Review Article |
Corresponding author: Michel Laurin ( michel.laurin@mnhn.fr ) Academic editor: Florian Witzmann
© 2024 Michel Laurin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Laurin M (2024) Habitat of early stegocephalians (Chordata, Vertebrata, Sarcopterygii): a little saltier than most paleontologists like? In: Witzmann F, Ruta M, Fröbisch N (Eds) The fish-to-tetrapod transition and the conquest of land by vertebrates . Fossil Record 27(3): 299-332. https://doi.org/10.3897/fr.27.123291
|
A controversy on the degree of marine influence in the paleoenvironments represented by many Paleozoic stegocephalian-bearing fossiliferous localities has persisted for decades. Many authors have equated the absence of a typical stenohaline marine fauna with freshwater environments, but this ignores continental salt lakes and the many transitional environments (deltaic, estuarine, lagoonal, and some epicontinental seas that receive much freshwater influx, like the Baltic Sea) that separate typical marine environments from freshwater environments. This is problematic because it seems plausible that many of the late Paleozoic sediments that have been preserved were deposited on coasts in deltas and estuaries. The author had compiled a dataset of paleoenvironmental interpretations of Devonian to Early Permian stegocephalian (“tetrapod”)-bearing fossiliferous localities in 2010. How have these interpretations withstood the test of time, especially in the face of new results from different kinds of evidence? An updated dataset and a new literature review show that the case for a marine origin of stegocephalians has strengthened, especially through additional discoveries or reinterpretations of fossils that suggest marine influence in various classical vertebrate-bearing Permo-Carboniferous localities traditionally interpreted as freshwater, and a recent analysis of stable isotopes in Late Devonian localities.
Amphibians, brackish, Carboniferous, Devonian, epicontinental seas, freshwater, marine environment, paleoenvironments, Permian, tetrapods
Most early studies on Paleozoic stegocephalians have assumed that these taxa normally inhabited freshwater or dry land, unless their remains were clearly associated with typically marine fossils. This is exemplified by this quote from
“Most of the British and North American tetrapod localities represent water bodies within non-marine swamps and have generally been assumed to be in the freshwater regions of fluviodeltaic systems, not least because of the presence of amphibians as presumed freshwater indicators together with the absence of unambiguously marine organisms.” (Emphasis mine in all quotes unless stated otherwise.)
Thus, the fact that most of the Devonian and Carboniferous stegocephalians were called “amphibians” (in the paraphyletic sense of “anamniotic limbed vertebrate”) probably played a role in this, even though parsimony does not validate the inference that these were freshwater taxa (
This quote highlights the need for a precise nomenclature. Indeed, if the word “amphibians” had been consistently used in the sense that is now established under the PhyloCode (
Many late Paleozoic fossiliferous localities lacking fossils of stenohaline, strictly marine organisms (such as echinoderms, cephalopods and coral reefs) have been interpreted as freshwater habitats, or the marine influence on such habitats has been minimized. However, factors other than low and fluctuating salinity may explain the absence of many stenohaline marine taxa; these include high sedimentation rates and turbidity (
Taxa found in these localities were often assumed to have been stenohaline, freshwater forms.
A Dimetrodon grandis chases an Eryops megacephalus and an Edaphosaurus pogonias through a Sigillaria forest. In the foreground, a Meganeuropsis flies near strobili of Equisetum hyemale; the ground is covered by mosses. Reconstruction of the Artinskian (Early Permian) in what is now Texas, USA. Drawing by Ruben Koops (Haarlem, Netherlands), Rafael Albo (Corumbá, Brazil), Jacek Major (Starachowice, Poland), and Amin Khaleghparast (a biologist from Tehran, Iran); coloring by Ruben Koops. Advisors for Dimetrodon: Tracy Lee Ford (San Diego, California, USA) and Russell J. Hawley (Casper, Wyoming, USA). Advice on plants was provided by Ryan Thummel and Paige K. Wilson Deibel (both at U. of Washington).
