Malformations in Trilobites from the Silurian and Devonian of Europe.

Malformations in fossils, such as pathologies caused by infections, scars left by recovered injuries, or teratologies caused by developmental problems, can tell us a lot about how extinct organisms grew and interacted with their environments, although when analysing these is clearly preferable to have access to non-malformed examples of the same species, or at least a close relative.

Trilobites were a diverse and abundant group of Arthropods which appeared early in the Cambrian, and survived until the End Permian Extinction. Their dorsal exoskeleton was heavily biomineralized, and was shed periodically to allow the animals to grow. This has lead to an extensive fossil record with Trilobites being extremely common in many Palaeozoic marine deposits. This abundant fossil record makes Trilobites an excellent candidate group for the study of malformations.

In a paper published in the journal Acta Palaeontologica Polonica on 22 April 2025, Russell Bicknell of the Division of Paleontology (Invertebrates) at the American Museum of Natural History, and the Palaeoscience Research Centre at the University of New England, Patrick Smith of the Palaeontology Department at the Australian Museum Research Institute, and the Department of Biological Sciences at Macquarie University, Lisa Amati of Paleontology at the New York State Museum, and Melanie Hopkins, also of the Division of Paleontology (Invertebrates) at the American Museum of Natural History, describe malformations in European Silurian and Devonian Trilobite specimens from the collections of the Natural History Museum in London and the New York State Museum.

The first specimen examined by Bicknell et al., NYSM 19739, is an isolated cephalon (head part) from a Harpetid Trilobite, Lioharpes venulosus, from the Early Devonian Koněprusy Limestone of the Czech Republic, in the collection of the New York State Museum. The cephalon is 26.2 mm long and 22.3 mm wide with a u-shaped indentation on its right marginal rim. This indentation is 5.6 mm long and extends 1.9 mm towards the midline. The marginal rim is covered in small circular pits, which around the indentation are irregular, ovate, and occasionally fused into larger pits.

Malformed Harpetid Trilobite Lioharpes venulosus, NYSM 19739 from the Koněprusy Limestone, Pragian, Lower Devonian, Koněprusy, Czech Republic. (A₁) complete cephalon; (A₂) close up showing U-shaped indentation (arrow). Specimen coated in ammonium chloride sublimate. Bicknell et al. (2025).

Bicknell et al. note that malformations to the cephalic fringes of Harpetid Trilobites have been recorded before, and that these are usealy attributed to injuries, an analysis with which they concur. However, they also observe that injuries can happen in a variety of ways, with fringe injuries having previously attributed to problems during moulting, failed predation attempts, or unknown causes. They suggest that a moulting injury is the most likely explanation for the injury to the Koněprusy specimen, with the delicate fringe likely torn during moulting, and the enlarged and fused pits being a result of fusion of the torn margin during healing. Various purposes have been suggested for the cephalic fringes of Harpetid Trilobites, including filtering for food, sensory roles, sediment ploughing, hydrostatic support, cephalic reinforcement, burrowing, and enhancing hydrodynamic efficiency. Whatever the purpose of this organ, an injury to it is likely to have been detrimental to the living Trilobite, and presumably repairing this injury would have been a priority during subsequent moults.

The second specimen, NHMUK PI In 65061, looked at is a Phacopid Trilobite, Calymene blumenbachii, from the Early Silurian Much Wenlock Limestone Formation of Shropshire, England, in the collection of the Natural History Museum. This specimen comprises a partial cephalon, thorax, and pygidium, with a total length of 92.9 mm and a width of 48.6 mm. The second thoracic axial ring (middle part of the second segment of the thorax) of this specimen is covered by a structure with closely spaced openings, which has an elevated round crater at its right extremity, with an opening 1.7 mm across.

Bicknell et al. interpret this as an encrusting Trepostome Bryozoan covering the 3rd thoracic tergite, with the larger opening being an ovate zoarium (specially modified zooid which produced eggs). The restriction of the encrustation to one tergite strongly suggests that this happened while the Trilobite was alive, and that the Bryozoan colony was therefore unable to overgrow the articulations between tergites.

Abnormal Calymenid Trilobites Calymene blumenbachii from the Much Wenlock Limestone Formation, Homerian, Wenlock, Silurian, England, UK. (A) NHMUK PI In 65061, (A₁) complete specimen; (A₂) close up showing the large bryozoan growth. (B) NHMUK PI In 19857 showing pygidial ribs that terminate early (white arrows) and are fused proximal to the medial lobe (black arrow).  Bicknell et al. (2025).

The next specimen examined, NHMUK PI In 19857, is another example of Calymene blumenbachii from the Much Wenlock Limestone. This specimen is a partial pygidium (tail part) 13.2 mm long and 18.2 mm wide. On the right side of this specimen the pygidial ribs are disrupted and irregular, with two ribs terminating 1.6 mm short of the pygidial margin, while another two fuse 1.2 mm from the pygidial axis.

Bicknel et al. observe that similar deformations to the pygidia have been observed in a wide range of other Trilobites, including Dalmanities pleuroptyx, Dechenella macrocephalus, Niobina sp., and Prionopeltis archiaci. They attribute these deformations to genetic or developmental issues, but suggest that the limited disruption to the pygidium they caused did not represent a major handicap.

Specimen NYSM 19740 is an Acastid Trilobite, Treveropyge sp., from the early Devonian Saint Céneré Formation of Mayenne in northwest France. this specimen is another isolated pygidium, s 11.6 mm long and 17.9 mm wide. It has a deformation to the axial lobe, which is asymmetric, with two of the axial rings malformed and curving to the right.

Malformed Acastid Trilobite Treveropyge sp., NYSM 19740 from the Saint Céneré Formation, Lochkovian, Lower Devonian, Mayenne, France. (A₁) Complete pygidium; (A₂) close up showing asymmetrical axial lobe and incomplete axial ring (arrows). Specimen coated in ammonium chloride sublimate. Bicknell et al. (2025).

Again, Bicknell et al. note that similar deformations have been seen in other Trilobites, such as Calliops marginatus, Dolicholeptus licticallis, and Sceptaspis lincolnensis. They suggest that these malformations are caused by genetic deformations, leading to incomplete development or non-functional somites.

The final specimen examined, NHMUK PI I 1108, is an external impression of a partial pygidium belonging to the Styginid Trilobite Scutellum (Scutellum) pardalios, from the Middle Devonin Barton Limestone Member of Devon, England. This impression is t is 59.5 mm long and 44.0 mm wide. On the right side of this impression (i.e. on the left side of the Trilobite) two ribs fuse into a single rib 29.1 mm from the pygidial axis. This single rib then terminates 4.8 mm from the pygidial margin.

Malformed Styginid Trilobite Scutellum (Scutellum) pardalios, NHMUK PI I 1108 from the Barton Limestone Member, Torquay Limestone Formation, Givetian, Middle Devonian, England, UK. (A₁) Pygidium preserved as external impression; (A₂) close up showing fused pygidial pleurae (arrows). Bicknell et al. (2025).

