Science Literature

A previous blog on Devonian tetrapods remarked on their aquatic lifestyles and noted their suite of mosaic characters that make discussion of evolutionary trajectories highly speculative. Few fossils from the lower Carboniferous were known, but the diversified forms from the middle and upper Carboniferous were clearly components of terrestrial faunas. So, we find a group of aquatic amphibians in the Upper Devonian and a diversified group of terrestrial amphibians in the middle-upper Carboniferous. The puzzle is the lack of any terrestrial fossil material in the lower Carboniferous, leaving evolutionary palaeontologists little or no data to work with. The absence of evidence has been so noticeable that this part of the record has been labelled Romer's Gap (after the distinguished American palaeontologist from the last Century). In 2006, Ward et al. proposed an explanation for the lack of terrestrial fossils that invoked low concentrations of atmospheric oxygen. This, they surmised, inhibited the evolutionary development of ecosystems on land. Since that paper, more discoveries have been made in Scotland in rocks representing Romer's Gap, and "a wealth of new tetrapod and arthropod fossils" have been recovered. The inference can be made that the Romer's Gap ecosystems were not impoverished but, for various reasons, only recently have palaeontologists discovered the evidence needed to warrant this conclusion.

"Rather than beginning immediately following "Romer's Gap", we can now test the hypothesis that diversification and terrestrialization of tetrapods had been taking place during the 15 or more million years that it represents. Our discoveries and other recent new records from elsewhere certainly suggest that many tetrapod lineages have their origins much earlier than previously appreciated, and their earliest appearances may well be extended back in time as the result of further research."

Five toes found near the Scottish fishing village Burnmouth belong to the new caches of fossils. (Credit: J. Clack, Source here)

It is undoubtedly exciting to have this new data to work with. Inevitably, the question to be answered is how the new fossil data affects our understanding of Devonian and Carboniferous tetrapods. The research paper provides an initial insight and the last sentence quoted above gives the gist of the findings. What follows attempts to highlight evidences documented in the research paper. Readers should be aware that the Tournasian stage is the lowest in the Carboniferous, and above this is the Visean stage. Romer's Gap extends from the base of the Tournasian to the middle part of the Visean.

The first locality to be reported is Burnmouth. Isolated tetrapod remains have been found in several horizons.

"The most significant of these is a small (10 mm across the metapodial series) pentadactylous autopod (identity as manus or pes cannot yet be determined). [. . .] Its morphology strongly suggests that its owner was a terrestrial tetrapod. [. . .] The proportions of the metapodials and preserved phalanges, being elongate and gracile, are most similar to those of the Visean forms Silvanerpeton, Eldeceeon, and Balanerpeton, as well as the Late Carboniferous anthracosaur Gephyrostegus, all of which are usually considered to have been terrestrial."

A few metres higher in the sequence, fossil material has been found that is very similar to Crassigyrinus.

"Crassigyrinus is a large tetrapod previously known only from the late Visean and early Namurian of Scotland. The newly discovered jaw ramus is almost exactly the same size as the known specimens, has almost identical external ornamentation, and differs from the known specimens in only minor details of the internal structure. [. . .] This Burnmouth horizon confirms the presence, by this early date, of large vertebrates whose affinities are with later Carboniferous rather than Devonian forms."

The same Formation outcropping at Burnmouth appears again at Willie's Hole to the south-west. Three distinct horizons have revealed tetrapods and many other fossils. In all, there are "100 samples of large and small semiarticulated tetrapod skeletons and isolated bones". In bed 1, reference is made to a small individual that can be provisionally reconstructed. "Its proportions most closely resemble those of the Visean Silvanerpeton or the Late Carboniferous Gephyrostegus.” Bed 2 has one of the largest specimens, but it cannot be assigned to an existing genus. There are indications that this animal was a mosaic of different characters allowing some similarities with other species to be recognised: "but only further study will elucidate their relationships". Another terapod from this bed had characters "reminiscent of that of a temnospondyl"(an upper Carboniferous group).

"[. . .] only further study could confirm or refute such an assignment. If corroborated, it would represent the earliest member of the group by about 15 million years."

Other sites are mentioned more briefly, but the fossil material does not change the position already noted. The authors are confident that their discoveries reveal Romer's Gap to be a collection failure. Consequently, it is not necessary to postulate low atmospheric oxygen levels to explain the absence of fossils.