Determining the habitat of long-extinct taxa is difficult because sedimentation and regression/transgression cycles cause shorelines to move quickly in geological terms (
Fortunately, some studies considered brackish habitats in their assessment of paleohabitats, but often still seem to have minimized the marine influence. This can be illustrated by the “Birthday Bonebed” of the Permo-Carboniferous Halgaito Formation (Utah), which was studied recently by
Within the bonebed, microconchids, xenacanths, actinopterygians, and the lungfish Sagenodus suggest an assemblage that was to some degree dependent on permanent standing water. We interpret these stream systems, particularly those associated with the major tiered channel bodies, as primarily freshwater with little marine influence, though the microconchids and xenacanths potentially leave open the possibility of proximity to marine-influenced channel reaches (
However,
I studied the problematic paleoenvironments inhabited by Permo-Carboniferous stegocephalians before (
There are reasons to believe that the extent of marine influence in the habitat of early stegocephalians has been significantly under-estimated in the literature (see below). A similar bias against marginal-marine environments has recently been argued to be present in the paleobotanical literature, at least in the Carboniferous.
“Tidal environments have been identified in other areas, including in European basinal settings (e.g. Fossil Grove: Gastaldo 1986, reinterpreted as a tidal setting). Such deposits are likely much more widespread than recognized because of the difficulty of identifying tidalites in their nascent, very finely laminated stages , particularly in mudrocks (most likely to be encountered in the basal coal-roof transition strata found in mines close to channel environments, see Fig. 10a). However, the limited recognition of tidal-flat settings also may reflect that most palaeobotanists are generally unfamiliar with these kinds of strata ”
This is hardly a new claim, though it remains highly relevant.
This bias in favor of a freshwater interpretation is not restricted to formations and sites that have yielded early stegocephalians. Even older strata have arguably been affected by a similar interpretation bias. A good example is provided by the Old Red Sandstone (ORS from here on; this ranges from late Silurian to Early Carboniferous in age), which yielded many early vertebrates.
“The first Silurian and Devonian fish remains described in the beginning of the nineteenth century were preserved in sandstones (e.g., the ‘‘Old Red Sandstone’’ of Britain and the Baltic States) and generally associated with plant remains, but rarely with marine invertebrates. In addition, these heavily armored fishes were regarded as ‘‘ganoids,’’ a group which classically included living bichirs, gars, and catfishes, all reputedly freshwater. Progressively, the received wisdom became that all these early fishes lived in freshwater and occasionally passed into the sea, when found in marine sediments.”
This may explain why
However, subsequent findings showed that some ORS was almost certainly deposited in a marginal marine environment as had been suspected already by some authors in the 1970s (e.g.,
The bias against marine influence is not restricted to vertebrate paleontologists. In her monograph on freshwater ecosystems in the fossil record,
“Similar in its defective logic and willingness to ignore the impressive data gathered by others concerning both depositional environment and characteristics of the biota, is Schultze’s (1972; Schultze and Arsenault, 1985) conclusion that the Late Devonian, freshwater-lacustrine, vertebrate faunas of the Escuminac Formation, Miguasha, Quebec, Canada are “coastal marine, based on the fauna present” merely because some of the genera are also found in undoubted marine deposits elsewhere in the world. Unwillingness to consider that any Devonian vertebrate might have been able to flourish in both marine and freshwater environments, as is the case with many taxa today, is biologically, as well as geologically, so unrealistic as scarcely to merit serious attention . With regard to the locality at Miguasha, the nearest known marine Upper Devonian beds are no closer than the Hudson Bay region , Canada, central New York , U.S.A., and the south of England .”
