What we missed #1

If you're reading this after all of of the other pages on this fossils learning resources website, well done for making it this far! Before we wrap up, lets start by looking at some important fossil groups that swe have missed in video one (there are four this week, FYI).


Key points to take away from this video are:

  • Sponges, or porifera, are important filter feeders in phanerozoic benthic communities.
  • They appear early in the fossil record (exactly how early is debated), and are major reef builders throughout the phanerozoic.
  • Bryozoa are an important group of colonial Lophotrochozoa, which are relatively common in the fossil record.
  • They appear in the Ordovician and have been around ever since. They often appear as net-like structures in Phanerozoic rocks.
  • Annelids are segmented worms, and a few groups in particular (machaeridians, scolecodonts, serpulids) are not uncommon as fossils.

Lets meet some of these groups

Want to see what these look like as fossils? If so, you are in luck, because (of course) I have dredged out some 3D models for you.


Let's start with the Hexactinellida (glass sponges) – these have siliceous, six-pointed spicules, and are often delicate in their construction. Whilst the glass sponges have their origins in the Cambrian, they reach peak diversity during the Cretaceous Period. This is true of the sponges as a whole, which also reached peak diversity in this period.

A fossil glass sponge (Hydnoceras tuberosum). This is Devonian in age (Steuben County, New York). Specimen ~26cm long.

Next up we have a demosponge – the Demospongiae is dominant in terms of today's sponge diversity – the group comprises more than 90% of all sponge species living today. Relatively few have hard skeletons, so they have a limited fossil record (they do have silaceous spicules though).

Fossil demosponge Hallirhoa costata from Wiltshire – Cretaceous in age. Max diameter ~9cm.


Moving on, let's meet a bryozoan fossil! I chose one which is the most similar to other examples I have seen in the UK.

A bryozoan (Polypora elliptica) that is Carboniferous in age, from the Thrifty Formation, Eastland County, Texas. ~4.5 cm max.

It's not unusual to find Bryozoa encrusting other organisms, such as this modern whelk snail shell, which is covered with our weird colonial creatures:

Modern whelk snail covered with encrusting Bryozoa. Shell ~20 cm long.


Let's finish with a quick serpulid worm for you to get a feel for what these look like:

Fossil polychaete Rotularia sp. from the Cretaceous of Seymour Island, Antarctica. Specimen 8cm long.

What we missed #2

Now this is the story all about how
My life got flipped, turned upside down
And I'd like to take a minute, just sit right there
I'll tell you ... the second bunch of fossils we missed.

(Yes, this is a reference, but it's to the Fresh Prince of Bel-Air which first screened in the early '90s – so is an unpopular culture reference? Either way, this what the early '90s was like. Mildly disappointing.)


Key points to take away from this video are:

  • Vertebrates have Cambrian origins, and moved onto land in the late Devonian.
  • Since then, the diapsids were very successful in the Mesozoic, with the mammals only radiating after the end-Cretaceous extinction.
  • Plants have been on land since the Ordovician, having evolved from the green algae.
  • A number of major evolutionary innovations see the rise of new groups and increased independence from water.
  • Fungi have been around equally long – if not longer – than both plants and animals, and deserve more study of their fossil record!

A bonus model

In general, the groups in this video are so diverse, that I don't think there is much utility of me lining 3D models up for you to check out.

But I did want to throw in one more model for you, which is a plant fossil typical of the Carboniferous coal measures, and one you stand a good chance of finding if you ever to fieldwork in northern England or Wales. It is called Lepidodendron, and represents the stem of an extinct lycopod (clubmoss) with leaf pads on it:

Fossil lycopod stem Lepidodendron sp., Carboniferous, found in Indiana. Stem with diamond-shaped leaf pads. ~20cm long.

If you would like to check out more 3D models, you can find lots of models at the following links:

Fossils as environmental indicators

Fossils can be used to tell us not only the age of a rock, via biostratigraphy, but also its environment of deposition. Here we take a look using water depth of clastic shelves as an example.


Key points to take away from this video are that, on clastic shelves:

  • We can work out the depth of water in which a sediment was deposited using changes in the fauna that we see preserved.
  • This fauna can be split into biofacies/benthic assemblages.
  • The makeup of these biofacies changes with time, and looking at animal communities through time allows us to be more nuanced in our interpretations.
  • This is arguably easier in the Palaeozoic: post-PT the situation gets a bit more complex.

Benthic assemblages

I mentioned biofacies and the associated benthic assemblages. The table below provides an overview of each of these benthic assemblages! It was published in the following textbook:

Brenchley, P.J., Brenchley, P. and Harper, D., 1998. Palaeoecology: Ecosystems, environments and evolution. CRC Press.