"Our new records, combined with those from trackways, suggest that tetrapods appear to have recovered relatively rapidly from the EDME [End-Devonian Mass Extinction] by the mid-Tournaisian. Fish groups had evolved, or reevolved, into new large forms (e.g., rhizodonts, lungfishes). By the mid-Visean, not only had tetrapods appeared that are usually considered terrestrial and the base of the crown group been established, but highly specialized secondarily aquatic forms had also evolved."

The important point to note is that the new finds do not reveal an evolutionary trajectory linking the Devonian forms with the Carboniferous forms. Romer's Gap does not have transitional forms but documents the earlier appearance of five-toed Carboniferous forms. As is so frequently found, new fossil finds do not document evolutionary transitions but extend the range of the more "modern" life forms. This is the pattern recognised by advocates of Punctuated Equilibria, not the pattern predicted by Darwinists. For the researchers, the real work is before them.

"These finds will allow us to put forward refined hypotheses, testable by further finds and analyses. The wealth of material from several different sites and environments will provide the opportunity to investigate the causes and consequences of the EDME. Our initial results suggest that reestablishment of at least some components of the tetrapod fauna was achieved within 10 million years. We have established that pentadactyly arose about 20 million years earlier than previously documented. Studies may now examine the interlinkage of environmental and atmospheric changes to faunal turnover, the timing of ecosystem recovery, the sequence of acquisition of terrestrial characters by tetrapods, resolution of the problems of relationships among early tetrapods (and thus the recalibration of the phylogenetic tree), and the time of appearance of crown group tetrapods, based on the presence, rather than the absence, of fossil data."

The last sentence is worthy of note. However, it is in tension with Figure 6 in their paper, which has a family tree of tetrapods. All the fossil evidence shows discontinuity, but evolutionary linkages are marked (all located within Romer's Gap) that are devoid of supporting data. We are still a long way from a science that majors "on the presence, rather than the absence, of fossil data".

Earliest Carboniferous tetrapod and arthropod faunas from Scotland populate Romer's Gap
Timothy R. Smithson, Stanley P. Wood, John E. A. Marshall, and Jennifer A. Clack.
Proceedings of the National Academy of Sciences, published 5 March 2012 | doi:www.pnas.org/cgi/doi/10.1073/pnas.1117332109

Devonian tetrapods (limbed vertebrates), known from an increasingly large number of localities, have been shown to be mainly aquatic with many primitive features. In contrast, the post-Devonian record is marked by an Early Mississippian temporal gap ranging from the earliest Carboniferous (Tournaisian and early Visean) to the mid-Visean. By the mid-Visean, tetrapods had become effectively terrestrial as attested by the presence of stem amniotes, developed an essentially modern aspect, and given rise to the crown group. Up to now, only two localities have yielded tetrapod specimens from the Tournaisian stage: one in Scotland with a single articulated skeleton and one in Nova Scotia with isolated bones, many of uncertain identity. We announce a series of discoveries of Tournaisian-age localities in Scotland that have yielded a wealth of new tetrapod and arthropod fossils. These include both terrestrial and aquatic forms and new taxa. We conclude that the gap in the fossil record has been an artifact of collection failure.

See also:

Fossil discoveries fill crucial gap in land animal evolution, by Tamera Jones (Planet Earth Online, 7 March 2012)

Fossil pushes back land-animal debut, by Devin Powell (ScienceNews: Monday, March 5th, 2012)

Tetrapod family tree looks like a bush, by David Tyler (ARN Literature blog, 29 April 2009)

Although fossil plants are well documented from the Silurian Period of Earth history, spores from land plants are known from the preceding Ordovician and Cambrian Periods. However, it is not until the Mid-Devonian that fossil forests appear in the fossil record. The Gilboa Forest from New York State was first described in the 1920s and it became known as the earliest fossil forest. It has the same status today. Only one plant was known from this forest - the Eospermatopteris, or "ancient seed fern" - thought to grow up to 10 metres above the ground. They were not woody, but they had characters that that suggest affinities with tree ferns. The original analysis reinforced the evolutionary assumption that the earliest terrestrial systems were essentially simple. Recent research has changed all this.