Yet, shortly after, a study based on isotopes of several chemical elements concluded that the Bothriolepis canadensis sample that they had included yielded “a strong marine signal” (
These developments illustrate the danger of relying on previous reconstructions of ancient coastlines to assess local paleoenvironments (
A possible bias against even the presence of a modest marine influence may be visible in previous works on the Permo-Carboniferous El Cobre Canyon in New Mexico, which is typically considered to be a freshwater (fluvial) environment (
“At the south side of the cañon, the junior author found a perfect cast of a Spirifer, identified by Professor Schuchert as S. rockymontanus Marcou, a form occurring in Colorado in the Pennsylvanian. Though the specimen was found free, so that its exact horizon could not be determined, its excellent preservation proves conclusively that it had not been carried far from its original bed, and inasmuch as vertebrate fossils are found in the deepest strata of the cañon it seems quite certain that the specimen came from an intercalated bed among those yielding so-called Permian vertebrates. No other explanation seems possible.”
One might tend to prefer the opinion of more recent studies that benefited from additional decades of research, but the statements by
The presence of this sole brachiopod fossil does not imply that sediments of all levels of the El Cobre Canyon were deposited in a marine-influenced environment; some data suggest otherwise. For instance,
Other evidence suggests that there might be marine influence in El Cobre Canyon.
An important consideration in paleoenvironmental studies is the autochtonous or allochtonous nature of the fossils preserved in a given locality. Only autochtonous fossils are informative in this respect. Allochtonous fossils provide information about more distant environments. In most cases, only fairly long-distance transport (several km) can be easily detected, through wear marks (erosion) on the surface of fossils, disarticulation, and the fragmentary nature of specimens (for instance, only a few isolated bones, rather than a nearly complete, articulated skeleton). Abundance of material can also be used, to an extent, in combination with quality of preservation (
In a few cases, transportation can be ruled out, even for fairly short distances. Some fossils of sessile organisms are obviously preserved in situ; this may occur for brachiopods, coral-forming organisms (especially cnidarians), some echinoderms and some mollusks, among others, but not vertebrates, unless they are found in burrows (a few examples are known, notably for dipnoans). Other compelling but rarer cases are found when evidence of predation is encountered, which suggests, minimally, that predator and prey occupied the same habitat, although this does not rule out transport of the bodies over a short distance. An example is described below, of a shark (Triodus sessilis) that ate a temnospondyl that had eaten an acanthoderm (
Another type of fossil that nearly always reflects local paleoenvironments are ichnofossils. The identity of the trackmakers is often poorly constrained, but some ichnofossils are associated with specific paleoenvironments and as such, they may be informative. They are even more useful when the identity of the trackmaker is reasonably well-constrained. An example is provided by the ichnofossils of Puertollano (Spain), which includes an interesting assemblage of trackways left by a stegocephalian and traces left by a finned vertebrate that swam above the substrate. The trackway, called Puertollanopus microdactylus, was left by a small stegocephalian, tentatively identified as an amphibian (a “microsaur”) or, less probably, an amniote. The traces left by a gnathostome that swam above the substrate, presumably in shallow water, are called Undichna britannica, and they were probably left by the xenacanthid chondrichtyan Orthacanthus, which is also known by skeletal remains from the locality (
The isotopic ratios of various elements have been used to assess paleoenvironments, notably to shed new light on the degree of marine influence and paleosalinity in various fossiliferous localities. We saw above that
As we will see below, some of these conclusions may rest on tenuous ground because the isotopic signature of strontium reflects freshwater input and flux between a given water body and the ocean, rather than salinity; a similar phenomenon has been documented for neodymium by
The 87Sr/86Sr ratio (87Sr being the radiogenic isotope) has varied through time, but oceanic mixing appears to have resulted in fairly homogeneous world-wide 87Sr/86Sr oceanic values at any given time. By contrast, freshwater bodies have much more variable ratios at any given time because these ratios depend on the 87Sr/86Sr signature of the soil and bedrock in the drainage basins. Thus, if the 87Sr/86Sr ratio of a given sediment matches the contemporary oceanic ratio, the sediments were probably deposited in oceans, although in a small minority of cases, the similarity might be coincidental. Conversely, a significant departure from the coeval 87Sr/86Sr oceanic ratio (beyond measurement error and outside two standard deviations) indicates that the Sr in the water body in which the sediments were deposited was not at equilibrium with oceanic values. This indicates that little or no exchange with the ocean took place. Several studies have interpreted such cases as representing freshwater, but this is only one of several other possibilities; the others include brackish water, which may be purely continental, far from the coasts, but also coastal (ponds, lagoons, estuaries and deltas), and even some epicontinental seas, such as the Baltic Sea.
