I’ve often remarked about the amazingness of sites of exceptional preservation (heck, I’ve even done work on the arthropods from one). I’ve already introduced an example. This is another one: the 425 Ma Silurian Herefordshire locality in the UK.
The Herefordshire locality preserves animals from a Silurian marine shelf-slope environment, entombed in volcanic ash. The animals are found, stiff, within carbonate nodules that precipitated very soon after the burial in the ash, prompted by a high pH causing calcite precipitation (Riley, 2008). The rapidity of this process meant that soft parts didn’t have time to decay and are preserved as sparry calcite in 3D – besides the calcite part, you can imagine this like Pompeii. The diagram above (Orr et al., 2000a) summarises the taphonomical processes underlying the preservation.
The downside to the amazing preservation is the amount of work required to actually see the fossils. You can’t hammer the fossils out, nor can you prepare them out. What’s done instead is serial thin sectioning; the hundreds of thin sections are then photographed and scanned, and in recent papers, the fossil is reconstructed using appropriate volume rendering software (Sutton et al., 2001a). The fossil is effectively only a digital dataset by the end of the process (although the sections are of course preserved). Pictured above are a complete nodule (f), cross-sections (g, h), and fossiliferous thin sections (a-d), from Orr et al. (2000a).
What follows is a basic run-down through most of the findings from here, with notes. The critical fact to keep in mind is that all these finds are allowed solely due to the exceptional nature of the preservation, emphasising the importance of such localities.
Offacolus kingi, thin-sections pictured above (Orr et al., 2000b), is the most common fossil found here. Sutton et al. (2002) interprets it as possessing chelicerae and opisthosomal opercula, placing it as a chelicerate. Its retention of biramous limbs places it at the very base (Dunlop, 2006).
Colymbosathon eplecticos – the amazing swimmer with a large penis – is a myodocopid ostracod classifiable in the extant Cylindroleberididae due to its possession of six gills. The large penis part of the name is also notable: this fossil has the to-date oldest preserved penis of any animal; you can see it labelled as (co) in the picture above (Siveter et al., 2003). The amazing swimmer part is probably an allusion to the first and second antennae, which were presumably used for swimming. Overall, this fossil shows that this ostracod family, as far as its soft part anatomy goes, have remained incredibly static over the course of its evolution.
Another Herefordshire myodocopid was described by Siveter et al. (2007b), Nymphatelina gravida, and recognisable as such by the compound eyes and the 7th limbs. The name suggests what’s remarkable about this specimen: it was found preserved with eggs and what are probably juveniles in a brood chamber (labelled ‘jv’ above, in yellow). Remember, this is 425 million year old material!
Another exceptional arthropod from Herefordshire is the pycnogonid Haliestes dasos (Siveter et al., 2004). This is a testament to the amazingness of the preservation here, since pycnogonids are as fragile as arthropods can be – not even 10 fossil species are known.
Cinerocaris magnifica, reconstructed above (Briggs et al., 2004), is a phyllocarid. Phyllocarids are known nowadays as small crustaceans, exemplified by Nebalia bipes, but back in the Palaeozoic, they were pretty big, as seen in this species (and other exceptionally-preserved phyllocarids of the time, for example from the Hunsrück Slates where I did some work).
Xylokoris chledophilia is a marrellomorph, a group of seemingly successful stem-group arthropods. The most famous marrellomorph is Marrella splendens, the most abundant macrofossil of the Burgess Shale. Marrellomorphs are a great group for demonstrating the utility of sites of exceptional preservation, having been found in all the major fossil localities except the Orsten. Findings of them go from the Cambrian, the Ordovician (Furca from Morocco and Czech Republic, see Van Roy et al. (2010) and Rak et al. (in press), respectively), the Silurian (Xylokoris from Herefordshire, see Siveter et al. (2007a)) and the Devonian (Vachonisia rogeri and Mimetaster hexagonalis from the Hunsrück Slates, see Kühl et al. (2008) and Kühl & Rust (2010), respectively). This gives us an excellent look at the evolution within this fossil-only group, a look made possible only by the fossils’ exceptional preservation.
Another stem-group fossil found in Herefordshire is Tanazios dokeron, described by Siveter et al. (2007c). They classify it as a stem-group crustacean. This can be contested by highlighting one of the flaws of the preservation in Herrefordshire: for all the awesomeness of having 3D soft parts, the resolution available for study isn’t high enough to be able to distinguish small aspects of morphology, as is needed to interpret the phylogenetic positioning of such early crustaceans.
The fossil pictured above and described in Briggs et al. (2005) is, to me, one of the most spectacular of all fossils. At first glance, it admittedly doesn’t look very exciting. That is until you realise that this fossil, Rhamphoverritor reduncus, is a cyprid larva caught in the process of metamorphosing into the barnacle adult. It proves that this life cycle had evolved very early in barnacle evolution.
