Fezouata Formation

The Fezouata Formation or Fezouata Shale is a geological formation in Morocco which dates to the Early Ordovician. It was deposited in a marine environment, and is known for its exceptionally preserved fossils, filling an important preservational window beyond the earlier and more common Cambrian Burgess shale-type deposits. The fauna of this geological unit is often described as the Fezouata biota, and the particular strata within the formation which exhibit exceptional preservation are generally termed the Fezouata Lagerstätte.

Biota
Over 1,500 non-mineralized specimens, representing 50 distinct taxa that have a composition similar to earlier Burgess Shale type biotas, have been recovered from the formations in addition to a less abundant shelly fauna. The make-up of the community varies significantly through the stratigraphic sequence, with both abundances and faunal composition changing as time progresses. Major burrowing is not present, but there are small (1-3 mm wide) burrows in the sediment, which may indicate that there is not enough oxygen in the water or sediment. Particularly notable is the presence of bryozoa and graptolites, forms that are absent in the Cambrian period. Diverse echinoderms indicate a normal range of salinity, and the overall shelly assemblage is not significantly different from the normal shelly fauna expected in open Ordovician waters. The non-mineralized cohort contains a range of forms familiar from the Burgess Shale: Demosponges, lobopods, barnacles, annelids, radiodonts (e.g. Aegirocassis), possible halkieriids, marrellomorphs, paleoscolecid worms, nektaspids, skaniids as well as the expected problematica. Other Ordovician oddballs are also present, including mitrates, machaeridians, cheloniellids and xiphosurans in abundance.

Depositional setting
The fossiliferous strata were deposited just above storm wave base (offshore to lower shoreface transition), at between 50 and 150 m water depth. Organisms were likely buried in situ. Because of its placement above storm wave base, storms would have mobilized sediment that could be quickly deposited, trapping animals and leading to their preservation. Consequently, the assemblage is dominated by benthic organisms.

Preservation
Fossils of the Fezouata Formation, which are usually squashed flat (although some do retain some degree of their original three-dimensionality) are often coated with a dusting of pyrite, and tin; this aspect of the fossil preservation is very similar to that at Chengjiang. Non-mineralized appendages are often preserved. While the formation as a whole is over 1000 m thick, only two intervals, 25 m and 15 m thick, provide exceptional preservation. Both of these intervals are located near the top of the lower formation, corresponding to the Araneograptus murrayi and Hunnegraptus copiosus graptolite zones respectively.

Location and stratigraphy
The fossils occur within an area of 500 km2, in southeast Morocco's Draa Valley, north of Zagora. Stratigraphically productive layers are found through a 1.1 km-thick column of rock that spans the Tremadocian and Floian epochs. Two stratigraphic intervals of the formation are fossiliferous: the lower is Late Tremadocian and sits 260 to 330 m above the base of the formation; the upper, at 570 to 620 m, is mid-Floian in age.

History
The Lagerstätten were first identified in the late 1990s when a local fossil collector, Ben Moula, showed some of the finds to a PhD student who was then working in the area.

IUGS geological heritage site
In respect of the 'exceptional fossil preservation bridging the Cambrian Explosion and the Great Ordovician Biodiversification', the International Union of Geological Sciences (IUGS) included the 'Ordovician Fezouata Shale Fossil Site at Jbeltizagzaouine' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'

Paleobiota
After and subsequent literature:

Radiodonts
Apart from the three named species of Fezouata radiodonts, three other unnamed species occur in the formation: a third species of Pseudoangustidontus, an aegirocassisine, and a predatory hurdiid formerly mistaken for a eurypterid.

Trilobites
The largest trilobite individuals in the Fezouata Formation tend to inhabit deep oxygenated waters with minimal influence from storms or larger predators.

Other arthropods
Many arthropods of the Fezouata Biota remain unnamed and undescribed. These include synziphosurines, xiphosurans (horseshoe crabs), eurypterids, chasmataspidids, phyllocarids, ostracods, a canadaspidid, a leanchoiliid, a cheloniellid (Eoduslia?), a possible retifaciid, and a lepadomorph barnacle.

Echinoderms
Many echinoderm species of the Fezouata Biota remain unnamed and undescribed. These include representatives of chauvelicystids, other cornutes, anomalacystitids, mitrocystitids, eocrinoids, rhenopyrgids, and somasteroids. Fezouata stylophoran fossils include soft tissue preserved among the skeletal elements, helping to unravel controversial details of their anatomy and ecology.

Specific echinoderm species may form dense fossil beds in some layers of the formation, a phenomenon which is particularly common in the mid-late Tremadocian (Araneograptus murrayi graptolite zone). Most echinoderm beds are dominated by only a few species, often representatives of stylophorans or the eocrinoid Rhopalocystis, with few other animal fossils. Through nearly the entire the formation, small Rhopalocystis species dominate "meadow"-like ecosystems in shallow waters impacted by storms. In the youngest layers of the formations, diploporites usurp this niche. Conversely, stylophorans are opportunistic colonizers of deeper low-oxygen seabeds in some intervals of the Araneograptus murrayi zone. Echinoderms are uncommon at intermediate depths, which have a higher proportion of brachiopod and trilobite fossils.

