopsis
The invasion of the land was a complex, protracted process, punctuated by mass extinctions, that involved multiple routes from marine environments. We integrate paleobiology, ichnology, sedimentology, and geomorphology to reconstruct Paleozoic terrestrialization. Cambrian landscapes were dominated by laterally mobile rivers with unstable banks in the absence of significant vegetation. Temporary incursions by arthropods and worm-like organisms into coastal environments apparently did not result in establishment of continental communities. Contemporaneous lacustrine faunas may have been inhibited by limited nutrient delivery and high sediment loads. The Ordovician appearance of early land plants triggered a shift in the primary locus of the global clay mineral factory, increasing the amount of mudrock on the continents. The Silurian-Devonian rise of vascular land plants, including the first forests and extensive root systems, was instrumental in further retaining fine sediment on alluvial plains. These innovations led to increased architectural complexity of braided and meandering rivers. Landscape changes were synchronous with establishment of freshwater and terrestrial arthropod faunas in overbank areas, abandoned fluvial channels, lake margins, ephemeral lakes, and inland deserts. Silurian-Devonian lakes experienced improved nutrient availability, due to increased phosphate weathering and terrestrial humic matter. All these changes favoured frequent invasions to permament establishment of jawless and jawed fishes in freshwater habitats and the subsequent tetrapod colonization of the land. The Carboniferous saw rapid diversification of tetrapods, mostly linked to aquatic reproduction, and land plants, including gymnosperms. Deeper root systems promoted further riverbank stabilization, contributing to the rise of anabranching rivers and braided systems with vegetated islands. New lineages of aquatic insects developed and expanded novel feeding modes, including herbivory. Late Paleozoic soils commonly contain pervasive root and millipede traces. Lacustrine animal communities diversified, accompanied by increased food-web complexity and improved food delivery which may have favored permanent colonization of offshore and deep-water lake environments. These trends continued in the Permian, but progressive aridification favored formation of hypersaline lakes, which were stressful for colonization. The Capitanian and end-Permian extinctions affected lacustrine and fluvial biotas, particularly the invertebrate infauna, although burrowing may have allowed some tetrapods to survive associated global warming and increased aridification.
... Euthycarcinoid arthropods have been suggested as likely producers of Protichnites (e.g., MacNaughton et al., 2002;Collette et al., , 2012. Both of these ichnogenera have been documented from deposits representing marginal marine, freshwater, and subaerial (high intertidal, aeolian dune) settings (MacNaughton et al., 2002;Braddy, 2004;Hagadorn and Seilacher, 2009;Hagadorn et al., 2011a, b). ...
Roadcuts, core, cliffs and even mountainside exposures often limit the lateral extent to which we can assess bioturbation on bedding planes, simply because exposures of such surfaces are generally rare or small. Such constraints may hamper paleoenvironmental and paleoecological interpretations drawn from facies where most bioturbation is bed-parallel or patchily distributed. To explore how acute such constraints are, we conducted a pilot study of bioturbation in three exceptionally large (>5 m²), well-exposed, paleoenvironmentally similar bedding planes from the Cambrian of Wisconsin. The goal was to test the hypothesis that trace fossils and biogenic sediment disruption can be distributed heterogeneously at the scale of meters to decameters. Mapping the size variation and distribution of trace fossils across these bedding planes reveals variable patchiness. Individual bioturbated patches tend to be large (m-scale), but the density of biogenic sediment disruption within these patches is quite heterogeneous. In addition, bioturbated areas are separated by zones of undisturbed sediment that vary in size from <0.5 m² to >3 m². These observations suggest that ancient bedding planes, like modern substrate surfaces, can exhibit considerable lateral bioturbation heterogeneity at a range of spatial scales. Caution is thus urged when extrapolating the distribution and diversity of trace fossils and other biogenic sedimentary structures across broad sedimentary horizons based on small (≤1 m²) bedding plane exposures.
... But terrestrial arthropods may have existed earlier, as suggested by trackways in coastal dune strata from the Middle Cambrian of New York (c.500 Ma; Hagadorn et al. 2011) and the Late Cambrian-Early Ordovician of Ontario (MacNaughton et al. 2002;Braddy 2004;Collette & Hagadorn 2010;Collette et al. 2012), and by molecular time trees (Rota-Stabelli et al. ...
Arthropods are the first animals to colonize land habitats, with myriapods and arachnids having done so at least by the Silurian. Much later several lineages of Pancrustacea (hexapods and the paraphyletic crustaceans) also venture onto land: the hexapods by the Early Devonian, and later at least four other groups of pancrustaceans, namely isopods, amphipods, ostracods and decapods, which most generally colonised the continental water bodies. All faced a series of challenges, in particular gas exchange, desiccation, reproduction, osmoregulation and exposure to ultraviolet radiation, resulting in many morphological, physiological and ecological adaptations. Nonetheless, whether they reached land via salt or freshwater remains poorly documented, mainly because relevant localities are few. The Famennian (Late Devonian) Strud locality in Belgium provided an exceptional source of information on early aquatic continental ecosystems and their plant, vertebrate and arthropod colonizers at a crucial step in the terrestrialization process. Here, we review and update its crustacean fauna, which inhabited floodplain and temporary pool waters. New anatomical details of the notostracan Strudops goldenbergi Lagebro et al., 2015, as well as a new genus and species of spinicaudatan, are described. We also discuss the ecology of this unique, early continental ecosystem and its insights into the terrestrialization process.
... But terrestrial arthropods may have existed earlier, as suggested by trackways in coastal dune strata from the Middle Cambrian of New York (c.500 Ma; Hagadorn et al. 2011) and the Late Cambrian-Early Ordovician of Ontario (MacNaughton et al. 2002;Braddy 2004;Collette & Hagadorn 2010;Collette et al. 2012), and by molecular time trees (Rota-Stabelli et al. ...
Arthropods were the first known animals to colonise land habitats, with myriapods and arachnids having done so at least by the Silurian. Much later, several lineages of Pancrustacea (hexapods and the paraphyletic crustaceans) also ventured onto land; the hexapods by the Early Devonian, and later at least four other groups of crustaceans, namely isopods, amphipods, ostracods and decapods, most of which generally colonised the continental water bodies. All faced a series of challenges (in particular: gas exchange; desiccation; reproduction; osmoregulation; and exposure to ultraviolet radiation), resulting in many morphological, physiological and ecological adaptations. Nonetheless, whether they reached land via saltwater or freshwater remains poorly documented, mainly because relevant localities are few. The Famennian (Late Devonian) Strud locality in Belgium provided an exceptional source of information on early aquatic continental ecosystems and their plant, vertebrate and arthropod colonisers at a crucial step in the terrestrialisation process. Here, we review and update its crustacean fauna, which inhabited floodplain and temporary pool waters. New anatomical details of the notostracan Strudops goldenbergi Lagebro et al. , 2015, as well as a new genus and species of spinicaudatan, are described. We also discuss the ecology of this unique, early continental ecosystem and the insights it gives into the terrestrialisation process.
... However, whether these animals were habitually terrestrial or were aquatic animals sprinting from one pool to another to survive desiccation is not clear. Moreover, evidence for the sediment being exposed to the air (e.g., mud cracks) does not necessarily tell us whether the tracks were made under water and then exposed, or the mud was already drying and cracking when the trackway was made (Braddy, 2004). Nevertheless, a Cambro-Ordovician origin of terrestrial animals was suggested by Rota-Stabelli et al. (2013) using molecular clock analyses of extant Ecdysozoa. ...