Bicknell et al. note that the surface of the pygidium was covered with ornamentation, with no visible break in this, which appears to  rule out the malformation having been formed by an unsuccessful predation attempt, or similar injury. Recovery from injury is the most commonly sited reason for malformations seen in Styginid Trilobites, followed by parasitic infections during early development, which seems equally unlikely. Bicknell et al. suggest instead that this deformity might be the result of a difficult moult, or possibly a genetic aberration. They do not believe this minor deformity would have significantly affected the living Trilobite.

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Wildfires in the Silurian.

Free oxygen first appeared in the Earth's atmosphere during the Great Oxidation Event, between 2.5 and 2.3 billion years ago, and has played a significant role in all biogeochemical cycles ever since. A soil-forming microbial cover is thought to have begun to form on the Earth's landmasses about 850 million years ago, which further contributed to the oxygen content of the atmosphere. Following this, atmospheric oxygen concentrations are thought to have remained fairly constant until the appearance of the first land Plants in the Ordovician. 

These first Plants emerged into a world already dominated by Fungi, a group which quickly acquired the ability to both break-down the tissues of dead plants, and to form mycorrhizal relationships with living ones, enhancing the flow of nutrients through these primitive terrestrial ecosystems, and causing another rise in oxygen levels. The oldest known Vascular Plant, Cooksonia, a leafless, dichotomously branching Plant reaching less than 10 cm high, appeared during the Wenlock Epoch of the Silurian, between 433.4 and 427.4 million years ago, and has been recorded from rocks in Ireland and the Czech Republic. However, Cooksonia is not the most common form in the deposits where it is found, it lived in a landscape dominated by Namatophytes, vegetative organisms comprised of aggregations of tubes and cuticles, which were almost certainly Fungi, and possibly Lichens, and fossils of which have been found around the world.

The appearance of terrestrial Plants is generally accepted to have provoked a rise in atmospheric oxygen levels, although the extent of this is hard to determine, and scientists have differing opinions upon when oxygen levels comparable to those seen today were first achieved. 

In a paper published in the journal Geology on 13 June 2022, Ian Glasspool and Robert Gastaldo of the Department of Geology at Colby College, attempt to reconstruct the oxygen content of the Silurian atmosphere, by using charcoal fragments from deposits in Wales and Poland as evidence of Silurian wildfires, which would have required a minimum level of oxygen to occur.

Charcoal can only form when the atmospheric oxygen pressure is between 70% and 140% of that of today's atmosphere. Lower than this, and plant matter will not combust, higher, and it will combust completely, turning all available carbon into CO₂. Charcoal first appears in the Ordovician, suggesting that the atmosphere had sufficient oxygen for it to form then, but disappears again in the (Early Devonian, 419.2-410.8 million years ago), and is absent for much of the the rest of the Devonian. 

Glasspool and Gastaldo examine microscopic charred plant fragments from the Middle Silurian Pen-y-lan Mudstone at Rumney in Wales, and which is about 10 million years older than the previous oldest charred vegetative material, from the Late Silurian Winnica Formation at Winnica in Poland.

Silurian palaeogeography with Rumney (Wales, UK) and Winnica (Poland) localities plotted against International Geological Congress time scale version 2021.7. Abbreviations: Ord., Ordovician; Sa, Sandbian; Ka, Katian; Hi, Hirnantian; Rh, Rhuddanian; Ae, Aeronian; Te, Telychian; Sh, Sheinwoodian; Ho, Homerian; Go, Gorstian; Lu, Ludfordian; Pd, Přídolí; Lo, Lochkovian; Pr, Pragian; Em, Emsian; Ei, Eifelian; Gi, Givetian; Fr, Frasnian; Fa, Famennian. Glasspool & Gastaldo (2022).

Organic material from Rumney is dominated by Nematophytes, and in particular members of the genus Pachytheca. Glasspool  and Gastaldo obtained charred vegetative fragments from rock samples obtained from a borehole, drilled by the British Geological Survey, at depths of between 314.71 and 316.3 m. 

Since the exact nature of Nematophytes is unclear, it was necessary to use proxies to estimate the temperature at which this material may have burned; previous research having found that charcoal can be made from wood at temperatures between 410 and 730°C, and from Bracket Fungi at between 440 and 940°C. Based upon the degree of charring, Glasspool  and Gastaldo conclude that the material from Rumney burned at an average of about 490°C if Namatophytes were woody, or about 540°C if they were Bracket Fungus-like, although the most charred were probably burned at about 640°C (if woody) or 820°C (if Bracket Fungus-like).

Select charred phytoclasts: Rumney (Wales, UK) Nematophytes in reflected light (A), (B) and scanning electron microscopy images of Nematophytes from Winnica (Poland) (C), (D). (A) High reflectance and brittle fracture in Prototaxites. (B) Outer cortex of Pachytheca. (C) Trilayered Nematophyte cf. Tristratothallus. (D) Aggregation of small and large tubes. Glasspool & Gastaldo (2022).

The development of an oxygen-rich atmosphere has been crucial for the development of life on Earth, and particularly so for the emergence of Animals onto land. However, there is no direct way of measuring the oxygen content of ancient atmospheres is impossible, forcing scientists to rely on proxies for this information. As the formation of charcoal requires atmospheric oxygen levels approaching those of today, charcoal is an excellent proxy for the development of an oxygen-rich atmosphere, and Glasspool and Gastaldo's evidence that this may have developed by the Wenlock Epoch of the Silurian Period. about 430 million years ago.

There are some significant carbon isotope excursions (short intervals in which the proportions of carbon isotopes in sedimentary rocks shift significantly) in the Silurian, including notable ones in the Wenlock to Přídolí epochs, which suggests that the Silurian carbon cycle was more prone to severe disruption than that of any other period of the Phanerozoic. While some fairly good models of oxygen fluctuations in the Silurian have been developed, these operate on a scale of tens of millions of years, and cannot provide helpful data on the sort of sudden, short lived events that would cause these isotope excursions. However, a stratigraphic model of fire intervals within the Silurian could potentially address this mystery, something Glasspool and Gastaldo propose creating.

Most current predictions of oxygen levels in the atmosphere suggest that the oxygen content remained below 15% from the Late Ordovician until the Early Devonian. However, this is inconsistent with evidence for wildfires in the Silurian, as an oxygen content of 16% or above is needed for the combustion of dry vegetative matter. One model (GEOCARBSULFOR) does allow for higher oxygen levels, but still below 18%, which would allow for small fires under very dry conditions, though Glasspool and Gastaldo question whether any Silurian Plant or Fungus was capable of surviving in such an environment. The deposits at Rumney record a fairly high proportion of charcoal in a marine sediment, which would generally imply a conflagration of some size, inconsistent with any current model of Silurian oxygen levels.