Kenostrychus clementsi, the complete animal seen in (h) above (Sutton et al., 2001b), is a polychaete. With the exception of tube-builders and the scolecodonts (hardened mouthparts), polychaetes are completely soft, mushy, and decay very quickly (Briggs & Kear, 1993), so any complete body fossil such as this is a spectacular find. This particular one is remarkable in that it’s not crushed. They’re found in a lot of Lagerstätten in various states of decay, so this impeccable state of the body is something quite unique. The only “better” polychaetes fossils I can think of are awesome specimens from the Jurassic of Solnhofen (the same place where all specimens of Archaeopteryx were found) with preserved 3D muscles. Phylogenetically, it’s a crown-group aciculate.
Sutton et al. (2005a) describe a brachiopod with its pedicle preserved, another structure that normally would never fossilise; although see Bassett et al. (2007) for an alternate interpretation of the pedicle as a pedicle sheath (which wouldn’t be remarkable since when present, they are calcified and found in normal fossils), and a reply to their criticisms by Sutton et al. (2007).
Sutton et al. (2011) describe a problematic lophophorate, Drakozoon kalumon. As the name suggests, lophphorates are animals with a lophophore, a tentaculate feeding organ, exceptionally preserved on the dorsal valve of a brachiopod in this fossil. There are four extant lophophorate phyla: brachiopods, bryozoans, entoprocts, and phoronids. Drakozoon doesn’t fit comfortably into any of them, but bears the most similarities with brachiopods, and so is interpreted by the authors as a stem-group brachiopod.
Sutton et al. (2005b) describe an asteroid. It’s notable for its tubefeet being unsuckered (see the end of structure Po in (h) above, there’s nothing at the end of it) – modern asteroids have suckers on their tubefeet, using them to stick to the ground (like geckos), or to grip their prey. If the tubefeet of this species had the same function, then they probably worked using chemical adhesion; Thomas & Hermans (1985) showed that some Recent asteroids can do this.
Acaenoplax hayae was described by Sutton et al. (2001b) as a mollusc. If this interpretation is correct – by no means a safe assumption to make, as Steiner & Salvini-Plawen (2001) point out – then this fossil is very informative for discussions of molluscan phylogeny (Sutton et al., 2004) as strong evidence for the Aculifera hypothesis, which states that these vermiform molluscs are secondary simplifications from a chiton-like ancestor. Acaenoplax offers compelling evidence with the presence seven serial shell plates, a similar to chitons.
A definite mollusc is pictured above, an undoubtable platyceratid snail (Sutton et al., 2006). The coloured structures are all soft parts you know from a garden snail, just so you can imagine how cool the preservation is.
Outside of this one spectacular locality, the rocks of Herefordshire have yielded some cool findings as well, including numerous eurypterid species (see listings in Tetlie (2007)). The oldest pedicellariae (echinoderm tubefeet, in this case from a sea urchin) known come from the Ludlow of Herefordshire, as part of a generally rich locality for echinoderms (see Shackleton (2005) for several species from there). The Devonian Old Red Sandstone of Herefordshire also contains examples of freshwater fish (Miles, 1973), maybe some of the oldest in the fossil record.
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Rak Š, Ortega-Hernández J & Legg DA. in press. A revision of the Late Ordovician marrellomorph arthropod Furca bohemica from Czech Republic. Acta Palaeontologica Polonica.
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Shackleton JD. 2005. SKELETAL HOMOLOGIES, PHYLOGENY AND CLASSIFICATION OF THE EARLIEST ASTEROZOAN ECHINODERMS. Journal of Systematic Palaeontology 3, 29-114.
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Sutton MD, Briggs DEG, Siveter DJ, Siveter DJ & Orr PJ. 2002. The arthropod Offacolus kingi(Chelicerata) from the Silurian of Herefordshire, England: computer based morphological reconstructions and phylogenetic affinities. Proc. R. Soc. B 269, 1195-1203.
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Sutton MD, Briggs DEG, Siveter DJ & Siveter DJ. 2006. Fossilized soft tissues in a Silurian platyceratid gastropod. Proc. R. Soc. B 273, 1039-1044.
Sutton MD, Briggs DEG, Siveter DJ & Siveter DJ. 2007. Pedicle preservation in a Silurian rhynchonelliformean brachiopod from Herefordshire, England: soft-tissue or an artefact of interpretation?—A Reply. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 98, 309-310.
Sutton MD, Briggs DEG, Siveter DJ & Siveter DJ. 2011. A soft-bodied lophophorate from the Silurian of England. Biology Letters 7, 146-149.
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ORR, P., BRIGGS, D., SIVETER, D., & SIVETER, D. (2000). Three-dimensional preservation of a non-biomineralized arthropod in concretions in Silurian volcaniclastic rocks from Herefordshire, England Journal of the Geological Society, 157 (1), 173-186 DOI: 10.1144/jgs.157.1.173