The emergence of plants and animals from the sea onto land – terrestrialization – was one of the major advances in the history of life on Earth. Evidence for early colonization is poor, but the fossil record reveals that, by mid-Paleozoic times, complex terrestrial ecosystems had become established. The earliest evidence of excursions onto land by animals comes from trace fossils of arthropods crossing damp sediment in the late Cambrian. These animals were unlikely to have been truly terrestrial. The first animals onto land were probably those capable of being blown by the wind from the sea into damp habitats (i.e., possibly tardigrades), but there is no fossil evidence for this. The earliest body fossil evidence of land animals comes from millipedes from the Silurian of Scotland (ca. 428 Ma). Early terrestrial ecosystems were dominated mainly by detritivorous and predatory arthropods. Tetrapods appeared in the late Devonian, and were terrestrial by early Carboniferous times. It was not until the latest Paleozoic that modern-type terrestrial ecosystems, with abundant herbivores in the food chain, had evolved.
... Continental ichnofaunas display secular changes in bioturbation through geologic time (Miller, 1984;Mángano, 1993, 1998;Miller et al., 2002;Miller and Labandeira, 2003;Braddy, 2004). One of these changes is the increase in degree and depth of bioturbation through the Phanerozoic. ...
Continental ichnofaunas display a progressive increase in bioturbation depth and intensity through the Phanerozoic. Ichnologic data from Cenozoic fluvial deposits of the Chaco Basin, Subandean zone of Bolivia, indicate widespread colonization of deep infaunal ecospace by the Miocene. Trace fossils are described from the Tariquia Formation, which records deposition in anastomosed fluvial systems. Although the Tariquia ichnofauna is of low diversity and does not display significant compositional variations throughout the succession, ichnofabric analysis reveals some degree of variability linked to different taphonomic pathways that helps to understand depositional dynamics and environmental conditions during accumulation of this fluvial unit. Intense and deep bioturbation occurs in medium- to very fine-grained crevasse sandstone and overbank mudstone. Less pervasive bioturbation is recorded in deposits of abandoned main channels. The Tariquia ichnofauna is dominated by Taenidium barretti, representing an example of the Scoyenia ichnofacies. Overbank deposits are totally bioturbated (BI = 6), showing complete destruction of the primary sedimentary fabric. Main-channel and crevasse-splay sandstones display an upward increase in degree of bioturbation. The top of the channel and crevasse-splay sandstone represents colonization surfaces that allow direct measurements of maximum burrowing depth. Taenidium barretti extends up to 2.2 m into the crevasse sand sheets. Depth and intensity of bioturbation of the main-channel and crevasse sands seem to be a function of time between depositional events. Main-channel and crevasse sandstones underlying thick packages of bioturbated overbank mudstones are intensely bioturbated, recording prolonged periods of low-energy sediment fallout between crevassing events. Conversely, the lowest degree of bioturbation is found in amalgamated channel sandstone units underlying thin intervals of overbank mudstones, reflecting high-frequency depositional episodes.
... Late Carboniferous eurypterids were mostly amphibious and largely represented by the genus Adelophthalmus (Selden and Nudds, 2004), a form typically found with freshwater and terrestrial animals and plants (Plotnick, 1997). Terrestrial excursions by eurypterids were unlikely to be the result of food scavenging, as they lacked suitable adaptations for terrestrial feeding (Braddy, 2004). A more likely, albeit speculative, explanation is that these excursions were driven by environmental factors, such as the drying up of a formerly inhabited body of water (Donovan, 2001) or to mate (Braddy, 1999). ...
In Cañon del Cobre of Rio Arriba County, northern New Mexico, red-bed siliciclastic sediments of the Late Pennsylvanian-Early Permian Cutler Group were deposited by synorogenic fluvial systems in upland and inland settings along the flank of the Uncompahgre uplift of the ancestral Rocky Mountains. Rare lacustrine strata (thinly laminated siltstones) of the El Cobre Canyon Formation of the Cutler Group represent shallow, localized floodplain lakes of the late Paleozoic Pangean tropics. We document the first ichnofossil assemblage known from such an intermontane lake deposit, in strata of Late Pennsylvanian age. This ichnofossil assemblage consists of arthropod trackways (Diplichnites x Diplopodichnus, Diplichnites gouldi, cf. Paleohelcura tridactyla, Protichnites isp.), invertebrate grazing and feeding traces (Gordia indianensis and Helminthoidichnites tenuis), tetrapod footprints (Batrachichnus salamandroides and aff. Amphisauropus isp.) and fish swimming trails (Undichna britannica). The numerically dominant ichnofossils are Helminthoidichnites and Diplichnites; the other ichnotaxa are represented by few specimens. The Cañon del Cobre ichnofossil assemblage is readily assigned to the Scoyenia ichnofacies and is composed of members of the Diplichnites and Mermia ichnoguilds, consistent with a shallow lacustrine setting. The relatively low ichnodiversity of the Cañon del Cobre ichnofossil assemblage indicates a short-lived water body colonized by a mobile invertebrate epifauna of detritus feeders and predators that provided a food source for tetrapod predators.
... Friend et al. (1976, p. 66) discussed but rejected an arthropod tracemaker because "no fossil of such large arthropods are known from fresh-water strata of Devonian age." However, even larger arthropod trackways assigned to Diplichnites are now known from Devonian strata (e.g., Smith et al., 2003;Braddy, 2004). Therefore, I feel confident in identifying the Kap Graah trackways as arthropod made, and they likely are assignable to Diplichnites. ...
Devonian tetrapod tracks and trackways can be recognized by three criteria: morphology of the manus and pes impressions that matches known Devonian tetrapod skeletal morphology, manus smaller than pes, and the alternating trackway pattern that results from lateral sequence walking in quadrupedal tetrapod locomotion. The first reported Devonian tetrapod track, named Thinopus antiquus, from Pennsylvania, is not a tetrapod track and is likely an impression of a fish coprolite(s). A critical review of the published Devonian track record indicates only three can be verified as produced by a tetrapod trackmaker—Genoa River, Australia; Easter Ross, Scotland; and Valentia Island, Ireland. The supposed tetrapod tracks from the Middle Devonian of the Zachełmie quarry, Poland, fail the criteria for identification as Devonian tetrapod tracks. Indeed, no convincing case has been made that the Zachełmie structures are tetrapod tracks. Instead, they are reinterpreted as fish nests/feeding traces (ichnogenus Piscichnus). The oldest Devonian tetrapod trackway is Givetian and this is the oldest record of a tetrapod, but the sparse record of Devonian tetrapod tracks is of no other biostratigraphic and little paleobiogeographic significance. Bona fide Devonian tetrapod tracks are from nonmarine facies, so they do not support a marginal marine origin of tetrapods. They indicate lateral sequence walking and pelvic-limb-propelled, fully terrestrial (subaerial) locomotion in freshwater environments by at least some Devonian tetrapods.
... Trace fossils (ichnofossils) cover a broad spectrum of fossilised animal activity, including faeces, burrows and nests, as well as locomotion traces from individual footprints through to fully developed trackways left by animals walking over the substrate. A review of the ichnological evidence for early life on land can be found in Braddy (2004), and a recent comprehensive case study based on the Siluro-Devonian Old Red Sandstone of Britain was published by Morrissey et al. (2012). Ichnologists refer to trackways made in a terrestrial environment ( Fig. 16.2) as 'subaerial', differentiating them from those left in sediments under water. ...