"Palaeoecological studies of other Devonian-period sites describe early vegetated terrestrial landscapes partitioned into a 'two-dimensional' suite of patches growing side by side, each composed of closely related plants with similar morphologies and life traits, and adapted to the same environmental conditions. This structure of Devonian landscapes has almost become a dogma in palaeobotany, but Stein and colleagues' report provides the first direct evidence that some early forests contained widely divergent groups of plants." (Meyer-Berthaud & Decombeix, 2012, 41)

New interpretation of the Gilboa fossil forest (source here)

The new findings have been made possible by a dam maintenance project that involved the removal of infill from the original quarry where the fossil forest was found. A 1200 square metre palaeosol surface has been exposed and the locations of stumps of Eospermatopteris trees have been mapped. Two other significant components of the forest have been identified. These are:

"a large rhizomatous plant (one with underground stems growing horizontally) belonging to the extinct aneurophytalean progymnosperms; and a tree with bark similar to that of the lycopsid trees that inhabited [Carboniferous] coal swamps." (Meyer-Berthaud & Decombeix, 2012, 41)

The rhizomatous plant is of great interest because secondary xylem was detected in the rhizomes, but not in the shoots. The researchers considered that the shoots were not bio-mechanically capable of self-support. Also,

"they show that the aneurophytalean subterranean system - consisting mainly of rhizomes up to 15 cm in diameter - comprised a large amount of wood and had significantly more mass than previously estimated." (Meyer-Berthaud & Decombeix, 2012, 41)

The lycopsid-like trees were clearly present, but the fossils were not well preserved. They provide, however, a faunal link with the forests of the Carboniferous Period. Diversity is clearly present in the Devonian forest ecosystems, which were "much more complex than previously thought".

The most significant element of this complexity is the "bifacial vascular cambium" that is found in so-called 'modern' trees today. The term refers to the way the central cambium divides to give off water conducting wood towards the inside and food conducting wood towards the outside (the inner layers of the bark). Although Aneurophylates are already known from other Devonian deposits, this is the time they have been shown to have secondary wood typical of both hardwood and softwood trees. Therefore two important features of 'modern' trees - bifacial cambium and secondary thickening - were present in the Devonian Period.

The evolutionary paradigm is so deeply entrenched that every pointer to complexity steers questions as to how it evolved along Darwinian lines. So, Meyer-Berthaud & Decombeix write: the finding of woody rhizomes "adds credibility to the hypothesis that, in early land plants, wood did not evolve as an adaptation for mechanical support" (p.41). One senses that a just-so story is being developed: one that is based on adaptation and the presumption that Darwinian mechanisms are necessary to establish plausibility. However, it should be asked whether an ecological perspective could provide a way forward, given that the Devonian world was very different from today.

Another finding with a bearing on ecology relates to the youthfulness of the forest: it sprang up rapidly in an environment that was tectonically unstable. Gone is the old idea of a tranquil swamp. The researchers relocate the forest close to a shoreline, and recognise "brutal episodes of seal-level rise" that repeatedly flooded the forest and covered plant life with sediment. They suggest that the sizes of the large trees were constrained by environmental disturbances and that they were potentially fast-growing. The inference is that there was little time for the accumulation of materials to form coal.

"Low-angle cross-bedded sandstones in the Gilboa region containing several Eospermatopteris levels suggest that the site was formed by recurrent, marine-influenced and possibly catastrophic, processes with a relatively high frequency of disturbance." (Stein et al. 2012. 81)

These are fascinating discoveries for anyone interested in the history of life on Earth. Wood provides evidences of modernity in the aneurophytaleans, there is unexpected complexity and Eospermatopteris trees indicate transient environments for growing followed by abrupt destruction of the forest. Further research is needed to establish whether an evolutionary framework is justified in order to understand the findings or whether an ecological framework is sufficient. At very least, we should note that the evolutionary approach has resulted in numerous surprises and significant contradictions to the expectations of researchers.

Surprisingly complex community discovered in the mid-Devonian fossil forest at Gilboa
William E. Stein, Christopher M. Berry, Linda VanAller Hernick & Frank Mannolini
Nature, 483, 78-81 (01 March 2012) | doi:10.1038/nature10819