Brackish Lagoonal Environments

Context Commonly associated with terrestrial, barrier bar, estuarine, marsh or shallow marine shoreface facies.
Lithology Typically shales though siltstone, sandstones and less commonly coarser grained sediments may be introduced by tidal or storm processes in flood-tidal deltas and washover fans.
Taphonomy Dispersed in situ or disturbed neighbourhood assemblages, typically as thin shell concentrations. Tidal and storm processes may introduce allochthonous assemblages from shoreface and inner shelf environments.
Taxonomy/Diversity In situ faunas typically of low diversity; usually ostracodes, bivalves and gastropods.
Abundance Variable, commonly low in thin shell accumulations, but high in thick, typically monospecific, shell beds.
Ichnology Imperfectly described; low diversity assemblages of the Psilonichnus, Glossifungites, Cruziana and Zoophycos ichnofacies may each occur depending on energy and salinity levels, grain size, substrate consistency and specific depositional environment within a lagoon.

Shoreface Environments (BA1)

Context Nearshore and associated beach facies, distinguished from inner shelf deposits by generally having only rare mudstones interbedded with sandstones. Shoreface facies commonly cap upward-coarsening shallow marine cycles.
Lithology Typically sandstones with locally developed mudstones. Sandstones exhibit planar- or cross-stratification formed by wave, tidal or wind-forced currents, or hummocky cross-stratification formed by oscillatory or combined flow.
Taphonomy Allochthonous shell concentrations that may occur as coquinas, on bedding planes or cross-stratified foresets.
Taxonomy/Diversity Palaeozoic assemblages characterized mainly by brachiopods with more rarely bivalves and gastropods. Mesozoic/Cenozoic assemblages characterized by varied bivalves (including deep burrowers and attached forms) and gastropods. Taxa affected by substrate mobility (e.g. Corals) are rare. In situ faunas rare. Diversity, though still generally low, can be elevated by introduction of allochthonous faunas by storm, tidal and wave activity.
Abundance Generally low in Palaeozoic rocks, but Cenozoic strata typically possess more abundant remains. In more subdued shoreface environments shells may be relatively common in coquinas.
Ichnology Typically Skolithos, more rarely Psilonichnus and Cruziana ichnofacies depending on substrate mobility and subtle gradients in hydrological, sedimentological and ecological parameters. Diversity typically low.

Inner (shallow) Shelf Environments (BA2)

Context Facies subject to frequent storm waves and tidal processes, commonly occurring below shoreface sandstones and above mid-shelf sequences in coarsening-upward cycles.
Lithology Sandstones dominate, but may occur in equal abundance with interbedded shales. Storm-influenced facies exhibit hummocky cross-stratification; tidal facies may exhibit herringbone cross-stratification, sigmoidal cross-stratification or mud drapes.
Taphonomy Allochthonous or disturbed neighbourhood assemblages at the base of storm sandstones, typically as coquinas. Tidally influenced sandstones commonly exhibit transported assemblages or shell lags distributed along foresets. Shales possess disarticulated and dispersed shells and bedding plane assemblages. In situ faunas rare or absent.
Taxonomy/Diversity Palaeozoic strata dominated by moderately but variably diverse assemblages of brachiopods, particularly orthoids and strophomenoids: Mesozoic/Cenozoic strata typically possess variable infaunal and epifaunal bivalves. Representatives of other benthic macroinvertebrates may occur, sometimes in abundance.
Abundance Generally moderate, but high in coquinas. Faunas can, however, be generally sparse where a freshwater influence is established, as, for example, in inner shelf environments in front of deltas.
Ichnology Typically moderate to high diversity assemblages of the Cruziana and/or Skolithos ichnofacies. Mudstones commonly intensely bioturbated and possess trace fossils typical of the Cruziana ichnofacies: associated sandstones contain representatives of the Skolithos ichnofacies providing substrate mobility does not result in their destruction.