Most of the data from Rumney implies a fairly low temperature fire (how low depending on whether the vegetation was Plant- or Fungus-like), but there are charcoal fragments which formed at far higher temperatures, suggesting at least pockets of high temperature fire. This seems implausible given the nature of the fuel, though Glasspool and Gastaldo suggest it it more plausible if Nematophytes were Fungal in nature, as Fungi naturally burn at higher temperatures.

The limiting factor for vegetative growth in the Silurian was probably the availability of phosphorus. Fungi and Bacteria can liberate this from rock to some extent, enabling the growth of Lichens and Plants (the later through symbiotic mycorrhizal relationships), but the phosphorus then becomes locked into the Plant or Fungus body, even after it dies. Fires would have had the ability to free up phosphorus locked into dead vegetation, making it available for new growth on land and in the oceans, where it could prompt blooms of photosynthetic algae, further rising the oxygen level. However, too frequent or severe fires would have damaged the soil structure, killing off microbial communities and creating a sterile environment.

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Investigating tides as an environmental driver during the Fish-Tetrapod transition.

Only once in Earth’s history did vertebrates make the transition from an aquatic to terrestrial environment; trackway evidence indicates this occurred approximately 393 million years ago, although the earliest definite Tetrapod body fossils are approximately 20 million years younger. By contrast, there have been multiple adaptive radiations of Vertebrates from land back to the ocean, e.g. separate groups of semi-aquatic Mammals becoming the earliest Cetaceans and Sirenians at around 50 million years ago. The origin of Tetrapods was itself part of the rapid early diversification of Bony Fish (Osteichthyes); shortly after their origin, the Osteichthyes split into Ray-finned Fish (Actinopterygii, the predominant Fish group today) and Lobe-finned Fish (Sarcopterygii), the latter giving rise to Tetrapods. The earliest known crown-group Usteichthyans come from the Late Silurian (425 million years ago) of South China, suggesting that the whole process took little more than 30 million years. Most of the terrestrial adaptations, including the modification of the pectoral and pelvic fins into weightbearing limbs, were acquired during the origin of Tetrapods. However, one key component, the lungs, is older and can be traced back to the origin of the Osteichthyes, where they evidently evolved for use as supplementary respiratory organs in an aquatic environment before being co-opted to support terrestrial life. The crown-group Osteichthyes most probably originated in South China, as the earliest known members are found there, and the Late Silurian to Early Devonian (starting 425 million years ago) faunas of the region contain a diversity of Osteichthyans that cannot be matched elsewhere. The origin of tetrapods is more difficult to pinpoint, but the two earliest known trackway localities are situated in present day Europe, which at the time was part of the ancient supercontinent Laurussia; the earliest body fossils are also Laurussian. Although the drivers behind the evolution of Osteichthyans and Tetrapods are as yet poorly understood and many hypotheses have been suggested to be behind these evolutionary events, it is known that the palaeoenvironment was rapidly transforming due to the emergence of macroscopic Plant communities on land and a period of overall marine regression occurring from the Late Silurian to Middle Devonian.

In a paper published in the Proceedings of the Royal Society Series A: Mathematical, Physical, and Engineering Sciences on 21 October 2020, Hannah Byrne of the School of Ocean Sciences at Bangor University, and the Department of Organismal Biology at Uppsala University, Mattias Green, also of the School of Ocean Sciences at Bangor University, Steven Balbus of the Department of Physics at the University of Oxford, and Per Ahlberg, also of the Department of Organismal Biology at Uppsala University, explore the hypothesis that tides were an important environmental adaptive pressure.

The influence of tides on the Fish-Tetrapod transition has been the subject of several studies by palaeontologists and developmental biologists, with Steven Balbus producing the most comprehensive intertidal hypothesis. The hypothesis, an elaboration on Alfred Romer’s classical ‘drying pools’ hypothesis, is that as the tide retreated, Fish became stranded in shallow water tidal-pool environments, where they would be subjected to raised temperatures and hypoxic conditions. If there was a large spring–neap variation in tides, which today occurs on a 14-day cycle, individuals trapped in upper-shore pools during spring tides could be stranded for several days or considerably longer, depending on the beat frequency of the solar and lunar tides. This would select for efficient air-breathing organs, as well as for appendages adapted for land navigation, so that the Fish could make their way to more frequently replenished pools closer to the sea. Experimental rearing of Polypterus (a basal member of Actinopterygians, the sister group to Sarcopterygians) in terrestrial conditions results in single-generation morphological adaptation to terrestrial locomotion by means of developmental plasticity, suggesting that environmental factors are powerful drivers of such evolutionary changes. While the expanse of estuaries and deltas is largely controlled by long-term sea-level fluctuations, a large tidal range would also help to maintain such regions, which provide an ideal transitory environment for the terrestrialization of Tetrapods. Many of the earliest Tetrapods, as well as the transitional ‘Elpistostegalians’ Panderichthys and Elpistostege (though not Tiktaalik), are found in sediments identified as deltaic or estuarine, and isotopic evidence supports a lifestyle adapted to a wide range of salinities. Furthermore, a recent study on ancestral Vertebrate habitats has suggested that many early Vertebrate clades originated in shallow intertidal–subtidal environments.

Byrne et al. investigated whether there is a detailed hydrodynamic basis for inferring that large tides did indeed exist during the Late Silurian to the early Late Devonian in locations where evidence for early Osteichthyans and early Tetrapods have been found. Byrne et al. have used recent global palaeogeographic reconstructions for the Late Silurian (420 million years ago), early Middle Devonian (400 million years ago) and early Late Devonian (380 million years ago) in an established state-of-the-art numerical tidal model. Byrne et al. evaluate the two dominant components of the contemporaneous tide: the principal lunar constituent and the principal solar constituent to allow us to compute spring–neap range variability. Neap tides occur when principal lunar constituent and the principal solar constituent are out of phase, and spring tides when they are in phase, so the spring–neap range difference is equal to the range of the principal solar constituent. Byrne et al. also discuss the simulated tidal ranges for both tidal constituents. They focus on two geographical areas in the reconstructions: the South China region for the 420 million years ago time slice, and Laurussia for the 400 million years ago time slice, because of their respective associations with the earliest Osteichthyans and the earliest trace fossil evidence of Tetrapods in the form of trackways. The 380 million years ago time slice is included to encompass the period in which body fossils of Elpistostegalians occur, during the late Givetian to mid-Frasnian. Like the earliest Tetrapod trackways, two of the three main Elpistostegalid genera (Panderichthys and Elpistostege) occur along the Southern coastline of Laurussia. The South China region for Byrne et al.'s study includes Indochina, as there is evidence that the South China and Indochina blocks were linked due to the presence of similar fauna in the fossil record. To test the robustness of our simulation outputs, Byrne et al. have identified three tidal proxies for each time slice which they have used for comparison. 