Thus, interpreting the isotopic signature of Sr in terms of salinity is not straightforward. Purely continental saltwater lakes have a Sr isotopic signature that reflects that of the rivers that flow into it, and brackish coastal environments, such as the Baltic Sea, show strong deviations from oceanic signatures, especially where the freshwater influx is greatest; for instance,
Epeiric seas may not reflect oceanic isotopic values, as exemplified by the extant Baltic Sea (
Sulfur (S) has also been uses as a paleosalinity indicator in some recent studies. As for Sr, S is highly variable in freshwater environments, with δ34S values ranging from − 20.0 to + 20.0‰, whereas current oceans have a δ34S value of about + 21.0‰ (
A third element that has been used to assess paleosalinity is oxygen (O). Current oceanic seawater has a relatively uniform δ18O value of 0 ± 1‰, although it is lower at high latitudes, ranging from about −3 to −1‰ (
Isotopic analyses have been used to assess the paleoenvironment of many fossiliferous localities, but their relevance in this context can be analyzed through the example of the Joggins Formation, which has yielded a rich metazoan fauna, including many vertebrates, including some stegocephalians. On the basis of isotopic analyses,
Isotopic data are also directly relevant to assess the habitat of early stegocephalians and their associated fauna. Among the latter, xenacanthiform chondrichthyans are especially relevant and are discussed below. Some xenacanthids were found in marine sediments (
The δ18O values obtained by
A slightly different picture emerges for the xenacanthiforms from Joggins; the few Sr isotopic analyses performed by
Today, the vast majority of extant chondrichthyans are strictly marine; only 43 species (less than 4% of elasmobranch species currently recognized) venture into freshwater (
Xenacanthiforms are the chondrichthyans most frequently associated with Permo-Carboniferous stegocephalians (Fig.
The temnospondyl Eryops megacephalus leaps to seize a small chondrichthyan (Xenacanthus). While the co-existence of both taxa is well established, the environment that they occupied (freshwater, brackish water, or even a marginal marine environment) remains enigmatic. Amin Khaleghparast (a biologist from Tehran, Iran) drew the figure, which was colored by Dmitry Bogdanov (a cardiologist and paleo-artist from Chelyabinsk, Russia); Roman Yevseyev (Moscow) adjusted the legs. Anatomical advice was provided by Tracy Lee Ford (San Diego, California, USA) and Bryan Riolo (Ocala, Florida, USA).
As we saw above (isotopic section), the δ34S results on xenacanthiform bioapatite obtained by
Collectively, xenacanthiforms appear to have inhabited both continental, plausibly freshwater (at Buxière-les-Mines and the Muse) and marine environments (represented in other localities), and most frequently environments transitional between these, but this does not imply that each xenacanthiform species inhabited all these environments. Perhaps, as in extant teleosts, there may have been freshwater, marine, and euryhaline taxa, but so far, we have a highly incomplete picture of xenacanthiform environmental preferences.
Microconchids (often called “Spirorbis” in the older literature) are often associated with Permo-Carboniferous stegocephalians. Their paleoenvironmental significance is thus relevant to assess the habitat of early stegocephalians. Note that microconchids have often been called “Spirorbis” in the older literature, but the latter is an extant marine annelid, and the coiled calcitic tubes encountered in fossiliferous localities older than the Cretaceous were not formed by annelids, but rather, by microconchids (
To assess the validity of these opposing claims, I checked the most relevant data presented by
The Ohio occurrence is justified by a conference abstract (
“Their presence indicates a marine connection and probable brackish influence (Schultz[e], 2009; Gierlowski-Kordesch and Cassle, 2015). Two occurrences of microconchid-bearing ‘nonmarine’ limestones in the Glenshaw Formation have been re-interpreted as brackish, clear water, nearshore facies (Morris, 1967; Busch and West, 1987)”
Indeed,
To support the presence of “freshwater” microconchids in the Westphalian (Late Carboniferous) of England,
Before closing this section, it may be useful to discuss a recent study that clearly sides with the interpretation that some microconchids occurred in freshwater, even though this concerns taxa that occur after the temporal interval considered in this paper (namely, in the latest Permian and early Triassic).
