The Souss lagerstätte of the Anti-Atlas, Morocco: discovery of the first Cambrian fossil lagerstätte from Africa

Episodic low oxygenated conditions on the sea-floor are likely responsible for exceptional preservation of animal remains in the upper Amouslek Formation (lower Cambrian, Stage 3) on the northern slope of the western Anti-Atlas, Morocco. This stratigraphic interval has yielded trilobite, brachiopod, and hyolith fossils with preserved soft parts, including some of the oldest known trilobite guts. The “Souss fossil lagerstätte” (newly proposed designation) represents the first Cambrian fossil lagerstätte in Cambrian strata known from Africa and is one of the oldest trilobite-bearing fossil lagerstätten on Earth. Inter-regional correlation of the Souss fossil lagerstätte in West Gondwana suggests its development during an interval of high eustatic levels recorded by dark shales that occur in informal upper Cambrian Series 2 in Siberia, South China, and East Gondwana.

. The Amouslek Formation crops out along the northern slope of the western Anti-Atlas of southern Morocco, with exceptionally preserved fossils of the here described Souss Lagerstätte being found in an area between approximately 30° 10′ to 30° 25′ N and 9° 00′ to 8° 40′ W. The strata with these exceptionally preserved fossils belong to part of the Daguinaspis Zone 24 which forms the upper part of the regional Issendalenian Stage corresponding approximately to the upper Lower Cambrian Stage 3. The formation is a succession of slightly argillaceous siltstones to fine-grained sandstones with only sparsely intercalated limestones (which elsewhere form frequent intercalations in the formation). These rocks are faintly laminated and have a light to bright yellow colour in outcrops due to weathering effects. These beds are macroscopically similar to other parts of the Amouslek Formation, but for the presence of exceptionally preserved fossils. Relatively fresh rock surfaces are medium grey in colour, and it is highly probable that this rock type, with limited evidence of bioturbation, represents short episodic low oxygenation events during deposition of the sediments.

Preservation and diversity of fossils
The typical Daguinaspis Zone consists of fine-grained siliciclastic rocks in the western Anti-Atlas and contains a moderately diverse fauna dominated by fallotaspidid, bigotinid, abadiellid, dolerolenid, and despujolsiid ("resseropid") trilobites [25][26][27][28] (Fig. 3c) and Perrector falloti (Fig. 3a,b,e) that show paired, metamerically-disposed, ovoid to club-shaped dark stains in the glabella and anterior part of the thoracic axis. These stains are interpreted to be incompletely preserved parts of the digestive tracts, particularly paired digestive glands and alimentary canal. Similar preservations can be seen in other specimens of Perrector falloti (Fig. 3d,g,i), in Perrector brevilimbatus (Fig. 3f) and in Marsaisia sp. (Fig. 3h). A different type of exceptional preservation of labile tissues is observed in specimens of the Mickwitzia-type brachiopod Microschedia amphitrite. Some of specimens of this species show remains of setae and dark colouration that correspond to parts of the visceral mass. The valves were demineralized during early diagenesis, and now show several preserved shell layers. In addition, the visceral mass was forced out from between the valves with compaction (Fig. 4a,e-g). Associated hyoliths (Fig. 4b-d) are variably preserved, with the shell material of the conch either demineralized or limonitized, but frequently with partial iron oxide moulds of the soft parts inside the conch. In addition, small fragments of arthropods without a mineralized dorsal carapace have been found on weathered bedding surfaces, along with a number of yet unidentified fragments that show labile tissues.
The limited number of isolated sclerites and the absence of sedimentary structures indicative of high-energy deposition would favour obrution as key mechanism to explain the preservation of these exceptional fossils. Taphonomic data such as very fine lamination and clayey lamina suggest restricted, but episodic wave-activity. Thus, the Souss lagerstätte must be classified as a konservat-lagerstätte sensu Seilacher 1 .

Global relation
Intercontinental correlation of the strata indicates that the Daguinaspis Zone of the Moroccan Atlas ranges is partly coeval with the upper Atdabanian Stage of the Siberian Platform, the Wutingaspis-Eoredlichia Assemblage Zone of the lowermost Nangaoan Stage of South China, the "Abadiella" huoi Zone of South Australia 31 , and the dark mudstones of the lower Calodiscus lobatus Zone of NE Laurentia 34 All four regions show relatively low oxygen levels in approximately coeval strata, with a pronounced development of black shales locally as in the Shujingtuo and Qiongzhusi formations of South China 32,33 . It is possible that coeval low-oxygen conditions could have developed synchronously and reflect high eustatic levels during a period of pronounced warm epeiric seas with varying and episidocally low levels of oxygen in solution 34 . Indications of such developments associated with low dissolved oxygen are provided by carbon isotope (δ 13 C carb ) signatures. For the upper Amouslek Formation in the western Anti-Atlas, reconnaissance sampling has shown generally negative values 35 , but due to the limited number of carbonate beds in this stratigraphic interval, the significance of these values is highly uncertain. A negative peak is recorded from the coeval upper Atdabanian-lowest Botoman of Siberia 36 . By contrast, the Chengjiang, Qinjiang and Sirius Passet lagerstätten were related to the so-called CARE positive carbon isotope excursion 37 , which is commonly but probably erroneously regarded as a lower to middle Atdabanian equivalent 31 . However, biostratigraphic correlation clearly indicates that at least for the Chengjiang and Qinjiang lagerstätten are late Atdabanian (= late part of global Stage 3) equivalents (Fig. 2).

Conclusions
This report features the first relatively abundant fossils with exceptional preservation from the Cambrian of Morocco (and Africa). The metazoan fossils now known with such exceptional preservation mostly belong to such characteristic invertebrate taxa as trilobites, brachiopods, and hyoliths, but the mode of preservation differs considerably among those groups. These fossils are found in a regionally and stratigraphically limited part of  . (a, b, d, e, i)

Data availability
The raw image files and all other data that support this study are available from the senior author.