 

Themodel bathymetry for 420 million years ago (a), 400 million years ago (d) and 380 million years ago (g), with depth saturating at 6000 m (Abyssal ocean is at 4200 m, with trenches at 6000 m). The major continents are as follows: Laurussia is highlighted as panels (c), (f) and (i), Gondwana is the major continent in the south of panels, and Siberia is located northeast of Laurussia denoted as S in panels (a), (d) and (g). The South China region is highlighted in panels (b), (e) and (h), with South China denoted as SC and Indochina as IC. The tidal proxies have been indicated in each time slices; Kez Fm, Keziertage Formation; Kar Fm, Karheen Formation; Man Fm, Manlius Formation; Pad Fm, Padeha Formation; Batt P Fm, Battery Point Formation; Pär and Rez Fms, Pärnu and Rēzekne Formations; Gau Fm, Gauja Formation; Ham gp, Hamilton Group and Esc Fm, Escuminac Formation. The stars in (f) indicate the locations of the two earliest fossil Tetrapod trackways. Zachelmie is denoted by Z and Valentia Island as V. Byrne et al. (2020).

The tides for the periods of interest were simulated using the Oregon State University Tidal Inversion Software, which has been used extensively to simulate deep-time, present day and future tides. The Oregon State University Tidal Inversion Software provides a numerical solution to the linearised shallow water equations, with the nonlinear advection and horizontal diffusion excluded without a loss in accuracy.

Byrne et al. generated close to 100 simulations using five different reconstructions of the bathymetry for present day, and for the 420 million year ago, 400 million year ago and 380 million year ago time slices. To replicate the relevant tidal forcing for the past time slices, the equilibrium tidal elevation and frequency of the tidal constituents were altered. These constituents allow the calculation for the tidal range and spring–neap range. For the Late Silurian (420 million year ago), the principal lunar constituent period used was 10.91 hours and the principal solar constituent period was 10.5 hours. For the early Middle Devonian (400 million year ago), slightly longer periods of 10.98 hours for principal lunar constituent and 10.7 hours for principal solar constituent were used, whereas the early Late Devonian (380 million year ago) had an principal lunar constituent period of 11.05 hours and an principal solar constituent period of 11.0 hours. These numbers are based on small changes to a contemporaneous lunar semi-major axis (average distance between the Earth and the Moon) of 365 000 km, and are consistent with studies on Silurian–Devonian Corals and Brachiopods growth increments (simulations run with present day values for these parameters show qualitatively similar overall results). Because the orbital periods are directly related to lunar distance, Byrne et al. increased the corresponding lunar force by 15%, but did not allow for this to vary between the time slices.

The bathymetric dataset for the present day simulations were a conglomerate of version 14 of the Smith and Sandwell topographic database, along with updated bathymetries for regions north of 79°N from the International Bathymetric Chart of the Arctic Ocean and south of 79°S from a recent research paper by Laurie Padman, Helen Fricker, Richard Coleman, Susan Howard, and Lana Erofeeva. The combined dataset was averaged to 1/4° in both latitude and longitude, to match that of the palaeobathymetry data. Simulations with this bathymetry are referred to as ‘present day control’.

There are several reconstructions of the palaeogeography available for the time periods in question. Byrne et al. have used the latest products from Deeptime Maps, representing 420 million years for the Late Silurian (Pridoli–Lochkovian), 400 million years for the late Early Devonian (Emsian) and 380 million years for the early Late Devonian (Middle Frasnian). There is a difficulty to directly turn the maps into numerical model grids due to a lack of bathymetry depth information for the deep time slices, beyond what is included in the published reconstructions. Byrne et al. have quantified the oceanic bathymetry using step-changes in depths of 150 m, 300 m, 800 m for the continental shelf, and a 4200 m deep abyssal plain. Byrne et al. refer to this simulation as ‘control’ in the following. The assumption for this choice of depths is that the period of study is at a similar point in the supercontinent cycle as present day, so the age of the oceanic plates would be comparable between the Devonian and present day.

This means that the mean depths of the abyssal plain and continental shelfs should be similar for both; this underpins Byrne et al.'s control bathymetry set The bathymetry outlines (e.g. what are shelf seas, continental slope) is determined by the palaeogeographic reconstructions. Because of the poorly constrained depths in the past reconstructions, Byrne et al. did a suite of sensitivity simulations where the depths were modified to check the robustness of our results. These are referred to as ‘shallow’ and ‘deep’ and have the depths shallower than 800 m from the mid-bathymetries halved or doubled, respectively. Byrne et al. also did a set of simulations were water shallower than 150 m in the mid-bathymetries were set to land (testing sensitivity to coastline locations), another two sets of simulations where water shallower than 800 m in the mid-bathymetries were set to wither 800 m or 150 m, respectively. Byrne et al. refer to these three sets as ‘no shelf’, ‘deep shelf’ and ‘shallow shelf’.

Stratification is also poorly constrained because there are as yet no ocean model simulations of the period published (although some are in progress). It has been shown that the tides are relatively insensitive to the buoyancy frequency, within an order of magnitude or so from present day values. Consequently, Byrne et al. used the standard globally averaged buoyancy profile used before in their simulations as well, and then did a series of sensitivity tests to explore robustness. In the sensitivity simulations, which were done for all six bathymetries (shallow, mid and deep, and no shelf, shallow shelf and deep shelf) for all three time slices, with the buoyancy frequency halved or doubled. As ongoing ocean model experiments are able to produce progressively more reliable estimates of Devonian stratification, Byrne et al. will revisit the details of their computations. For now, the sensitivity simulations show a degree of robustness that warrants the support of theirr emphasis on the role of tides in the evolution of Osteichthyans and early Tetrapods. The shelf simulations, and the stratification sensitivity simulations are mainly used for statistics of the robustness of the tidal dynamics.

Byrne et al. also introduced degraded present day bathymetries based on the method for the Devonian simulations. In these, the same depth ranges were used as in the Devonian bathymetries, i.e. any water shallower then 150 m was set to 150 m, anything in the range 150–300 m or 300–800 m was set to 300 m and 800 m, respectively, and anything deeper than 800 m was set to 4200 m (Byrne et al.'s abyssal depth). Byrne et al. refer to this as present day mid, and again computed deep and shallow bathymetries.

Themodel output consists of the amplitudes and phases of the surface elevations and velocities for each simulated tidal constituent. Both the present day control simulation and degraded present day simulation, were then compared to the TPXO9 satellite altimetry constrained product, giving a globally averaged root-mean-square error of 12 cm and 20 cm, respectively, for the principal lunar constituent amplitudes. The results suggested that Byrne et al. should expect an over-estimate in tidal ranges located in shelf seas for their palaeotidal simulations. Byrne et al. discuss a classification of tidal ranges, and say that micro-tidal refers to a range of 0–2 m, a meso-tidal range is 2–4 m, a macro-tidal range sits between 4–8m and a mega-tidal range is larger than 8 m.