The case for a freshwater community in the latest Permian Tunguska Basin, also described by
Thus, in this paper, I will consider that the presence of microconchids implies marine influence.
The habitat of Permo-Carboniferous xiphosurans, which are sometimes associated with early stegocephalians, has proven particularly controversial. Extant xiphosurans (only four currently recognized species) are basically marine, even though they frequently enter brackish estuaries and less frequently, rivers where the water is almost fresh; this is documented, for Carcinoscorpius rotundicauda (named Limulus rotundicauda, in the older literature), in the Hughli river at least as far as Calcutta (
Some paleontologists argued for an early invasion of freshwater habitats by xiphosurans. For instance,
Xiphosurans occur in what was once called the Braidwood Mazon Creek fauna (
“it was collected from a Coal Measures site near Dudley, Worcs. The Coal Measures strata in this area are Westphalian B in age (Upper Carboniferous) but unfortunately, this is the only stratigraphic detail available . The nearby site of Coseley (Westphalian B) has yielded Bellinurus koenigianus WOODWARD, 1872, Bellinurus bellulus KONIG 1851 (see SCHULTKA 1994: 347), and Pringlia birtwelli (WOODWARD, 1872).”
Thus, the locality data for this specimen are vague, stratigraphy is worse, and there is no associated fauna. The other specimen, LL 11133, has fairly precise locality data (the Bickershawe Complex colliery tip near Leigh, Wigan), but the reported associated fauna (mostly terrestrial, with the exception of the bivalve Naiadites) is only moderately reliable because “Unfortunately, the material is not preserved in situ, however all of the material listed above comes from a single, constrainable area of the spoil tip, and as such is likely to reflect original association.” (
What should we conclude from all this? What seems to be reasonably well-established is that chelicerates most likely originated in the marine environment (
“The main habitats of limulids have always been in the shallow sea; but the fossilization potential for both, carcasses and tracks, was so much lower in the true biotope than in marginal and partly nonmarine environments that the fossil representation of the limulids is now stronger in these than in their main biotopes”.
Eurypterids are rare, but a good proportion of the known Permo-Carboniferous specimens have been found in localities that yielded stegocephalians, like Joggins (
The habitat of eurypterids, as for several other extinct taxa, has proven difficult to assess.
Pentecopterus, one of the oldest (Darriwilian, mid-Ordovician) eurypteryds. Drawing by Patrick Lynch, published on Wikimedia commons (https://en.wikipedia.org/wiki/Eurypterid#/media/File:Eurypterids_Pentecopterus_Vertical.jpg) under the CC0 1.0 DEED licence.
Nevertheless, Permo-Carboniferous eurypterids have often been interpreted as freshwater taxa (
Furthermore,
1 corresponds to the intertidal environment, as well as the brackish plus estuarine; 2 to the high subtidal; 3 to the remainder of the subtidal photic-phytal zone; 4 and 5 to the mid- and outer-shelf zone; and 6 to the shelf margin to upper bathyal region.