Nevertheless, there are quite a few exceptions which contradict these broad rules. Many sites record diverse deep-water ecosystems protected from both storms and insufficient oxygen. Numerous species of echinoderms and other invertebrates coexist at these localities, even if one echinoderm species outnumbers other fossils at any given time. Large Rhopalocystis species, Macrocystella, Plasiacystis, and Balantiocystis are common components of these assemblages.

Fezouata reconstructs the uneven nature of the Cambrian-Ordovician transition of echinoderm faunas. Cosmopolitan Late Cambrian hallmarks (such as cornute stylophorans) maintian their abundance in oxygen-poor areas, while newer groups (crinoids, diploporites, asterozoans) make inroads into more oxygenated waters. The rarity of carbonate platforms and hard substrates in the Gondwanan area delayed the diversification of crinoids and edrioasteroids in the region. This also provided more space for the establishment of a distinctive South Polar ecosystem dominated by eocrinoids, mitrates, solutans, and eventually diploporites.

Conodonts
Conodonts from the Fezouata Formation are mostly coniform elements of Early Ordovician species.  Overall diversity is rather low, and species which were common in temperate and tropical seas are apparently absent. The Fezouata Formation appears to be an exemplar of the 'subpolar domain', an assemblage of cold-water coastal conodonts native to the South Polar region of the Early Ordovician. Similar conodont faunas are known from Early Ordovician deposits in Central Europe, which was also located near the South Pole. The 'subpolar domain' survived into the Middle Ordovician and expanded into areas now found in the Middle East. 

Fezouata conodonts are difficult to correlate to biostratigraphic systems in nearby temperate regions such as Baltica. Most species appear to correspond to the time interval stretching from the Oelandodus elongatus-Acodus deltatus subzone of the Paroistodus proteus zone (uppermost Tremadocian) up through the Prioniodus oepiki zone (lower Floian). 

Graptolites
Graptolites in the Fezouata Formation are important for biostratigraphic correlation to other regions. Most species found in the formation are assigned to planktic (graptoloid) groups, though some mesh-shaped taxa (like Araneograptus and Rhabdinopora) may have been transitional between sessile (dendroid) graptolites and free-floating graptoloids.  Sessile forms, such as Didymograptus, Dictyonema, Webbyites, and rhabdopleurids, are also present but much more rare.

The graptolites of the Fezouata Formation are distributed over 10 biozones. In order, these zones are: the Anisograptus matanensis zone (1), Rhabdinopora flabelliformis anglica zone (2), “Adelograptus” tenellus zone (3), Aorograptus victoriae zone (4), Araneograptus murrayi zone (5), Hunnegraptus copiosus zone (6), ?Cymatograptus protobalticus zone (7), ?Baltograptus jacksoni zone (8), Baltograptus minutus zone (9), and the “Azygograptus interval” (10). Not all of the index taxa which these zones are named for are known from the Fezouata Formation. The Tremadocian-Floian boundary is approximately at the level between the Hunnegraptus copiosus and ?Cymatograptus probalticus zones. 

The most well-preserved fossils in the Fezouata Formation generally come from strata of the late Tremadocian (late Araneograptus murrayi zone and early Hunnegraptus copiosus zone) and the mid-late Floian (late Baltograptus minutus zone and early "Azygograptus interval"). 

Sponges
Many sponges from the Fezouata biota remain unnamed, including protomonaxonid demosponges (leptomitids, "choiids", hamptoniids, piraniids), a hazeliid, reticulosan hexactinellids (asthenospongiids,  etc.), and other indeterminate forms. Many of the sponges have affinities with Cambrian taxa common in Burgess Shale-type faunas.  Though at least 27 sponge species have been recorded in the biota, nearly all occurrences are monospecific death assemblages, with the exception of Pirania auraeum, which has a broader and less dense distribution in the formation. Periodic unstable seafloor conditions (potentially related to seasonal disruptions) would have favored species-poor colonization events over short time periods, rather than a stable and diverse equilibrium.

This is unusual relative to other Ordovician sponge ecosystems, such as the Builth Inlier of Wales. Another difference is how Fezouata's sponge fauna consists mostly of protomonaxonids, with a few reticulosans occupying course-grained shallow seabeds. Conversely, in Wales there is a clear succession of diverse and sturdy lithistids and thick-walled hexactinellids in shallow reefs and other energetic areas, with protomonaxonids at intermediate depths, and reticulosans in the deepest and calmest environments.

Other animals
Many animals from the Fezouata biota remain unnamed and undescribed, including bryozoans, chordate "dermal plates", annelid worms, priapulids, problematica, and at least five new species of lobopod, including an armored form.

Other organisms
Chitinozoans, acritarchs, and algae have been recorded from the formation.