 

(a), (b) Show the modelled principal lunar constituent tidal ranges (in metres) for the present day control (a) and present day reconstructed simulations (b). The root-mean-square error values between the modelled and the TPXO M2 amplitudes are approximately 12 cm for present day and approximately 20 cm for present day reconstructed. (c), (d) as in (a) and (b) but for the principal solar constituent. Byrne et al. (2020).

The extraction of palaeotidal data from the geological record can be difficult and uncertain, but there are tidal deposits described in the literature for the periods of study. Byrne et al. have identified three deposits per time slice that can be used to test the robustness of our simulations. They have used the tidal depositional systems and relative tidal ranges classification from a recent paper by Sergio Longhitano, Donatella Mellere, Ronald Steel, and Bruce Aimsworth, to quantify tidal regimes represented in the tidal deposits.

For the 420 million year time slice, two of the tidal proxies are situated in Laurussia and one near Gondwana. The Keziertage Formation is part of the Tarim Basin, which belongs to the Late Pridoli (420 million years) as determined by zircon dating, and represents a tidal flat environment, likely representing a meso–macro (i.e. larger than 2 m) tidal regime. The Manlius Formation is a lagoonal deposit fromthe Silurian–Devonian boundary at around 419 million years, currently situated in New York, USA, and represents a micro-tidal regime. The Karheen Formation dates to the Early Lochkovian (around 419–415 million years), is located in present day Prince of Wales Island, Alaska, and is an intertidal flat deposition likely representing a meso–macro-tidal regime.

For the 400 million year time slice, two of the proxies are again from Laurussia and one from Gondwana. The Battery Point Formation of Eastern Canada, dating to the Late Emsian (approximately 400–393 million years old), is a deposit made of sedimentary structures representing a meso-tidal environment. The Padeha Formation, dating to the Emsian–Eifelian boundary (approximately 393 million years old), belongs to the Central block of Iran and is a tidal flat deposit, likely showing a meso-macro-tidal regime. The Rēzekne and Pärnu Formations, dating to the Late Emsian to Early Eifelian (approximately 395–390 million years old), belong to the Baltic Basin, a vast delta which measured about 250 × 500 km. These formations indicate that the delta was tidally dominated at this stage, suggesting a meso–macro-tidal regime.

For the 380 million year time slice, all three proxies are located in Laurussia. The Gauja Formation is also part of the succession of deposits from the Baltic Delta, dating to the Late Givetian (approximately 385–383 million years old). It indicates that the Baltic Delta has gone from being tidally dominated, as shown in the earlier Rēzekne and Pärnu Formations, to being tidally influenced, and hence experiencing a shift to a micro–meso-tidal regime (0–4 m). The Appalachian Foreland basin, now in the Eastern USA, was a large epeiric sea, and is well-known for containing vast Coral Reef systems and several shale deposits in the Hamilton Group from the Givetian (388–383 million years old), indicative of a micro-tidal regime. Lastly, the Escuminac Formation from Eastern Canada, is well-known as the location for the Elpistostegalian, Elpistostege watsoni, and Tetrapodomorph Fish, Eusthenopteron foordi. The deposit dates to the Middle Frasnian (approximately 378 million years old) and represents a wave-dominated estuary associated with a micro-tidal regime.

The positioning of the proxy locations on the relevant palaeogeographic reconstructions were done using the present day locations of each proxy in conjunction with palaeogeographic reconstructions which had present day country outlines superimposed, which were provided from Deeptime Maps. Precise placement of the tidal proxy locations on the palaeogeographic reconstructions was unattainable due to the coarse resolution of the reconstructions, and so the location markers are approximate. In the future, Byrne et al. plan to have higher-resolution simulations concentrated in these regions with higher-resolution and smaller-scale palaeogeographic reconstructions.

In the 420 million year control simulation, the principle lunar constituent tidal response shows several localised macro-tidal areas near West and East Laurussia, and around East Siberia. Several distinct macro-tidal areas are also found around East Gondwana, with the majority occurring in Byrne et al.'s region of interest. The maximum principle lunar constituent range for the South China region is mega-tidal and is located around the Indochina block. The principle lunar constituent tide is generally weak away from coastlines and in the strait between the middle and west islands of Laurussia, although Byrne et al. find the maximum global principle lunar constituent range at West Laurussia (13 m). Meso-tidal spring–neap ranges are seen in multiple areas throughout Laurussia and Gondwana, occurring in areas where principle lunar constituent macro-tidal ranges are found. Laurussia is home to several meso-tidal areas, reaching almost macro-tidal ranges along West Laurussia. The South China region has three distinct meso-tidal spring–neap range areas, with a maximum of over 3 m reached around Indochina. The meso-tidal ranges, or larger, in both principle lunar constituent and principle solar constituent tides around the South China region show a large tidal variability occurring in the region and at the time of the origin and diversification of Osteichthyans.

 

The 420 million year simulation with tidal range (colour, range in metres) for the principle lunar constituent (a)–(c) and principle solar constituent (d)–(f ). Enlarged areas of evolutionary interest are shown in (b) and (e) for the South China region and (c) and (f ) for Laurussia. Note that the principle lunar constituent range is equal to the spring–neap range difference, so panels (d)–(f) show the spring–neap range difference as well. Byrne et al. (2020).

The depth sensitivity simulations show a similar picture in terms of the spatial patterns, but there are expected variations in range. For the 420 million year shallow bathymetry simulation, the principle lunar constituent tide is much less energetic compared to the control, particularly around East Gondwana. There are again meso-tidal spring–neap ranges found in the principle lunar constituent macro-tidal areas, having the same global average and a reduced maximum range compared with the control. By contrast, the deep bathymetry simulation is muchmore tidally energetic (i.e. experiences larger tidal ranges) for the principle lunar constituent, with more and larger macro-tidal areas seen around the coastlines of all three continents. This trend is also observed for the spring–neap range.

 

The 420 million year simulation, using the shallow bathymetry. Byrne et al. (2020).

The globally averaged principle lunar constituent ranges for the control and shallow bathymetries are similar (0.4 m and 0.5 m, respectively), whereas the deep bathymetry comes in at 0.7 m. The maximum principle lunar constituent range found in the 420 million year simulations vary from 7.9 to 13 m, and it is evident that the deep. However, despite this global amplification, the maximum values for both the principle lunar constituent and spring–neap ranges are lower than the control simulation bathymetry creates a general amplification of the principle lunar constituent and principle solar constituent tide. However, despite this global amplification, the maximum values for both the principle lunar constituent and spring–neap ranges are lower than the control simulation.

 

The 420 million year simulation, using the deep bathymetry. Byrne et al. (2020).