The origin of trees by the mid-Devonian epoch (398-385 million years ago) signals a major change in terrestrial ecosystems with potential long-term consequences including increased weathering, drop in atmospheric CO2, modified climate, changes in sedimentation patterns and mass extinction. However, little is known about the ecology of early forests or how changes in early terrestrial ecosystems influenced global processes. One of the most famous palaeontological records for this time is the 'oldest fossil forest' at Riverside Quarry, Gilboa, New York, USA, discovered in the 1920s. Hundreds of large Eospermatopteris sandstone casts, now thought to represent the bases of standing cladoxylopsid trees, were recovered from a horizon that was originally interpreted as a muddy swamp. [. . .] Here we describe a 1,200 m2 map showing numerous Eospermatopteris root systems in life position within a mixed-age stand of trees. Unexpectedly, large woody rhizomes with adventitious roots and aerial branch systems identified as aneurophytalean progymnosperms run between, and probably climb into, Eospermatopteris trees. We describe the overall habit for these surprisingly large aneurophytaleans, the earliest fossil group having wood produced by a bifacial vascular cambium. The site also provides evidence for arborescence within lycopsids, extending the North American range for trees in this ecologically critical group. The rooting horizon is a dark grey sandy mudstone showing limited root penetration. Although clearly belonging to a wetland coastal plain environment, the forest was probably limited in duration and subject to periodic disturbance. These observations provide fundamental clarification of the palaeoecology of this mixed-group early forest, with important implications for interpreting coeval assemblage data worldwide.

In the shade of the oldest forest
Brigitte Meyer-Berthaud & Anne-Laure Decombeix
Nature, 483, 41-42 (01 March 2012) | doi:10.1038/483041a

The uncovering of a large soil surface preserved under sediment for 390 million years has exposed plant remains which show that the world's earliest forests were much more complex than previously thought.

See also:

World's Oldest Fossilised Forest Unearthed in NY, William Stein (Binghampton University, 28 February 2012)

Floor of Oldest Fossilized Forest Discovered: 385 Million Years Old, ScienceDaily (Mar. 1, 2012)

The abrupt appearance of Cambrian life forms in the Cambrian Period of Earth history continues to provide us with spectacular evidence of sophistication. New research on fossils recovered from petroleum exploration drill cores assigned to the Deadwood Formation of western Canada documents "a cryptic but significant diversity of Cambrian crustaceans". Previously, palaeontologists have had hints of these animals from the nonmineralised remains of minute organisms (<2mm). The new finds are of disarticulated body parts that are unambiguously crustacean, representing branchiopods, copepods and ostracods. They are part of an assemblage known as SCFs (small carbonaceous fossils). They show many signs of modernity.

"The fresh taphonomic perspective of SCFs provides the only direct evidence for sophisticated particle-handling in larger-bodied Cambrian arthropods. This characteristically crustacean-type ecology at the interface of micro- and macroscopic nutrient cycling has otherwise been loosely inferred from overall body form and the proxy record of phytoplankton diversification. The detailed adaptations described here represent the acme of Cambrian differentiation within appendages, an alternative (and potentially correlative) measure of evolving arthropod complexity to the larger-scale tagmosis that has been the focus of previous studies. In part, the new fossils reinforce a picture of early origination and subsequent conservation in crustacean form and function."

Fossil branchiopod-type limbs from the Middle Cambrian Deadwood Formation. (for detailed explanation and the PNAS source, go to Figure 3 here)

Branchiopod-type mandibles have been recovered that have overall shapes and ornamentation that are "conspicuously similar to those of branchiopod crustaceans". There are "striking similarities" with mandibles of "various extant anostracan branchiopods". These body parts are considered to be "adapted for enhanced food-grinding efficiency". The inference from the size of these parts is that these organisms had a body size of at least 10-15 mm and formed part of a more complex ecosystem than is normally envisaged for the Cambrian:

"The presence in the first and second morphotypes of a moderately sized posterior tooth and an asymmetric "tooth-groove" system points to an ecology of mixed benthic scraping and suspension feeding, as opposed to more exclusive predation or suspension feeding."

Some of the most beautifully preserved body parts are the filter plates of branchiopods that are essentially modern in form. Together with other material, the authors have reconstructed the branchiopod crustacean.

It has "a long series of filtering thoracic appendages, [and] an overall body length of at least several millimeters is likely for the more articulated arrays, although a centimetric body size is suggested by isolated filters constructed from substantially larger setae. A mixed scraping/filtering ecology (rather than a wholly planktic mode of life) is suggested by the juxtaposition of filter plates and saw-toothed armatures."

Copepod-type mandibles have been recovered where there are many "close matches" with living species.

"In particular, the prominent projecting seta is comparable in form and position to the potentially homologous "dorsal seta" (sometimes a pair of setae) found in every major order of nonparasitic copepods [i.e., Calanoida, Cyclopoida, Platycopioida, Misophrioida, Harpacticoida and Mormonilloida."

Ostracod-type mandibles closely resemble those belonging to both ostracod subgroups - the Myodocopa and Podocopa.