Arthropods are, by a considerable margin, the most species-rich group of animals alive today and have long been a major component of the Earth’s biodiversity. Exact counts of the total number of species are not easy to come by, but Zhang (2011) offered a recent summary. Together the ca. 1,023,559 described living species of hexapods, 11,885 myriapods and 110,615 arachnids—most of which live on land—massively outnumber the ca. 66,914 recorded crustaceans, 1,322 sea spiders and the four species of horseshoe crab. Put bluntly, in terms of raw species numbers, the primarily terrestrial lineages (Hexapoda, Myriapoda, Arachnida) outnumber the primarily aquatic ones (‘Crustacea’, Pycnogonida, Xiphosura) by a factor of almost seventeen to one. In fairness, there is a degree of bias in these figures.
... In the initial phase of terrestrialization, extending from ,470 to 510 Ma, the tracemakers are unknown; but candidates include chasmataspids, euthycarcinoids, phyllocarids, or stem-group arthropods (Dunlop et al., 2004;. This early phase of colonization is primarily documented by trace fossils (Braddy, 2001(Braddy, , 2004. ...
Exposures of the middle Cambrian Potsdam Group of northern New York, including the type section, represent a suite of interfingering eolian dune and aquatic deposits that record the activities of early land-going arthropods. Quartz arenites at these exposures are dominantly fine to medium grained, well sorted, and have hematite-coated well-rounded, high-sphericity quartz grains characterized by secondary optically continuous quartz cement overgrowths. Eolian beds are laminated and dominated by m- to dm-scale foresets characterized by large-scale, laterally extensive cross-beds that dip >∼15°–34° and contain reverse-graded lamination, en-echelon microfaulted slumps, adhesion structures, and very high ripple-index asymmetric ripples that lie on the foresets with crests that trend downdip. Lower parts of foresets are interpreted as toesets, and contain unusual Diplichnites and Protichnites trackways, which record the uphill, downhill, crest-parallel, and switchback-style movement of arthropods in dry or damp sand. Trackways that ascend dune faces do not possess a medial tail drag, whereas trackways that descend do, and turns of trackways often have deep continuous medial impressions; these features suggest arthropods leaned into the slope when turning downhill in dry sand. More shallowly dipping (∼4°–15°) cross-beds are interpreted as bottomsets. Like nearby intertidal Potsdam deposits, these beds contain Protichnites trackways and Arenicolites burrows. Paleocurrent analyses imply a coastline in which offshore and mixed-direction winds moved dunes seaward. Considered together, these strata record migration of coastal dunes into aquatic environments and flooding and reworking of distal dune bottomsets. In this setting, the same suite of epifaunal arthropods inhabited dry, damp, and aquatic marine environments.
... Evidence for arthropod excursions on land presented here (Fig. 10) supports the view of Buatois et al. (1998) that early arthropods ventured on land to exploit new resources. This is not to deny alternative views of continental excursions into saline groundwater (Maples & Archer 1989), to avoid predation (Trewin & McNamara 1994) and to mate without being disturbed (Braddy 2004). Evidence of deep burrowing in early Cambrian coastal palaeosols (Retallack 2008) supports the idea that some animals lived below the water-table of coastal soils normally dry at the surface (Maples & Archer 1989). ...
Newly discovered trace and body fossils from the Grindstone Range Sandstone of South Australia reveal evidence of megascopic life on land during the Cambrian–Ordovician. Arthropod trackways (Diplichnites gouldi) are interpreted here to have formed on land. The most persuasive evidence for this view is that footprints vary in clarity along the length of the trackway as it traversed moist then dry silt, then biological soil crust. Compatible, though not diagnostic of walking on land is trackway symmetry, without one side buoyed up by current. The footprints bulge outward and are partially filled with miniature talus cones. Footprints also are alternate as in walking, rather than opposite as in sculling. Arthropod resting traces (Selenichnites sp. indet.) have 11 lateral furrows, and footprints are bundled into sets of 8–11, most like euthycarcinoids. No arthropod dwelling burrows were found in associated palaeosols, so the track maker was more likely amphibious than fully terrestrial. Associated trace fossils include a new ichnotaxon of burrow, Myrowichnus arenaceus gen. et sp. nov. Thallose impressions (Farghera robusta gen. et sp. nov.) have the radiating dichotomous form of lichens, algae and liverworts. All these trace and body fossils were found in weakly developed palaeosols. Other palaeosols in the same formation are evidence of terrestrial ecosystems of modest biomass, weathering, carbon sequestration and stability in dry tropical regions.
... Nonetheless, reference to the Juniata burrows appears in numerous reviews regarding evolutionary paleoecology. The Potters Mills locality has variously been cited as containing the oldest evidence for complex animal life on land (Signor and Vermeij, 1994;Kenrick and Crane, 1997;Bronger and Catt, 1998;Droser and Finnegan, 2003;McNamara, 2008), the oldest evidence for subaerial backfilled burrows (Donovan, 1994;MacNaughton et al., 1997;Braddy, 2004;Counts and Hasiotis, 2009), the oldest indirect evidence for the evolution of millipedes (Edgecombe, 2004;Melchor et al., 2006), the oldest evidence for paleosols created by the bio-invasion of the land (Hasiotis, 2002;Masuda and Ezaki, 2009), the oldest evidence for terrestrial detritivory (Anderson and Trewin, 2003;Habgood et al., 2004), and the oldest evidence for root-forming plants (Jutras et al., 2009). ...
The Juniata Formation comprises Upper Ordovician sandstone and mudstone that crops out in the Appalachian region of the eastern United States from Pennsylvania to Tennessee. An outcrop at Potters Mills, central Pennsylvania, has previously been attributed to a terrestrial environment. Because this outcrop contains numerous sub-vertical burrows and evidence for pedogenesis, it has regularly been cited as the oldest evidence for several aspects of continental ecosystem development, including the first evidence for terrestrial infauna and animal-plant interactions. We present evidence from both original fieldwork and published literature that collectively sheds considerable doubt on previous interpretations. The evidence suggests that the Juniata Formation at Potters Mills was deposited in a marginal marine setting and, as such, no evidence for early life on land can be inferred from its strata. This has significant implications for the numerous studies that have cited the Juniata Formation as providing a key record of early terrestrial evolution.Removing it from the dataset of studies that deal with the history of life on land, we conclude that currently the majority of fossil evidence from localities worldwide supports the appearance of terrestrial infauna and animal-plant interactions in the Silurian-Devonian.
... The presence of lingulids in some beds may indicate that the general environment was one of a coastal plain (Williams and Hillier, 2004) with brackish water pools or tidal creeks with marine animals, including semi-aquatic arthropods like eurypterids, periodically invading a predominantly freshwater environment. The trace fossil assemblage is typical of other examples from Lower Palaeozoic transitional marine/coastal settings dominated by epifaunal or shallow infaunal traces (Maples and Archer, 1989;Braddy, 2005), and it lacks trace fossils such as Zoophycos Massalongo, 1855 that are typical of fully marine environments. ...
The Upper Silurian/Lower Devonian Old Red Sandstone of the southwest Wales is dominated by fluvial sediments deposited in a mud-rich, low gradient, dryland setting that, apart from rare finds of fish head shields and spines, is lacking in fossil fauna. The Moor Cliffs Formation contains several fine-grained tuff horizons that act as regional markers for correlation, three of these are substantial in thickness (0.5–4 m), laterally persistent and contain abundant trace fossils. The tuffs record sudden volcanic events associated with the convergence of Avalonia with Laurentia, possibly accompanied by tsunami. The trace fossils preserved on the tops of individual falls include the locally profuse development of ovoid faecal pellets in close association with the tops of trumpet-shaped burrows. Surfaces also preserve arthropod locomotion and foraging traces (Palmichnium antarcticum, Diplopodichnus biformis, rare Cruziana sp., and bilobed trails). Vertical and horizontal burrows (Beaconites antarcticus) are common. Other traces found are microbially generated wrinkle marks (mat grounds) and “cauliflower” structures. Traces in the tuffs record a greater diversity of faunal activity than that observed in the encasing dryland environment sediments and indicate a possible preservational bias provided by deposition of the air fall tuffs or colonization of the tuff deposits by opportunist populations following phytoplankton blooms associated with the volcanic events. They remain the main indicators of biodiversity in this relatively fossil-poor continental stratigraphic interval.