For the 400 million yeara control simulation, there are several principle lunar constituent macro-tidal areas located along North Laurussia and Siberia and around East Gondwana. There is one distinct macrotidal region around South China, with several more localized upper meso-tidal ranges around Indochina, with the region being less energetic compared with the 420 million year control simulation. Around Laurussia, there are several macro-tidal areas across the north, with a weaker principle lunar constituent tide in the south. This simulation shows a weakened principle lunar constituent tide along the south and west coast of Laurussia between 420 and 400 million years ago. The spring–neap range at 400 million years shows a similar distribution as in the 420 million years control simulation, located in principle lunar constituent macro-tidal areas. The South China region again experiences a smaller spring–neap range compared to that in the 420 million years control simulation; it also has a smaller average and maximum range. As in South China, the spring–neap raAs in figure 3 but for the 400 Ma simulation.nge is smaller around much of Laurussia compared to in the 420 million years control simulation.

 

The 400 million year simulation. Byrne et al. (2020).

The 400 million years shallow bathymetry simulation is much less energetic, for both the principal lunar constituent and principal solar constituent tide, than the control and deep bathymetry simulations of the same time slice. There are fewer principal lunar constituent macro-tidal areas and they are more localized, with the global average principal lunar constituent range being some 75% of that found in the control and deep bathymetry simulations. A similar trend occurs for the spring–neap range. The Deep 400 million year bathymetry simulation is similar to that of the control bathymetry for both principal lunar constituent and principal solar constituent. For Laurussia, the principal lunar constituent tide appears to be less energetic around the north coast and more energetic towards the west and south coast, with a macro-tidal range occurring at the Baltic Basin. The South China region is more tidally energetic in the deep bathymetry simulation, with the global maximum principal lunar constituent range occurring here. Globally, the spring–neap range is largest in the deep bathymetry simulation, with the maximum found in East Gondwana.

 

The 400 million year simulation, using the shallow bathymetry. Byrne et al. (2020).

The simulation for 380 million year shows a slightly reduced global tidal range for both principal lunar constituent and principal solar constituent compared with simulations from the other two time slices, whereas the tides in South China and Laurussia are on par with those in the 400 million year simulation of the same region. There are, however, a few local hotspots in the 380 million year simulations, where the islands in the north-west experience principal lunar constituent macro-tidal ranges over 8 m. Around Laurussia, the tides are still macro-tidal, albeit weaker than in the earlier time slices.

 

The 400 million year simulation, using the deep bathymetry. Byrne et al. (2020).

The 380 million year shallow simulation has a similar global tidal range output as the control simulation, though produces lower maximum ranges for both principal lunar constituent and principal solar constituent, with a similar trend observed in the regions of interest. The deep simulation is more energetic than both the control and shallow bathymetry simulations, producing tidal ranges comparable with the deep bathymetry simulations from the previous two time slices.

 

The 380 million year simulation. Byrne et al. (2020).

The 420 million year control simulation fits best with the tidal proxy ranges for the time, with macrotidal ranges occurring in the Karheen Formation region, micro-tidal ranges at the Manlius Formation region and macro-tidal ranges at the Keziertage Formation region. In the shallow bathymetry simulation, tidal ranges for both the Karheen and Keziertage Formation locations are smaller than the proxy ranges and for the deep bathymetry simulation, the Keziertage Formation region has smaller ranges than the proxy. For the 400 million year simulations, the control matches reasonably well with all three proxies: it shows a meso-tidal regime at the Battery Point Formation locality and a meso-tidal regime in the region of the Padeha Formation. However, the control simulation does not agree with the tidal proxy of the Rēzekne and Pärnu Formations. The proxy represents a meso–macro-tidal regime, with the simulation showing micro-tidal conditions. The shallow bathymetry simulation produces tidal ranges smaller than all three proxy tidal regimes and the although the deep bathymetry fits well with both the Pärnu and Rēzekne and the Padeha Formation proxies, it does not fit with the Battery Point Formation proxy, with the simulation underestimating the tidal regime at that location. In the 380 million years time slice, the control simulation fits well with all three proxies, with micro-tidal regimes for the Escuminac Formation and Hamilton Group regions and a micro–meso-tidal regime occurring in the Baltic Basin area, where the Gauja Formation is located. The shallow bathymetry simulation is less tidally energetic than the control simulation, and also fits well with the three proxies, though has a slightly smaller tidal range output in the Baltic Baltic region. The deep bathymetry produced tidal regimes much greater than the tidal proxies, particularly in the region of the Escuminac Formation.

The earlier time slices for our period of study (420–400 million years) and presnt day are believed to be at roughly similar central points in their respective super-continental cycles, whereas the 380 millio year slice is closer to the formation of a supercontinent (Pangea in this case) than the modern continents currently are. This central position in the cycle is associated with multiple ocean basins, and thus an increased chance of ocean resonances in one or multiple basins which would lead to the tides becoming more energetic. At present we are experiencing a tidal maximum due to the near resonance of the North Atlantic, whereas the period of study occurs after a tidal maximum, shown in other simulations to have occurred at around 440 million years ago. This is important as tides can be sensitive to small-scale changes in bathymetry when the ocean is near resonance, but as this is not the case for our period of study, our results are not prone to this sensitivity. The similar positioning within a super-continent cycle of our period of study with present day would also suggest that the contemporaneous oceanic crust would have been of similar age to the present day crust; consequently, we based the control bathymetry on present day bathymetry values. The sensitivity simulations show that the results are generally robust when the depths are changed.

The control simulation produces the best fit for the three tidal proxies for 420 million years ago, and although only the deep bathymetry simulation produced a meso-tidal regime matching the Baltic Basin tidal proxy for 400 million years ago, it is not a representative bathymetry for this time slice. This is due to the early Middle Devonian being in a period of lowered sea-level caused by marine regression occurring from the Late Silurian. Byrne et al. therefore argue that the control simulation is still a valid baseline for the 400 million year time slice. Higher-resolution simulations are required to resolve the tides of the Baltic Basin for the control bathymetry, as it is common for the local full tidal range not to be captured in global tidal simulations, like the Bay of Fundy of the present day, which is dominated by a small-scale resonance. For the 380 million years time slice, the control simulation also fits well with the three tidal proxies for that period, as does the shallow bathymetry simulation.

For the 420 million year time slice, the South China region is consistently associated with multiple principal lunar constituent macro-tidal areas across the sensitivity simulations. Furthermore, multiple spring–neap meso-tidal areas also persist, implying a large tidal variability during the time of the origin of Osteichthyans. It should also be noted that a macro-tidal regime also occurs along the coastline of Indochina in conjunction with South China. Combined with evidence of shared fauna between the two blocks, this warrants further palaeontological exploration of present day countries belonging to the Indochina block: Vietnam, Laos, Cambodia and Thailand. The Van Canh and Dong Tho sandstone Formations, which represent the Silurian–Devonian of Eastern Indochina, show indications of extensive tidal zones and are associated with early Dipnomorph Fish (members of the Lungfish lineage, the extant sister group to Tetrapods).