To this, Plotnick added BA0, for “probable nonmarine occurrences”. The faunal composition of some of the localities seems to match imperfectly the assigned assemblage. Out of the 94 localities, 8 are scored as BA 0, sometimes “or BA 1”, and 46 are classified as possibly BA 1, sometimes “or BA 2”, which reflects substantial uncertainty. The localities classified as BA 0 (including those that might be BA1) exhibit some signs of marine influence; some localities (like no. 2) have no associated fauna, or only plants, presumably terrestrial (like no. 62), some are associated with vertebrates (ex. localities 1, 17, 51, 54, 64), and only one (71) is associated with a more diversified biota, which includes xiphosurans. But note that while these vertebrates were once considered freshwater because they occur in the Old Red Sandstone (then considered to represent freshwater deposits), the argument no longer holds because many authors consider the ORS as a marginal marine environment (
The localities classified as BA 1 show stronger marine influence, like brachiopods, mostly inarticulate, especially lingulids, or more rarely, articulate brachiopods like Hindella, Atrypa, Dalejina (like localities 7, 18, 26, 27, etc.). Some, like localities 26–30, 32–35, 38, 48, even yielded cephalopods, which suggests a fairly typical marine environment (see below). Some have corals, in addition to cephalopods, like localities 3,2 68, and 79, or crinoids, like localities 37, 48. A few BA 1 localities have acritarchs (no 42), stromatolites (66), trilobite fragments (88), or possible cirripedes (92). Some BA 1 localities are hypersaline (91). Thus, most of the localities considered BA1 by
Last but not least, paleobiogeographic data show that at least some eurypterids, namely, the pterygotoids “apparently could cross open oceans, and are found throughout the world in the short time span of their existence (~40 Ma)”, as
Thus, the case for a freshwater interpretation of some eurypterids seems to be weak.
Some bias in favor of a freshwater interpretation was built into
“variously interpreted as representing an upper intertidal portion of a sabkha to subtidal sequence (Hamell, 1982), a near shore lagoon showing fluctuating salinity (Heckel, 1972; Copeland and Bolton, 1985), a wide lagoonal system behind a reef (Ruedemann, 1925; Monahan, 1931), a brackish to freshwater lagoon or estuarine deposit (Kindle, 1934), or a deltaic environment (O’Connell, 1916).”
Yet,
The eurypterids that co-occur with microconchids probably inhabited marine-influenced habitats (see above, section on microconchids). One such co-occurrence with microconchids (reported erroneously as “Spirorbis”) is in the Middle Devonian Gaspé Sandstone Series of New Brunswick and Québec provinces, in Canada (
More importantly, at least four specimens (at least one of which belongs to the mycteropoid Mycterops whitei) have been found in three Late Pennsylvanian localities in two formations (Hushpuckney Shale, Swope Formation, Iowa, and Stark Shale Member, Dennis Formation, Nebraska) that have yielded “abundant conodonts” (
More recent research suggests that Permo-Carboniferous eurypterids may have lived closer to the seas than previously thought. Thus,
I conclude from the above that the presence of eurypterids in the seas and marginal-marine environments is much better established than in truly freshwater deposits; I have not seen a single solid case for the latter, although localities where Adelophthalmus occurs without any other faunal element suggesting marine influence (if such localities exist) could plausibly represent a freshwater environment. However, I have not studied all eurypterid-bearing localities, so my conclusions on Permo-Carboniferous eurypterid paleoenvironments remain tentative; as
These enigmatic arthropods of uncertain affinities (
“Fossil remains of these animals were found in many regions of the world, generally in rocks derived from marine or coastal environments, such as Mazon Creek for instance. Thus their presence in the intermontane basin of Montceau-les-Mines poses a problem.”