... Goldring and Seilacher (1971) reported walking traces from sediments deposited at moderate depths while other workers insisted on their subaerial or shallow-water origin; or their formation in the absence of traveling waves (Caster, 1938;Rudloe and Herrnkind, 1980;Mickelson et al., 2006). Shoreline attachments of limuloidae instars have been documented by many workers (Botton et al., 2003;Braddy 2004 and references therein). Permian paleolimuloids are so far described as marine forms (cf. ...
Abstract: Present work is the first report of Early Permian xiphosurid trackways from India. Surface trackways and
undertracks, preserved within ice-marginal storm-affected, shoreface sediments of the Talchir Formation, Jayanti coalbasin
closely resemble Kouphichnium Nopsca 1923. Trackways reveal distinct heteropody with foliate pusher impressions,
paired series of semi-circular, oval, V- or Y-shaped chilate (walking) leg impressions. Varied ventral impressions also
include telson drag marks and telson dents; spine drag marks, book -gills impression, genal gouges as well as symmetric,
paired lateral genal furrows, partial cubichnial impressions of prosoma and spinose opisthosoma. A tentative early
Permian age (Asselian/Sakmarian) and marginal marine facies association points towards a paleolimuloidae affinity for
the trace makers, which is in conformity with their inferred widths of exoskeleton, sub-rounded prosoma and subtriangular
spinose opisthosoma with predictable movable spines. Flexed pusher impressions and co-dominant pincer impressions
as well as, other ventral impressions in abundance, confirm crawling as their dominant mode of locomotion. Inferred
presence of juvenile crawlers in the community suggests a probable shoreline or marginal marine ecospace.
Palmichnium gallowayi (Sharpe, 1932) new combination from the Middle Ordovician Martinsburg Formation (proximal deltaic facies) of Rondout, near Kingston, New York State, is redescribed. It consists of opposing series of five tracks, the outer two large and pear-shaped, the inner three smaller and elliptical, arranged in a chevron converging in the direction of travel, on either side of a wide medial impression. It is attributed to a medium-sized stylonurid eurypterid using a decapodous gait, crawling onto the shoreline, traversing the intertidal zone, a behavior interpreted as part of its reproductive life cycle. This provides the earliest ichnological evidence for the ‘mass-molt-mate’ hypothesis, which proposes that eurypterids migrated en masse into nearshore environments to molt and mate.
Ichnologic data provide insights into evolutionary paleoecology, including the nature of Ediacaran ecosystems, diversification events such as the Cambrian explosion and the Ordovician radiation, and the colonization of various habitats including brackish-water and continental environments. In many cases, trace fossil evidence demonstrates much greater evidence of ecologic change than that revealed by body fossils alone. Trace fossil distribution through geologic time reveals a process of colonization, resulting from the exploitation of empty or underutilized ecospace. Secular trends include increase in the diversity of biogenic structures, increase in the intensity of bioturbation, addition of new invaders, environmental expansion, and faunal turnovers. This chapter reviews the potential of ichnology to provide insights into five major issues in evolutionary paleoecology: ediacaran ecosystems, the Cambrian explosion, the Ordovician radiation, colonization of brackish water environments, and colonization of continental environments. The distribution of biogenic structures through geologic time reveals a process of colonization resulting from the exploitation of empty or under-utilized ecospace.
In geological terms, the Oslo region is a graben structure containing downfaulted fossiliferous, Lower Palaeozoic rocks in a belt 40-70 km in width and extending 115 km north and south of the city of Oslo. Shelly, graptolitic and early vertebrate faunas together with microfaunas and -floras offer a detailed biostratigraphy and time scale for the Caledonide tectonics and associated events. The provided correlation charts reflect a preferred Baltoscandian terminology for the Cambrian and Ordovician successions and a standard British system for the Silurian. Reference to recent biostratigraphic and sedimentological studies allows speculation on changes in sedimentary rates having both global and local causes based on the fact that the Oslo Region occupied an intermediate position between the stable platform to the east and the developing orogen to the west. Sedimentary rates, were high with dominantly mudstones and limestones and local thicknesses up to 1 km in the Ordovician and nearly twice this amount in the Silurian where siliceous rocks in a red-bed facies first appear around the Wenlock-Ludlow boundary. Caledonian tectonics in the Oslo Region activated the Osen-Røa detachment along which the major displacement was to take place. This structure underlies the entire Oslo Region, but dies out to the south in the Skien-Langesund area. In the Oslo Region, the Osen-Røa detachment lies within the late Cambrian Alum Shale and is developed as an intensely deformed thrust plane, from which numerous faults splay up-section into the Ordovician and Silurian strata, forming a duplex structure. Although the strain intensity decreases towards the south and towards the upper part of the Cambro-Silurian section, three major structural levels, in addition to the basal Osen-Røa detachment are identified. These are partly associated with flats of semi-regional significance in the nappe pile. The bulk transport is towards the south-southeast, but areas of southerly (Klekken area) and southeasterly transport (southern Ringerike) are also prominent. The detailed timing of the deformation is not well established, but the first sedimentary response to the growing mountain chain to the northwest is believed to be the fine-grained sandstones and siltstones of the Elnes Formation of late Middle Ordovician (Darriwilian) age, whereas the first siliciclastic sediments of more significant thickness date to the latest Ordovician. Finally, it is evident that the up to 1250 m-thick, Upper Wenlock-Lower Ludlow sandstones of the Ringerike Group have been affected by the contractional deformation, defining a maximum age for the latest Caledonian (Scandian) orogenic movements.
Late Silurian to Early Devonian deposits from the Lower Old Red Sandstone Anglo-Welsh Basin preserve a diverse assemblage of trace fossils that collectively comprise an important record of early terrestrialisation. Five main palaeoenvironmental categories are represented: (1) perennial rivers, (2) ephemeral rivers, (3) floodplain lakes, (4) alluvial fan, and (5) marginal marine. Ichnocoenoses mainly reflect the activity of aquatic and/or semi-aquatic animals. Fluvial deposits preserve evidence for colonisation by eurypterids, fish and myriapods. Ephemeral river channel margins/splays preserve the most diverse ichnofauna composed of deposit feeders' burrows and trackways produced by arthropods feeding and foraging in channel-margin soft grounds that were colonised after fluvial flood events. High densities of deposit-feeding structures may indicate of explosive population growth in response to periodic, acyclical replenishment of nutrients in these aquatic settings. Ichnotaxa common to both perennial and ephemeral fluvial environments suggest that there might have been only one aquatic ecosystem. The degree to which the channel-margin ichnofaunas record a truly subaerial terrestrial arthropod signature is difficult to ascertain and most trace makers were probably semi-aquatic. Some terrestrial animals invaded channel-margin areas to feed and were also capable of inhabiting high-energy, alluvial fan environments. However the fans contain low diversity ichnological assemblages and were likely to have been inhabited by an opportunistic fauna. Possibly the only truly subaerial terrestrial ichnocoenosis is that of pedogenically modified mudrock with Scoyenia, which represents an early soil ichnofauna. The majority of ichnocoenoses described from continental palaeoenvironments could be classified under the Scoyenia ichnofacies. Marginal marine deposits contain an impoverished Skolithos–Cruziana ichnofacies typical of Silurian–Devonian brackish water assemblages.