Middle Shelf Environments (BA3 and BA4)

Context Commonly form portions of upward-coarsening sequences, but may also form part of a predominantly mudstone sequence. Below fair weather wave base, above storm wave base.
Lithology Mudstones, typically intensely bioturbated, and interbedded sandstone tempestites with hummocky cross-stratification are common.
Taphonomy Mudstones may contain in situ clumped or dispersed shells that are still commonly articulated, or as thin stringers or locally reworked disturbed neighbourhood assemblages. Sandstones commonly contain allochthonous lag assemblages or coquinas that are taxonomically similar or dissimilar to those assemblages within the associated mudstones.
Taxonomy/Diversity Palaeozoic strata possess moderate to high diversity assemblages dominated by brachiopods represented by several orders. Crinoids, bryozoans, corals, trilobites and gastropods also common. Mesozoic/Cenozoic strata dominated by varied infaunal and epifaunal bivalve assemblages with gastropods, echinoderms, bryozoans and less commonly brachiopods also present.
Abundance Generally high, decreasing with depth.
Ichnology Similar to inner shelf environments with high diversity assemblages typical of the Arenicolites ichnofacies in sandstones, typically as opportunists following storm activity, and intense bioturbation by members of the Cruziana ichnofacies in associated mudstones.

Outer (deep) shelf environments (BA5)

Context Mudrock-dominated sequences in association with shelf or slope/basin environments. Mainly below effective storm wave base, though extreme storms have disturbed shells.
Lithology Bioturbated mudstones with uncommon, and where present typically thin, distal siltstone or sandstone tempestites.
Taphonomy In situ dispersed, articulated faunas predominate. Thin stringers and bedding plane assemblages also common: disturbed neighbourhood assemblages may occur. Distal tempestites may contain allochthonous shells. though not commonly.
Taxonomy/Diversity Palaeozoic strata dominated by brachiopods and trilobites: Mesozoic/Cenozoic strata by bivalves, particularly infaunal species - nuculoids are very common. Other groups also occur but in lower numbers. Pelagic forms eg. graptolites (Palaeozoic), ammonites, belemnites (Mesozoic) also commonly present. Diversity variable depending on parameters such as nutrients, oxygen levels, etc., but can be high, then typically markedly decreases at the shelf edge.
Abundance Typically low.
Ichnology Discrete ichnotaxa commonly difficult to recognise as a result of intense bioturbation. Representatives of Cruziana, Zoophycos or rarely Nereites ichnofacies may predominate depending on specific environmental conditions. Bioturbation intense though diversity commonly low to moderate.

Slope and Basin (BA6 and below)

Context Mudrock-dominated sequences with associated sediment gravity flows particularly, but not exclusively, turbidites. Submarine canyons and associated fans commonly occur in association with slope and continental rise sequences. Contourites also common on slope and continental rise environments.
Lithology Pelagic and hemipelagic mudstones, turbiditic sandstones, and siltstones. Canyons and fans typically possess coarser grained lithofacies.
Taphonomy Autochthonous faunas, typically dispersed and articulated, in pelagic/hemipelagic mudstones. Allochthonous faunas introduced particularly by turbidity currents.
Taxonomy/Diversity Dominated by pelagic macroinvertebrates (eg. Palaeozoic - trilobites, graptolites, nautiloids; Mesozoic - ammonites, belemnites). In situ forms include Palaeozoic trilobites, hyolithids, etc. and Mesozoic/Cenozoic bivalves, gastropods and crinoids. Other groups may occur but are uncommon. Diversity is low.
Abundance Very low, but can be higher in sediment gravity flow deposits.
Ichnology Variable, with representatives of the Skolithos, Arenicolites and/or Glossifungites ichnofacies commonly occurring in association with well-oxygenated submarine canyons and fans; Zoophycos ichnofacies with slopes with restricted circulation and low oxygen levels and the Nereites ichnofacies in more stable, classical flysch-like settings in generally quiet, but oxygenated waters.

Trace fossils as indicators of environment

Let's finish by looking at how trace fossils can help us understand depositional environments.


Key points to take away from this video are:

  • We can also use trace fossils to understand the water depth on clastic shelves – albeit with caveats.
  • Different assemblages of trace fossils can be collected into ichnofacies, that correlate with depth.
  • But, there can also be confounding factors making this more difficult.
  • With both this and benthic assemblages, a nuanced understanding based on localised knowledge is really valuable.

If you would like to check out those ichnnofacies in a bit more detail, this image provides an overview for you:

Also from Brenchley, P.J., Brenchley, P. and Harper, D., 1998. Palaeoecology: Ecosystems, environments and evolution. CRC Press. Indeed, if you want to read more about palaeoecology as a whole, this is a great place to start.

3D models

Given that we had extra videos this week, I thought it would be a bit mean for me to put more 3D models to look at associated with this last – bonus – video. But if you would like to check out some of these trace fossils in 3D, you can do so here.


I hope you have found these learning resources on fossils useful. If you ever have any questions about fossils, give me a shout – either by email, or you can find me using the links in the footer below.