Similarly,
Brachiopods occasionally co-occur with stegocephalians in Permo-Carboniferous localities. Extant brachiopods are exclusively marine animals, with a fairly low tolerance to salinity variations. Some studies have suggested that in the Paleozoic, they may have lived in slightly hypersaline or slightly brackish water (
Echinoderms are infrequently associated with stegocephalians, but a few such associations are mentioned below, so it is worth reviewing briefly their osmotic tolerance. This taxon is overwhelmingly marine, a constraint probably linked to the fact that echinoderms have poorly developed circulatory, excretory, and gas exchange systems (
Extant cephalopods are among the most strictly marine mollusks. Parsimony suggests that crown-cephalopods in general were also marine, and this crown-group is ancient, given that the divergence between nautilids and coleoids harks back deep into the Paleozoic. Indeed, some molecular studies placing it around the Late Devonian (
Many articles that describe supposedly fully continental (i.e., without any marine influence) deposits reported “freshwater jellyfishes” (e.g.,
Most Devonian localities that have yielded stegocephalians were long interpreted as freshwater by most authors. Thus,
The Famennian Strud locality (Belgium), which recently yielded a stegocephalian (
“The depositional setting approximately corresponds to a ramp with both an increase in the marine influence and a deepening southwards (
Indeed, faunal elements that indicate a fairly typical marine environment occur at various levels of the Strud succession and more generally, in the Dinant Synclinorium (
The stegocephalian from Strud was found in beds B and D of the Royseux Member of the Evieux Formation (
The early Moscovian (Pennsylvanian) Minto Formation of New Brunswick has yielded an interesting metazoan fauna that includes a few stegocephalian remains, including a jaw that plausibly belongs to a colosteid, small limb bones, and a vertebral centrum that has been plausibly attributed to an embolomere, in addition to finned tetrapodomorph material, mostly assigned to Megalichthys (
Joggins has yielded many stegocephalian remains, including the oldest known amniotes (
Other recent studies support marine influence in the Joggins Formation (
The OW facies has been interpreted as reflecting a “brackish sea” in recent studies (e.g.,
This fossil Konservat-Lagerstätte from Kansas preserves a Stephanian coastal community (
The Hamilton locality displays three main lithologies: a conglomerate, an ostracode wackestone, and an assemblage of laminated limestones and mudstones. Most marine and brackish fossils mentioned above occur in all three lithologies, except for the echinoderms, which may occur only as lithoclasts of wackestone, in the laminated limestones and mudstones, which also includes the vertebrate fossils. Patterns in the lamination suggest that the limestones and mudstones were deposited in a tidal environment, and
Much has been written about the biota and paleoenvironment of this locality, including in various sections (above) of the present paper. Here, only a few additional points need to be added. Above, I indicated that many taxa found in Mazon Creek may have been euryhaline, but exceptions exist. Beyond the obvious case of allochtonous (mostly terrestrial) taxa, a fairly diverse cephalopod assemblage is documented (
Thus, Mazon Creek undoubtedly exhibits a stronger degree of marine influence than most other classical Permo-Carboniferous stegocephalian-bearing localities. This is paradoxical because a detailed study of tidal rhythmites of various Carboniferous and Holocene localities led
Other than the cephalopods, the obviously allochtonous elements include the remains of terrestrial embryophytes and of terrestrial arthropods, such as myriapods, arachnids (including scorpions), and insects (
Although not among the classical stegocephalian-bearing Permo-Carboniferous localities, the Red Tanks Member of the Bursum Formation recently yielded a fairly diversified assemblage of late Pennsylvanian stegocephalians (the temnospondyls Eryops, Trimerorhachis and an undetermined taxon, the embolomere Archeria, a diadectid, a caseid, Edaphosaurus and Dimetrodon cf. D. milleri) and other vertebrates (the chondrichtyans Petalodus and Deltodus, undetermined actinopterygians, as well as the dipnoan Gnathorhiza bothrotreta) from “mixed marine-nonmarine sequences” (
Montceau-les-Mines has been considered by many authors to represent an intramontane, freshwater basin (e.g.,
The isotopic analyses by
Another possible indicator of marine influence is Myxineidus gononorum, based on a fossil that was initially described as a hagfish, even though a more recent study raised doubts about its identity and suggested that it might be a lamprey (
Stegocphalians are represented by temnospondyls (Branchiosaurus petrolei, Micromelerpeton boyi, and fragments of Actinodon), nectrideans (Sauravus costei, S. spinosus, and Montcellia longicaudata), the aistopod Phlegethontia longissima, and fragments of a synapsid (Stereorachis? blanziacensis); none of these are very well-preserved, even though several specimens preserve traces of soft tissues (
In my previous review (
In the central German basins, which extend from the Thüringer Wald Permo-Carboniferous basins to the Döhlen Basin farther east,
It is impossible to assess the allochtonous or autochtonous nature of all stegocephalian remains found in all localities of the German basins, but the Glanochthon latirostre and Archegosaurus decheni found in a shark (Triodus sessilis) in the Lower Permian Lake Humberg, in the Saar-Nahe Basin, probably all lived in the same environment and there is no reason to infer significant transport (
The Bohemian basins in the Czech Republic were already discussed by
This literature survey illustrates a recurring theme that pervades the history of paleontological research on the Paleozoic paleoenvironments. The absence of typically marine indicators, such as coral reefs, echinoderms, and a diversified brachiopod fauna has been interpreted as indicating a “non-marine environment”, which was often implicitly or explicitly assumed to be freshwater. However, “non-marine environments” thus defined (very broadly) include estuaries, deltas, coastal mangroves, lagoons and salt marshes, which occur between truly marine and freshwater environments on, or near the coast, as well as brackish or salt lakes, which occur even far from coasts. This seems to have been too often forgotten. Thus, the paleoenvironment of many Permo-Carboniferous localities that have yielded stegocephalians need to be reassessed, even in the comparatively well-studied Permian redbeds of Texas (Fig.