Late Cambrian arthropod trackways from the Potsdam Sandstone have been known since the 1850s. A site in Clinton County, New York, USA, exposes Protichnites in fine-grained, quartz-rich, rippled, micro-laminated Potsdam Sandstone. The study area includes evidence of microbial mat growth on the original surface where the trackways were produced. Ripple marks presumably underlie, and therefore were generated prior to, the microbial mat. Trackway preservation is variable over the outcrop and depositional setting indicates a high intertidal or a low supratidal environment with growth of benthic microbial mats. At least eleven distinguishable trackways of multi-legged, telson-bearing individuals show a range of widths. The trackways consist of repetitive sets of seven pairs of tracks converging in the direction of motion of the organism. A telson impression, either discontinuous or continuous, divides the trackways longitudinally and is nearly centered throughout the lengths of the trackways. The trackways are consistent in number of tracks per series, arrangement, and stride lengths with the ichnospecies holotype from the original description of Protichnites septemnotatus Owen, 1852. Variable preservation probably resulted from varying thickness of the microbial mat and/or varying water depth or wind and wave action in an intertidal pool.
The Paseky Shale (Lower Cambrian, Central Bohemia) yielded the oldest known non-marine or brackish ichnoassemblage. It consists mostly of fodinichnia and repichnia of endemic arthropods Kodymirus vagans Chlupac et Havlicek, 1965 and Kockurus grandis Chlupac, 1995. These traces were attributed to ichnotaxa Monomorphichnus biserialis ichnosp. nov., M. semilineatus ichnosp. nov., M. multilineatus Alpert, 1976, M. lineatus Crimes et al., 1977, M. bilinearis Crimes, 1970, ?Rusophycus ichnosp. A, ?R. ichnosp B, ?Dimorphichnus ichnosp., and Diplichnites ichnosp. Presence of non-arthropod fauna is suggested only by finds of ?Bergaueria ichnosp. and ?bromalites ichnogen. indet. Except for M. lineatus and M. multilineatus, ichnotaxa occurring typically in the Cambrian marine environments are not present. -Author
The combined study of continental trace fossils and associated sedimentary facies provides valuable evidence of colonization trends and events throughout the Phanerozoic. Colonization of continental environments was linked to the exploitation of empty or under-utilized ecospace. Although the nonmarine trace fossil record probably begins during the Late Ordovician, significant invasion of nonmarine biotopes began close to the Silurian-Devonian transition with the establishment of a mobile arthropod epifauna (Diplichnites ichnoguild) in coastal marine to alluvial plain settings. Additionally, the presence of vertical burrows in Devonian high-energy fluvial deposits reflects the establishment of a stationary, deep suspension-feeding infauna of the Skolithos ichnoguild. The earliest evidence of plant-arthropod interaction occurred close to the Silurian-Devonian boundary, but widespread and varied feeding patterns are known from the Carboniferous. During the Carboniferous, permanent subaqueous lacustrine settings were colonized by a diverse, mobile detritus-feeding epifauna of the Mermia ichnoguild, which reflects a significant palaeoenvironmental expansion of trace fossils. Paleozoic ichnologic evidence supports direct routes to the land from marginal marine environments, and migration to lakes from land settings. All nonmarine sedimentary environments were colonized by the Carboniferous, and subsequent patterns indicate an increase in ecospace utilization within already colonized depositional settings. During the Permian, back-filled traces of the Scoyenia ichnoguild record the establishment of a mobile, intermediate-depth, deposit-feeding in-fauna in alluvial and transitional alluvial-lacustrine sediment. Diversification of land plants and the establishment of ecologically diverse plant communities through time provided new niches to be exploited by arthropods. Nevertheless, most ot the evolutionary feeding innovations took place relatively early, during the Late Palaeozoic or early Mesozoic. A stationary deep unfauna, the Camborygma ichnoguild, was developed in Triassic transitional alluvial-lacustrinbe deposits. Terrestrial environments hosted the rise of complex social behavioral patterns, as suggested by the probable presence of hymenopteran and isopteran nests in Triassic paleosols. An increase in diversity of trace fossils is detected in Triassic-Jurassic eolian deposits, where the ichnofauna displays more varied behavioral patterns than their Paleozoic counterparts. Also, a mobile, intermediate-depth, deposit-feeding infauna, the Vagorichnus ichnoguild, was established in deep lake environments during the Jurassic. In contrast to Paleozoic permanent subaqueous assemblages typified by surface trails, Jurassic ichnocoenoses are dominated by infaunal burrows. High density of infaunal deposit-feeding traces of the Planolites ichnoguild caused major disruption of lacustrine sedimentary fabrics during the Cretaceous. Most insect mouthpart classes, functional feeding groups, and dietary guilds were established by the end of the Cretaceous. Diversification of modern insects is recorded by the abundance and complexity of structures produced by wasps, bees, dung-beetles, and termites in Cretaceous-Tertiary paleosols. The increase in bioturbation migrated from fluvial and lake-margin settings to permanent subaqueous lacustrine environments through time.
New millipede specimens from the Paleozoic of Scotland are described, including Archidesmus macnicoli Peach, 1882, from the Lower Devonian (Lochkovian) Tillywhandland Quarry SSSI and three new taxa—Albadesmus almondi, Pneumodesmus newmani, and Cowiedesmus eroticopodus—from the mid Silurian (late Wenlock—early Ludlow) Cowie Formation at Cowie Harbour. Cowiedesmus eroticopodus new species is placed within the new Cowiedesmidae within the new order Cowiedesmida. Kampecaris tuberculata Brade-Birks from the Lower Devonian (Siegenian) of the Lanark Basin near Dunure is shown not to be a kampecarid myriapod, redescribed as Palaeodesmus tuberculata and placed order incertae sedis within Archipolypoda. Anthracodesmus macconochiei Peach is also redescribed and tentatively placed order incertae sedis within Archipolypoda. Archidesmus macnicoli, Albadesmus almondi, and Palaeodesmus tuberculata are each demonstrated to have broad sternites with laterally placed coxal sockets and paramedian pores containing paired valves. These pores are interpreted as having housed eversible vesicles. Some specimens of Archidesmus macnicoli and Cowiedesmus eroticopodus are male and have a pair of modified legs on trunk segment 8, identified as leg pairs 10 and 11, respectively. The presence of modified anterior legs restricted to segment 8 increases the range of variability known in modified appendage location in male millipedes and compounds existing uncertainty about using the presence of gonopods on trunk segment 7 as a synapomorphy of Helminthomorpha. An affinity between Archidesmida and Cowiedesmida is suggested based on possession of modified legs on segment 8 and Archidesmida + Cowiedesmida is placed along with Euphoberiida in Archipolypoda based on possession of free, broad sternites with bivalved paramedian pores and fused pleurotergites. The oldest known evidence of spiracles is demonstrated in Pneumodesmus newmani, proving that the oldest known millipedes were fully terrestrial.