In the 400 million year time slice, the tidal regimes vary throughout the simulations in areas where the earliest Tetrapod trackways are located in Southern Laurussia, and these results are supported by the later 380 million year simulation. The Zachelmie trackway locality lies on the western margin of the entrance to the Baltic Basin; in the control simulation, the Baltic Basin is located in a micro-tidal area but changes to a macro-tidal area in the Deep bathymetry simulation. The Baltic Basin was a shallow epicontinental sea which existed from the Silurian into the Early Carboniferous. Tidal regimes within ancient epicontinental seas have been greatly debated, with arguments for the weakening of the propagating tide due to shallow depths and the vast expansion of the seaways, leading to micro-tidal conditions. Offsetting this, other studies have found evidence for tide-domination in both extant and extinct epicontinental seas. Numerical models of ancient seaways have produced varied results; the Late Devonian Catskill seaway of Southern Laurussia is expected to have experienced meso-tidal ranges, whereas largely micro-tidal conditions are expected in the Late Carboniferous seaway of northwest Europe and the Early Jurassic Laurasian Seaway. Tidalites from the Pärnu and Rēzekne Formations suggest a meso-macro-tidal regime, which will be investigated further in future studies using higher-resolution simulations for the Baltic Basin. 

Byrne et al.'s principal conclusion is that simulations representing ocean tides for the time periods of the evolution of Osteichthyans and the emergence of tetrapods are broadly consistent with the hypothesis that tides were an important environmental and evolutionary driver for these events. Of particular significance is the fact that those areas with some of the largest tidal ranges and tidal variability in the palaeotidal simulations coincide with fossil proxy sites, i.e. South China from 420 million years ago. From the fossil record, it is apparent that tidal environments are closely associated with the fossils of Elpistostegalians and stem-Tetrapods. This stimulates the need for high-resolution tidal simulations to access tidal regimes in these regions in more detail, e.g. the Balsic Basin and Escuminac Formation sites. Extended tidal simulation studies using a variety of palaeogeographic reconstructions at more finely sliced time intervals, as well as at higher spatial resolution around areas of palaeontological interest, will more fully elucidate whether differing tidal regimes are correlated with the origin and diversification of other early Vertebrate clades. More generally, establishing the role of palaeotides in influencing major evolutionary events is a field holding great promise, a novel blend of fluid dynamics and palaeobiology that is still very much in its infancy.

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Dating the earliest Myriapods.

Understanding how organisms colonised the land, is crucial to clarify extant biodiversity and biological adaptation. But, evaluating the rate and pattern of land colonisation requires precise dating of the fossil of early land biotas and reconciling them with evolutionary divergence based on morphology and molecular clocks. The frequent striking inconsistency between the ages of fossils and their phylogenies limits our understanding of macroevolution, and it reduces confidence in phylogenetic inference: this is especially the case for the first land Animals. Arthropoda (Insects, Spiders, Centipedes and their allies) were the first, and are the largest group of land Animals in both numbers and biomass. The first fossil land Arthropod-Plant assemblages are rare and only found in Late Silurian (about 425 million years old) to Early Devonian (about 410 million years old) equatorial terrestrial and freshwater sediments within and marginal to the ancient Caledonian mountains which stretched from New York to Germany. The earliest fossil land Arthropods are Myriapods (Millipedes and Centipedes) from the latest Silurian (about 425 million years old) of Scotland and Wales. 

Yet, while Arachnid molecular phylogenies fit the fossils, Myriapod molecular clock phylogenies do not and suggest a late Cambrian (500 million years) monophyletic origin and divergence of the Myriapod classes, which implies not only a marine origin of the classes and possible independent land colonisation events during Myriapod evolution, but also that pre-late Silurian Myriapods are not preserved in the geological record. Though the first land Arthropods were initially very small with thin cuticle, and are difficult to preserve except under unusual conditions, nevertheless, there are enough organic-rich sediments with soft bodied fossils in appropriate environments in Cambrian to early Silurian times, that any existing land Arthropods would be at least occasionally preserved if present. But they are not. Apart from the doubtful Diplopod (Millipede), Casiogrammu ichthyeros, from the Hagshaw Hills, none occur in the Silurian Fish beds of the Midland Valley of Scotland, famed for their diverse freshwater Arthropods and oldest complete Fish fossils (but with no vascular plant remains), and currently assigned on fossils spores to the Wenlock (about 430 million years old). In any case, given that the dating of the Silurian Fish beds is so uncertain, this Diplopod? Could be the same age as or only slightly older than the Kerrera fossils, and similarly possibly aquatic. Pre-late Silurian tracks and trails of supposed land Arthropods are suspect and may represent, as is common today, only temporary excursion onto land. The earliest land Arthropod fossils are Millipedes associated with the first Vascular Plants in the latest Silurian (Pridoli) around 420 million years ago. But, while the earliest fossil Vascular Plant fossils are found at about 425 million years old (Upper Silurian), molecular phylogenies indicate a 515 million years old to 470 million years old (Late Ordovician-Silurian) origin. The precise timing of the appearance of the first land Arthropod and Plant fossils is difficult to determine because of the problem of correlating the deposits of the lakes, river and coasts, in which they occur, with the standard marine-based geological time scale, and there are almost no radiometric dates from associated sediments to help in this.

In a paper published in the journal Historical Biology on 15 May 2020, Michael Brookfield and Elizabeth Catlos of the Department of Geological Sciences at the University of Texas at Austin, and Stephanie Suarez of the Department of Earth and Atmospheric Sciences at the University of Houston, present the latest results in an ongoing study which aims to provide accurate dates for these first land biotas.

 

Caledonian mountains with cited localities. Ron Blakey in Brookfield et al. (2020).