"The complexity and form of the gnathal edge appear to be shared in particular with halocyprid myodocopes, some of which express a similar suite of characters including a raised toothed blade with adjacent long setae, an intermediate region with a hook-shaped spine, and a protruding grinding surface."

In their discussion, the researchers draw attention to the important evidences of modernity in the fossil material. It is customary to refer to stem-group and crown-group organisms, with the latter representing life forms that have reached a mature phase of evolutionary development.

"[T]he Deadwood fossils risk being assigned to inappropriately derived positions because of their "modern" appearance but disarticulated condition. Therefore, we conservatively assign them to comparatively inclusive clades, identifying crown groups via a synapomorphy shared with a subset of the crown (. . .) Taken together, our results provide unambiguous evidence for a substantial branching by the Late Cambrian of within-crown (pan)crustacean lineages - a largely cryptic component of the Cambrian "explosion" ."

The distinction between stem-group and crown-group forms draws heavily on the presupposition of evolutionary transformation from simpler "primitive" precursors. However, there are many evidences that are not a comfortable fit within this theoretical framework. The Deadwood assemblage is a case in point. All the groups identified demonstrate modernity of form extending back to the Middle Cambrian. Evidence for stem-group characters does not exist. Sophisticated structures are apparent in the first appearance of these organisms.

"In part, the new fossils reinforce a picture of early origination and subsequent conservation in crustacean form and function."

Interestingly, this research points to an ecological (rather than an evolutionary) theoretical framework for understanding the fossil data. The authors refer to an "unanticipated ecologic turnover". This is from their concluding paragraph:

"At the same time, however, the small carbonaceous record provides evidence for unanticipated ecologic turnover. In the modern oceans, branchiopods are represented by a just a few species of small, secondarily marine cladocerans; larger forms, comparable in size to those of the Deadwood (up to ~15 mm or more) and Mount Cap (~50 mm), are now entirely nonmarine. Furthermore, modern free-living copepods are almost all much smaller than the ~5- to 10-mm (plus) Deadwood taxon. In the modern world, visual predators - especially teleost fish - drive down body size in planktic freshwater crustacean communities and strongly constrain the complex behaviors and distribution patterns of krill, a group that shares with the Cambrian branchiopods the attributes of centimetric body size, marine habitat, and (by convergence) thoracic filtering. Significantly, the Deadwood and Mount Cap fossils reveal a contrasting pattern of crustacean distribution in the comparatively "unescalated" Cambrian biosphere."

The scenario, then, is one of adaptation to changing environments, where organisms are affected by environmental factors (including predation). The adaptations do not result in evolutionary novelties, but to changes in size, lifestyles and ability to thrive in waters of different salinity. These changes are not surprising, given the phenomenon of phenotypic plasticity. If some want to describe this as "evolution", then they should note that they are working with a concept that does not begin to explain the origin of branchiopods, copepods and ostracods.

Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation
Thomas H. P. Harvey, Maria I. Velez, and Nicholas J. Butterfield
PNAS, Published online before print January 17, 2012, doi: 10.1073/pnas.1115244109

Abstract: The early history of crustaceans is obscured by strong biases in fossil preservation, but a previously overlooked taphonomic mode yields important complementary insights. Here we describe diverse crustacean appendages of Middle and Late Cambrian age from shallow-marine mudstones of the Deadwood Formation in western Canada. The fossils occur as flattened and fragmentary carbonaceous cuticles but provide a suite of phylogenetic and ecological data by virtue of their detailed preservation. In addition to an unprecedented range of complex, largely articulated filtering limbs, we identify at least four distinct types of mandible. Together, these fossils provide the earliest evidence for crown-group branchiopods and total-group copepods and ostracods, extending the respective ranges of these clades back from the Devonian, Pennsylvanian, and Ordovician. Detailed similarities with living forms demonstrate the early origins and subsequent conservation of various complex food-handling adaptations, including a directional mandibular asymmetry that has persisted through half a billion years of evolution. At the same time, the Deadwood fossils indicate profound secular changes in crustacean ecology in terms of body size and environmental distribution. The earliest radiation of crustaceans is largely cryptic in the fossil record, but "small carbonaceous fossils" reveal organisms of surprisingly modern aspect operating in an unfamiliar biosphere.

See also:

Butterfield, N. J. and Harvey, T. H. P. (2012) Small carbonaceous fossils (SCFs): a new measure of early Paleozoic paleobiology. Geology, 40(1). 71-74.

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