Paleozoic nonmarine trace fossils evidence paleoenvironmental trends
that are linked directly to progressive ecospace utilization (spatial
and functional). These trends resulted in a considerable increase of
ichnodiversity throughout the Paleozoic. Colonization of nonmarine
settings began as early as the Late Ordovician, as recorded by the
presence of terrestrial burrow systems associated with ancient soils. In
the other cases, the presence of "nonmarine" trace fossils in Cambrian
and Ordovician sedimentary rocks was the result of temporary
colonization by marginal marine organisms and does not reflect
establishment of true fresh-water faunas. Devonian trace fossils have
been reported from alluvial and alluvial-lacustrine transitional
environments. Surface arthropod trackways are dominant in marginal lake
settings, representing the development of true nonmarine biotas. The
presence of vertical burrows in fluvial deposits records the
establishment of deep suspension-feeding infaunal organisms. Trace
fossils from the Carboniferous reflect migration farther into the lakes.
Ichnofossils have been reported from shallow and deep lake deposits and
from alluvial and alluvial-lacustrine transitional environments. Lake
assemblages are dominated by surface grazing trails. The Permian was
characterized by the establishment of a deep deposit-feeding infauna,
commonly recorded by the Scovenia ichnofacies
represented by backfilled meniscate burrows.
Trackways are reported for the first time from Silurian rocks in Newfoundland. They occur in the Clam Bank Formation of the Port au Port Peninsula. At least one of the trackways represents one of the earliest known occurrences of a large subaerial arthropod trace fossil. It is preserved in hyporelief in a red fine-grained sandstone. The trackway, Diplichnites Dawson, 1873, consists of two parallel series of clearly defined imprints. It is attributed to a myriapod similar to the Lower Devonian Eoarthropleura. The sandstone contains halite pseudomorphs that indicate that the sediment horizon had been subject to desiccation immediately prior to the formation of the trackway. The arthropod walked on a red siltstone deposited in an alluvial flats environment. Analysis of the trackway suggests that the arthropod was approximately 23 cm long and had 17 pairs of walking appendages. It appears that a high-geared gait with a ratio of forestroke:backstroke of 8:2 was used in walking across the substrate, that is, approximately 20% of walking appendages were in contact with the substrate at any one time. This rapid gait, and the lack of a body drag mark, indicates that the arthropod was well adapted to subaerial locomotion. Knowledge of modern myriapod gaits has been used to extrapolate a theoretical trail, which compares well with the fossil trackway. Smaller, less well preserved trackways occur elsewhere in the Clam Bank Formation and may have been made by a similar arthropod.
A diverse arthropod-dominated ichnofauna, associated with a poorly preserved crustacean fauna and soft-bodied ?medusoid impressions, is described from the Blaiklock Glacier Group of the north-western Shackleton Range (Coats Land), Antarctica. The ichnofauna consists of Asaphoidichnus, Beaconites, Didymaulichnus, Diplichnites, Gordia, ?Laevicyclus, Merostomichnites, Monomorphichnus, Palaeophycus, Planolites, Rusophycus, Selenichnites , and Taphrhelminthoides (ichnogen nov.). Three new ichnotaxa are recognised: Taphrhelminthoides antarcticus n. ichnogen. et ichnosp. is a bilobate trail, composed of two parallel flat lobes, separated by a median ridge with a characteristic figure-of-eight pattern. Merostomichnites gracilis n. ichnosp. is characterised by its proportions (external:internal width ratio >3) and series of 10 to 12, thin, linear tracks. Selenichnites antarcticus n. ichnosp. is characterised by small elongate horseshoe-shaped marks, the medial portion showing three to five transverse scratch-marks.
The palaeoenvironment is interpreted as extremely shallow marine water, possibly a tide-dominated estuary, based on sedimentological evidence and the composition of the ichnofauna. Radiometric and palaeomagnetic data indicate that this assemblage is Lower Ordovician in age, representing the first autochthonous Ordovician fossiliferous succession to be described from Antarctica. The succession shows several sedimentological and palaeontological similarities with the basal units of the Ordovician Table Mountain Group in South Africa, supporting palaeogeographic models placing the Palaeozoic Blaiklock basin close to the Ordovician Table Mountain basin.
Fossil burrows within newly recognized buried soils in the Late Ordovician Juniata Formation, near Potters Mills in central
Pennsylvania, represent the oldest reported nonmarine trace fossils. They are thought to have been an original part of the
soil because their greater density toward the top of the paleosols corresponds with mineralogical, microstructural, and chemical
changes attributed to ancient weathering and because about half the burrows are encrusted with nodular carbonate, interpreted
as caliche. Associated fossil caliche, the size distribution of the burrows, and their W-shaped backfills are evidence that
the burrows may have been excavated by bilaterally symmetrical organisms that grew in well-defined growth increments and were
able to withstand desiccation. Among well-known soil organisms, millipedes are burrowing animals that satisfy these requirements,
but have a fossil record not quite this old. This trace fossil evidence for animals on land, together with recent palynological
evidence for land plants of a bryophytic grade of evolution during Late Ordovician time, are indications of terrestrial ecosystems
of slightly greater antiquity and complexity than hitherto suspected.
Upper Silurian carbonates on Somerset, Cornwallis and Devon islands contain numerous trace fossils. A Polarichnus-Bergaueria ichnoassociation characterizes limestones and dolostones deposited in the intertidal zone (Cape Storm and Leopold formations), a Fuersichnus-Uchirites ichnoassociation characterizes limestones deposited under shallow subtidal conditions (Douro Formation), and a Neonereites ichnoassociation characterizes basin-slope limestones (tongue of ?Cape Phillips Formation).
Trace fossils in planar-bedded calcisiltite and dolosiltite were preserved through toponomic
processes, which resulted in diverse ichnoassemblages similar to those reported from contemporaneous shallow marine siliciclastic rocks. In contrast, trace fossils in bathymetrically equivalent rubbly (nodular) calcilutite were preserved through diagenetic processes, which resulted in ichnoassemblages largely restricted to infaunal feeding and dwelling traces. Although the mineralogic composition of the substrate is probably relatively unimportant in controlling the distribution of trace-making organisms, the greater susceptibility of carbonates to early and late diagenetic processes can significantly affect the nature of the ichnocoenose.
Characters resulting from formational and preservational processes of trace fossils suggest that the calcilutite substrates were generally firm, whereas calcisiltite substrates ranged from firm to thixotropic. Simple, "non-diagnostic" burrows such as Palaeophycus can contribute a wealth of information on substrate consistency, much of it not readily available from more traditional environmental indicators.
The recent discovery of abundant trace fossils in the Lower Devonian Muth Formation (Pin Valley, Spiti, northern India) contributes to our understanding of the diversity and distribution of Early Devonian arthropod activities in a marginal marine environment. The ichnoassemblage consists of abundant Palmichnium antarcticum and Diplichnites gouldi with rarer Diplopodichnus biformis, Taenidium barretti, Didymaulichnus cf. lyelli, Didymaulyponomos cf. rowei, Selenichnites isp., and vertical burrows of unclear affinity. The abundance of trackways enables the documentation of size- and gait-variations among producers of single trackway ichnotaxa. The paleoenvironment of the Muth Formation is interpreted as a barrier island system. Four different facies associations are recognized; arthropod trackways occur only in beach to coastal dune environments (facies association 2). Most of the Palmichnium and Diplichnites trackways are interpreted as subaerial. The abundance and frequently sub-parallel orientation of the Palmichnium antarcticum trackways, predominantly perpendicular to the paleocoastline, suggest that stylonurid eurypterids, which are interpreted as their pruducer, were migrating across the shoreline and climbing up slip faces of barrier island dunes. The similarity of this ichnofauna to others in marginal marine environments of similar age in Antarctica and Australia allows the identification of a recurrent Lower Devonian ichnocoenosis around the margins of eastern Gondwana.