Brookfield et al. determined the ages of these first land arthropods, with uranium/lead dating of zircons in the earliest Millipede-bearing sediments from three places, in western England (Ludlow) and Scotland (Cowie, Kerrera). These sediments are associated with contemporary explosive volcanic activity, so the youngest concordant zircons give not only the maximum age of the enclosing sediment  (a sediment cannot be older than the youngest thing in it, though it can be younger) but also a good estimate of the actual age of the sediment from the age of the contemporary volcanic rocks Zircons were separated from sediment samples taken either just above and below the arthropod-bearing beds (Kerrera and Cowie) or from heavy mineral separates obtained for other studies (Ludlow). Both the Kerrera and Ludlow dates are new. The Cowie date comes from an earlier study by Stephanie Suarez, Michael Brookfield, Elizabeth Catlos, and Daniel Stöckli, also of the Department of Geological Sciences at the University of Texas at Austin, and is included by Brookfield et al. to show the apparently rapid progressive coevolution of land Arthropods and Floras in semi-arid continental environments during the latest Silurian. The Kerrera Arthropods come from a temporary lake deposit with Anapsid Fish, interbedded with coarse semi-arid intermontane basin sediments, at the base of the Lorne Plateau Lavas. The only previous radiometric ages for Kerrera come from a lava at the top of the Lorne Plateau Lavas, 600 metres above the Kerrera sediments. Their uranium/lead zircon age of 425 ± 0.7 million years is a uranium/lead thermal ionisation mass spectrometry concordia age from two zircons which give individual ages of 425.4 ± 0.8 million years and 424.5 ± 0.8 million year. Zircons, however, are among the first minerals to crystallise from a cooling magma chamber, may be mixed populations from separate batches of magma, and can be several 100 000 years older than the lava eruption at the surface. The youngest concordant zircon gives a maximum age for the eruption of 424.5 ± 0.6 million years. The time taken for 600 metres of lavas to erupt is not known, but from other more recent similar lava piles, which accumulated over a fairly short time geologically, we can make an estimate. The average 1000 metre thick Grand Ronde Basalt lavas of the Miocene Columbia River Plateau were erupted over a 400 000 years period between 16.5 and 16.1 million years ago. A comparable duration is likely for the Lorne Plateau lavas, which seem to have erupted fairly continuously. Of the 52 zircons Brookfield et al. analysed, the youngest concordant ages were 426.5 ± 4.5 million years and 425.4 ± 4.8 million years, which are statistically indistinguishable. The 1 million years between the midpoints of the youngest Kerrera zircon and the youngest top Lorne Lava zircon is thus a reasonable estimate of the time taken for the Lorne Lava pile to accumulate.

 

Sections at Ludlow, Kerrera, and Cowie with Youngest dated zircons. Brookfield et al. (2020).

The Ludlow Arthropods, accompanied by a fragmentary Cooksonia flora come from one organic-rich siltstone lens filling ripple troughs in fine sandstone just above the Ludlow bone bed lag deposit, a sandstone marking the change from shallow marine to semi-arid continental environments. Brookfield et al. analysed 29 zircons from heavy mineral concentrations, obtained for Conodont analysis, from two samples of the basal Bone Bed near Ludlow. Nineteen of these were within the expected latest Silurian (Pridoli) biostratigraphic age. The youngest dates of 420.0 ± 8.9 million years and 420.3 ± 8.1 million years are, like those from Kerrera, statistically indistinguishable. The Cowie dates of 413.7 ± 4.4 million years and 414.3 ± 7.1 million years which bracket the Millipede-bearing Fish bed, are younger than both the Kerrera and Ludlow.

These ages are consistent with the evolutionary stages shown by the fossil Millipedes and associated Arthropods. 

The Kerrera and Ludlow Myriapods are Kampecarids which show no obvious structural adaptation to land. The Kerrera Kampecarids, Kampecaris obanensis and Archidesmus sp. occur with an early vascular ‘Cooksonia’ flora, Eurypterids and Anapsid fish in a freshwater lake deposit in a semi-arid fluvial environment. Kampecarids also sporadically occur in late Silurian to early Devonian freshwater lake environments throughout Scotland. The Ludlow Millipedes are also accompanied by the early vascular Cooksonia flora, and by a more diverse fauna of Arachnids, a Centipede and Eurypterids, eroded and redeposited either by a coastal storm deposit from a back-barrier environment or by a tsunami from more inland semiarid coastal plain environments. The most diverse Cowie Millipedes belong to the extinct Archipolyploda superorder and consist of Cowiedesmus eroticopodus which derives its name from preserved male gonopods, and Albadesmus almondi and Pneumodesmus newmani which do not have modified appendages preserved. Pneumodesmus newmani, however, has spiracles on the lateral parts of the sternites which are direct evidence of air breathing, and, before our dating, was taken to be the oldest fully terrestrial Animal. The Cowie deposit was laid down in a temporary lake environment related to braided and meandering streams in a semi-arid intermontane basin, but curiously, contains no identifiable Plant remains or spores. There is thus a progressive change in the Arthropod faunas across the Silurian/Devonian (Pridoli/Lochkovian series) boundary around 419 to 421 million years ago; from Kerrera (about 425 million years old, Ludlow series), through Ludlow (about 420 million years old, Pridoli series), to Cowie (about 414 million years old. Lochkovian series), and this increased diversification continues into younger more diverse early Devonian (Emsian) land biotas, like the Rhynie Chert intermontane basin hot springs biota from Aberdeenshire, and the Gaspé, eastern Canada and Alken, Germany, delta marsh biotas. The diverse Rhynie Chert, with an argon⁴⁰/argon³⁹ age of 407.1 ± 2.2 million years, has a diverse flora and fauna dominated by the Vascular Cryptogam Plants, Rhynia, Aglaophyton, and Horneophyton, and Arthropods; Arachnids (Trigonotarbida), Mites (Acariformes), Harvestmen (Opiliones), a possible Millipede (Diplopoda), Centipedes (Chilopoda), and Springtails (Collembola). Neither the Gaspé nor the Alken biotas have been dated radiometrically. But both have somewhat similar biota to Rhynie, despite their very different environment, with Vascular Cryptogams of the slightly more evolved Zosterophyllum flora, accompanied not only by Arachnids (Trigonotarbids), and Centipedes (Chilopoda), but also by freshwater Eurypterids and Anapsid Fish. By the Middle Devonian (about 385 million years ago), complex forests with ten-metre-tall trees and associated diverse arthropod communities had become established, for example at Cairo and Gilboa, New York. Nevertheless, the composition of the land biotas does not differ significantly from upland lake margins to delta front marshes from latest Silurian to mid-Devonian, a rapidly evolving successions of pioneer communities of Arthropods/Vascular Plants seems to have exploded over the Acadian landscape in a very few million years.

 

The Kampecarid Millipede, Kampecaris obanensis, from the 425-million-year-old Kerrera deposits of Scotland, currently the oldest known Myriapod. British Geological Survey.

From fossil evidence, the time from initial colonisation by Arthropod-Vascular Plant communities to complex forest communities took place over less than 40 million years (425–385 million years ago). An essentially intermontane lake margin Pridoli FW/land biota in the Acadian mountains centred on Scotland evolved rapidly and migrated, undoubtedly via rivers, to colonise floodplain marshes in the adjacent lowlands by the Emsian, as there is little difference in contemporary biotas. Molecular phylogenies of the Myriapods, however, indicate a monophyletic origin and divergence about 100 million years earlier, while the origin of Vascular Plants is similarly placed from 100 to 50 million years prior to their first body fossils. Which is correct?

Though non-Vascular Plants and possibly amphibious Arthropods were around in the Ordovician, the fossil evidence is clear that Vascular Plant/land Arthropod biotas evolved together in the Late Silurian (about 425 million years ago) and not earlier. If the molecular clock timing were correct, then land Arthropods should be found under exceptional circumstances in earlier deposits. These might be meiofaunal. So, such Arthropod should be looked for in silicified pre-Late Silurian organic soils or hot springs deposits like the Devonian Rhynie Chert, though we know of no such deposits or biotas.

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