Freshwater ecosystems may well be very ancient;
On the contrary, marginal-marine environments, where much sedimentation occurs, should be fairly well-represented in the fossil record, but they may be difficult to interpret because coastlines can vary quickly, especially in deltas, and even on a daily basis, tides result in short-term salinity variations in some coastal habitats. Thus, the exact environmental preferences and tolerances of long-extinct organisms that inhabited these coastal environments are difficult to assess. These organisms appear to have included many Paleozoic stegocephalians. Of course, this does not mean that all Paleozoic stegocephalians lived in brackish or normal-marine salt water. Like extant teleosts that occupy a great range of aquatic environments, many stegocephalians may have been adapted to freshwater habitats, and in some cases, independent evidence exists for this (e.g.,
Above, I raised the question of a bias in favor of freshwater interpretation of localities devoid of typically marine fossils, and discussed some cases for which marginal-marine, brackish environments seem more plausible. While I focused on the body fossil record which I know best, I note that
This review mostly supports the preliminary conclusions that I presented more than a decade ago (
Optimization of habitat in stegocephalians (with a few other sarcopterygians provided to better optimize near the base of Stegocephali) on a phylogeny. This figure is slightly updated from
The picture that emerges from all this is that early stegocephalian diversification seems to have occurred to a large extent close to coasts, including those of epicontinental seas, and to a lesser extent farther inland, and on land and possibly in freshwater. Is this pattern genuine, or does it reflect a taphonomic artefact that reflects the extent of sedimentation in deltas of the largest rivers on the coasts, along with erosion of sediments deposited farther from the coasts? If the latter is correct, a large evolutionary radiation of stegocephalians may have occurred in freshwater habitats but be poorly known because of taphonomic bias. Some localities, like Buxière-les-Mines, the Muse and Nýřany, may represent these freshwater localities, as suggested by the traditional interpretations. What was the salinity of the coastal environments in which stegocephalians diversified? As we saw above, the mere fact that tides occurred, as shown by tidal rhythmites, does not necessarily indicate brackish water because tidal effects can propagate inland along rivers (
This survey may have raised more problems than it has solved, and unfortunately, time constraints prevented me from reassessing the paleoenvironment of the many Carboniferous stegocephalian-bearing localities and formations, such as the Garnett quarry, and of the more numerous Cisuralian localities. Hopefully, this review has shown that it is time to have a fresh look at the development of continental ecosystems from the Late Devonian through the Cisuralian.
I thank Amin Khaleghparast (Tehran, Iran) for sending me two images that he had contributed to create to use as figures, Jean Goedert (CR2P, Paris) for information about isotopic methods to assess paleosalinity, Gloria Arratia (U. of Kansas, Lawrence) for commenting on the short section of the draft on the habitat of early actinopterygians, and Simon Braddy (Manorbier, U.K.) for sending one of his papers on eurypterids and providing the e-mail address of a retired colleague. Two reviewers and the editor, Florian Witzmann, provided feedback that improved the paper. However, the opinions expressed in this paper and the errors that the text may contain are my sole responsibility.