Neoproterozoic to subtrilobite Cambrian strata in the Mackenzie Mountains, Canada, provide a test case for the current paradigm of evolutionary stages in the development of the early infauna. Three predominantly siliciclastic formations (Ingta, Backbone Ranges, and Vampire formations) collectively exhibit 13 facies associations representing nonmarine to distal shelf environments, and can, be subdivided into 18 simple sequences, each of 800,000-1,000,000 years duration. Trace fossil occurrences were digitized at the level of simple sequences and plotted on a matrix with facies associations (environment) on the x-axis and simple sequences (time) on the y-axis. This analysis shows that evolution was a first-order control on trace fossil distribution, and that environmental factors exerted an important second-order control. Trace fossils were most abundant in normal marine environments and Less so in marginal-marine settings with fluctuating or brackish salinity. Eolian and fresh-water deposits were barren, presumably because animals able to tolerate these conditions had not evolved. Turbulence excluded organisms from shoreface environments. Several biostratigraphically important ichnotaxa (e.g., Treptichnus and Rusophycus) however, show broad environmental tolerance across the marine shelf: Taphonomic factors led to fine preservation in interbedded sandstone-mudstone facies, but to diagenetic obliteration of trace fossils in most carbonate facies. Trace fossils provide a consistent indicator of the Neoproterozoic-Cambrian boundary Four evolutionary zones can be recognized: (I) Simple burrows, (II) Treptichnuspedum Zone, (III) Rusophycus avalonensis Zone, and (IV) Cruziana tenella Zone. Global biostratigraphic and chemostratigraphic correlations imply that Zone I is terminal Neoproterozoic and the overlying three zones subtrilobite Cambrian in age. These zones are present in, the Newfoundland Neoproterozoic-Cambrian boundary GSSP, the East European Platform, and elsewhere.
The trace fossils of the Tumblagooda Sandstone (?late Silurian) of Kalbarri, Western Australia are spectacular in their variety and preservation. They provide a unique insight into the activities of the early invaders of terrestrial environments, and reveal the presence of a diverse fauna dominated by arthropods. Within the Formation trace fossil assemblages can be related to fluvial, aeolian and marine sand-dominated environments. Two distinct and diverse ichnofaunas are recognised.
The Heimdallia–Diplichnites Ichnofauna occurs in sandstones deposited in broad low sinuosity braided fluvial channels, between which were mixed aeolian and waterlain sandsheets, small aeolian dunes and flooded interdune and deflation hollows. Heimdallia is the major bioturbator, favouring shallow pools. Other burrows include Tumblagoodichnus (gen. nov.), Diplocraterion, Skolithos, Beaconites and Didymaulyponomos . Arthropod trackways ( Diplichnites ) occur on surfaces of waterlain sands and on foreset bedding of aeolian dunes, and represent some of the earliest reported terrestrial trackways. Other trackways include Paleohelcura and Protichnites , and the digging traces Selenichnites and Rusophycus are also present. At least ten types of arthropods are required to produce the observed traces. Myriapods, eurypterids, euthycarcinoids, xiphosurids and scorpionids are considered responsible for the trackway assemblage.
The Skolithos–Diplocraterion Ichnofauna occurs at the top of the exposed section in sandstones that overlie a thick fluvial sequence containing few traces. The strata are considered to represent marine influence at a fluvial/marine transition. They show variable trough cross-bedding, complex planar cross-bedding with down-climbing sets, ripple lamination, and fining-up sequences with bioturbated tops. Traces are dominated by crowded Skolithos up to 1 m long, together with two forms of Diplocraterion. Daedalus and Lunatubichnus (gen. nov.) burrows occur in a few beds and Aulichnites trails cover some foreset surfaces of cross-bedding.
The trace fossils and the sedimentology of the Tumblagooda Sandstone bear a remarkable similarity to those of the lower part of the Taylor Group of Antarctica, which is probably Devonian in age. It is suggested that the two represent a similar age, stratigraphy, and range of environments on the margins of Gondwana. Large unvegetated fluvial outwash plains with variable aeolian influence were essentially coastal in character and fluvial/marine transitions occur in sand-rich environments. The animals responsible for the traces inhabited coastal areas but many could survive outwith marine influence, and arthropods responsible for some types of Diplichnites trackways walked out of water.
The rich diversity of trackways attributable to arthropods illustrate that the invasion of terrestrial environments by arthropods, particularly large forms, was well-established by the beginning of the Devonian. The basis of the food chain was algal and bacterial films which bound the surface sediment in freshwater pools.
Basal terrestrial deposits in the Cambrian-Ordovician Nepean Formation (Potsdam Group) near Kingston, Ontario, contain arthropod-produced trackways that extend the record of the first arthropod landfall back by as much as 40 m.y. The presence of large, simple cross-beds and of wind-produced structures, including adhesion ripples and wind-ripple lamination, indicates that the host strata were deposited in an eolian dune field, probably in a marginal-marine setting. The trackways were preserved mainly as undertracks and record the activities of large, amphibious arthropods, possibly euthycarcinoids.
Abundant arthropod trackways, assigned to Diplichnitesgouldi, are described from the Lower Old Red Sandstone (Early Devonian; Lochkovian) of Pant-y-Maes quarry, Brecon Beacons, South Wales. The trackways are preserved on bedding planes of finely laminated planar and rippled siltstones. The sedimentology of the succession indicates that these units represent bar top and marginal deposits in a braided fluvial setting. Two trackway types are recognized (Type A and B); comparisons with contemporaneous myriapodous producers favour kampecarid and eoarthropleurid myriapods, respectively. Functional analysis of the trackways indicates that the producers were not using the most efficient, stable, walking techniques, but instead utilized in-phase -like gaits. Together with their occurrence on rippled surfaces, and lateral displacement of some trackways (attributed to currents), this indicates that they were produced sub-aqueously.
In 1909 John Smith, a Scottish naturalist and geologist,described 23 «genera and 51 «species of trace fossils from small patches of sediment associated with andesite lava flows at Dunure, Ayrshire. He interpreted the traces as evidence of a diverse invertebrate fauna which inhabited small pools and fissures in the lava surface between eruptions. Smith's collection (c. 300 specimens) was presented to the British Geological Survey, Edinburgh but has remained largely unstudied. Re-examination of the Smith collection shows thatit came from 3 separate localities which differ in composition of the ichnofaunas, associated sediments and sedimentary structures. Arthropod trackways dominate the ichnofauna occurring in laminated siltstone frequently with ripple marks and foam marks suggesting formation in shallow lacustrine conditions. Preservational and behavioural analysis of trace fossils reveals about 10 valid ichnogenera of locomotion and swimming trackways, resting traces, feeding trails but few burrows. Presence of early terrestrial arthropod traces is uncertain. Palaeocological interpretation is of ephemeral lakes in distal braid-plain situation subject to subsurface invasion of andesite lava producing fluidization and deformation of wet sediment. The Dunure ichnocoenosis shows unique diversity of Devonian arthropod trace fossils.
Abundant small tracks from ripple-marked bedding planes on coarse-grained siltstones of the Ringerike Group near Sundvollen are described and illustrated. They occur about 450 m above the base of the Ringerike Group, much higher in the succession than previously described tracks from this Group. They differ from tracks of the eurypterid Hughmilleria in having parallel epichnial ridges and grooves rather than paired depressions, and are here named Steinsfjordichnus brutoni. Their age is probably lower Devonian.
Eurypterid (Eurypterida, Chelicerata) palaeoecology is reviewed, in light of a statistical analysis of 74 Silurian and Early Devonian eurypterid assemblages. Kjellesvig-Waering's three eurypterid ‘biofacies’ model is rejected as this analysis indicates considerable mixing of eurypterids from each supposed biofacies. A new model is proposed, which regards each eurypterid genus as having a broad palaeoecological range, determined by several environmental and palaeophysiological criteria, including an ‘ontogenetic gradient’, whereby juveniles preferentially adopted marginal and near shore habits (here interpreted as representing a true population distribution and not a taphonomic or collecting bias). A ‘mass–moult–mate’ hypothesis is proposed, which depicts eurypterids as having migrated en masse into nearshore and marginal environments (e.g. lagoons) to moult and mate. This hypothesis is evidenced by: (i) the occurrence of abundant accumulations of eurypterid remains (e.g. the Bertie and Saarema assemblages), which are interpreted as concentrations of exuviae in the absence of any evidence that these remains represent mass mortalities. (ii) Eurypterid respiratory and reproductory palaeobiology (i.e. eurypterids possessed accessory aerial respiratory tissues, enabling amphibious excursions and they were capable of spermatophore exchange and storage, allowing the delay of fertilisation of their eggs). (iii) Occurrences of abundant, variously sized, sub-parallel, eurypterid trackways, indicating migrations en masse, in a similar direction, across marginal environments. Comparisons with modern analogues (semi-terrestrial crabs and xiphosurans) are made, which employ a similar behaviour.
I wish to emphasize the im portance of trace fossil evidence in studying the terrestrialization of invertebrates. Associations of trace fossils of arthropod origin are known from the late Silurian and Devonian non-marine sediments in the Welsh Borders, Scotland (Midland Valley and Orcadian basins), Norway (Ringerike and Hornelen basins), Spitzbergen, Appalachians of North America and Antarctica (Pollard et al . 1982, figure 15; Pollard & W alker 1984, figure 3).
Eurypterid trackways in paralic facies of the Graafwater and Peninsula Formations, lower Table Mountain Group (Ordovician) of the Western Cape Province, South Africa, are reviewed. The ichnotaxonomy of trackways preserved on the ‘Brandenburg slab’ from the Graafwater area is reassessed, with the recognition of two distinct forms: (1) Palmichnium capensis (Anderson, 1975), consisting of a series of four tracks, many accessory imprints and (usually) an intermittent bilobed medial impression; and (2) Petalichnus brandenburgensis isp. nov., a much narrower trackway with nine to ten tracks per series and no medial impression. Palmichnium capensis is interpreted as the locomotory trace of an onychopterellan eurypterid (Chelicerata: Eurypterida). Petalichnus brandenbergensis is provisionally ascribed to the Trilobita. The likely walking techniques employed by onychopterellan eurypterids are assessed from the revised ichnological evidence, comparisons with extant taxa and modelling procedures; subaqueous locomotion was achieved by a series of short hops across the substrate and on land these animals were slow-moving and cumbersome, employing an ungainly, undulatory gait.
Arthropod trace fossils are described from the Borrowdale Volcanic Group, a sequence otherwise devoid of faunal evidence. Two forms, both made by the same probably myriapod-like organism, are assigned to the ichnogenera Diplichnites and Diplopodichnus. The lithologies preserving the trace fossils are non-marine and may have been deposited in a freshwater lacustrine environment; some of the traces were probably made in temporarily emergent conditions. The change from one form of trace to the other reflects drying out of the substrate. The trace fossils probably record some of the earliest freshwater arthropods, before the widespread colonization of land by the group.
An assemblage of terrestrial trace fossils is described from the Lower Old Red Sandstone of Pembrokeshire, southwest Wales, preserved in mainly fine-grained alluvial facies (channel and overbank sediments, deposited by predominantly ephemeral flows within a semi-arid environment), and thin tuffaceous horizons. The ichnofauna is dominated by an extensive, but low diversity Beaconites ichnocoenosis, comprising the meniscate backfilled burrow Beaconites barretti. Concentrations of these burrows (up to 30 per square metre) show normal size distributions, representing periodic colonization events (inferred as a response to seasonal desiccation) of subaerially exposed (partly indurated) sediments, probably by a population of eoarthropleurid myriapods penetrating the substrate to the level of the water table in order to aestivate and/or moult. Arthropod trackways also characterize an active arthropod epifauna of arachnids (Paleohelcura; first Welsh record) and myriapods (two forms of Diplichnites up to 160 mm wide) and Diplopodichnus. Additional ichnotaxa include arthropod foraging and resting traces (bilobed trails, Tumblagoodichnus and Selenichnites), ‘scratch arrays’ and worm burrows (Cochlichnus, and Palaeophycus) and faecal pellets all representative of the Scoyenia ichnofacies. Copyright © 2004 John Wiley & Sons, Ltd.
Scoyenia beerboweri is a new ichnospecies of burrow from the late Ordovician (Ashgill) Juniata Formation in central Pennsylvania, USA. The burrows are abundant in red calcareous palaeosols, and were created by animals living at the time of soil formation, because they are filled with red sediment like that of the palaeosol matrix, and both cut across, and are cut by, nodules of pedogenic carbonate. The isotopically light carbon and oxygen of carbonate in the palaeosols indicate a terrestrial ecosystem of well-drained floodplains in a tropical seasonally-dry semi-arid palaeoclimate. Backfill layering within the burrows is evidence of a bilaterally symmetrical animal. Size distribution of the burrows reveals discontinuous growth, as found in arthropods. Ferruginized faecal pellets in the burrows indicate that they ingested sediment. For these reasons the burrows of Scoyenia beerboweri are most likely to be the work of millipedes. The nature of vegetation supporting them is unknown, although a single problematic plant-like fossil cast was found, and liverwort spores are widespread in rocks of this age. Vegetative biomass was limited judging from the degree of chemical weathering, extent of burial gleization and isotopic composition of carbon in the palaeosols. These distinctive respiration-dominated liverwort-millipede polsterlands lived at a time of global greenhouse climate, following Precambrian–Cambrian lichen-algal microbial earths and supplanted by Silurian brakelands of early vascular land plants.
Many Late Paleozoic environments have been interpreted as marine because of the co-occurrence of supposedly exclusively marine trace fossils. Beginning in the Late Ordovician, however, nonmarine trace-fossil diversity increased throughout the Paleozoic. This diversification of nonmarine organisms and nonmarine trace fossils was especially prevalent in Devonian and later times. Diversification of freshwater organisms is indicated by the large number of freshwater fish, arthropods, annelids and molluscs that had developed by the Carboniferous. In addition to diverse freshwater assemblages, entirely terrestrial vertebrate and invertebrate ecosystems had developed by the Devonian. This rapid diversification of freshwater and terrestrial organisms is inherently linked to development and diversification of land plants and subsequent shedding of large quantities of organic detritus in nonmarine and marginal-marine areas. Nearshore marine organisms and their larvae that are able to tolerate relatively short periods of lowered salinities will follow salt-water wedges inland during times of reduced freshwater discharge. Similarly, amphidromous marine organisms will migrate periodically inland into nonmarine environments. Undoubtedly, both of these processes were active in the Paleozoic. However, both processes are restricted to stream/distributary channels, interdistributary bays, or estuaries. Therefore, the presence of diverse trace-fossil assemblages in association with floodplain deposits is interpreted to reflect true nonmarine adaptation and diversity. Conversely, diverse trace-fossil assemblages in association with stream/distributary channel deposits, interdistributary-bay deposits, or estuarine deposits may reflect migration of salt-water wedges inland, or migration of marine organisms into freshwater environments (amphidromy), or both.
The stratigraphy and facies association of trace fossils in some Cambrian and Ordovician rocks of northwestern Spain
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