Overview
About a kilometre from Coombs Hill and extending for several kilometres to the west, and exposed during the same roadworks project in 2015-'16, Rabbit Ridge contains a unique fossil site with the only brachiopods (lamp shells) that we know of from the Witpoort Formation.
Thousands of lingulid brachiopods (Dignomia) are preserved either buried their in their burrows, or scattered about on bedding planes (former sediment surfaces) in a muddy lagoon bottom deposit. Additionally, abundant plant remains in the deposit have been transported and damaged to a greater extent than those at the other nearby Witpoort Formation localities. Trace fossils are very well preserved in thin sandstone interbeds in the mud layer and represent typically marine types of traces. Also found are rare fish bones, representing an undescribed placorderm taxon not previously known from the Witpoort Formation. This occurrence is interesting because lingulids have not been found in a Witpoort Formation mudstone before, and these are the youngest brachiopods known from the Cape Supergroup (1).
Lingulids are the only brachiopods that burrow, and it is perhaps due to this infaunal habit that they have managed to survive in the same manner for over 500 million years, since shortly after the 'Cambrian Explosion' until today. They exhibit extreme evolutionary conservativism, and this led Charles Darwin to cite the living genus Lingula as a prime example of a 'living fossil' (2). Superficially brachiopods can resemble clams, but belong to an entirely different phylum of animals.
Lingulids are very resilient creatures, which can withstand a wide range of shallow marine conditions (all brachiopods are restricted to marine habitats) as well as the intertidal zone. They have an ability to withdraw down in their burrows during harsh conditions using a pedicle (foot) anchored at the base, and in this way can 'aestivate' for days when conditions are unfavourable. This makes them unusually tolerant to salinity and temperature fluctuations and anoxia (low oxygen levels). Thus the deposit seems to indicate a stressful environment in which the lingulids have thrived due to competitive release (absence of competitors).
This is borne out by the trace fossils including Nereites and Zoophycos, traces which are typical of poorly oxygenated, stressful conditions. These ichnogenera are typical of deeper marine deposits in post-Palaeozoic strata (ca. 250 million years ago and younger), whereas in the Palaeozoic such traces were common in the intertidal zone, as observed on Rabbit Ridge.
We hypothesise that the dominance of lingulids at this horizon, compared with bivalve molluscs at other Witpoort Formation localities (Coombs Hill and Waterloo Farm) reflects salinity gradients and other typical shallow water ecological stresses amongst a spectrum of coastal estuaries and lagoons developed along the Agulhas Sea shoreline during the Famennian.
References
1) Harris C, and R. W. Gess, 2022, Insights from a monospecific lingulid brachiopod bed in the Late Devonian of South Africa, Palaios, 37(9), 471-485.
2) Darwin C., 1859, On the origin of species by means of natural selection, or, The preservation of favoured races in the struggle for life. J. Murray: London.
Thousands of lingulid brachiopods (Dignomia) are preserved either buried their in their burrows, or scattered about on bedding planes (former sediment surfaces) in a muddy lagoon bottom deposit. Additionally, abundant plant remains in the deposit have been transported and damaged to a greater extent than those at the other nearby Witpoort Formation localities. Trace fossils are very well preserved in thin sandstone interbeds in the mud layer and represent typically marine types of traces. Also found are rare fish bones, representing an undescribed placorderm taxon not previously known from the Witpoort Formation. This occurrence is interesting because lingulids have not been found in a Witpoort Formation mudstone before, and these are the youngest brachiopods known from the Cape Supergroup (1).
Lingulids are the only brachiopods that burrow, and it is perhaps due to this infaunal habit that they have managed to survive in the same manner for over 500 million years, since shortly after the 'Cambrian Explosion' until today. They exhibit extreme evolutionary conservativism, and this led Charles Darwin to cite the living genus Lingula as a prime example of a 'living fossil' (2). Superficially brachiopods can resemble clams, but belong to an entirely different phylum of animals.
Lingulids are very resilient creatures, which can withstand a wide range of shallow marine conditions (all brachiopods are restricted to marine habitats) as well as the intertidal zone. They have an ability to withdraw down in their burrows during harsh conditions using a pedicle (foot) anchored at the base, and in this way can 'aestivate' for days when conditions are unfavourable. This makes them unusually tolerant to salinity and temperature fluctuations and anoxia (low oxygen levels). Thus the deposit seems to indicate a stressful environment in which the lingulids have thrived due to competitive release (absence of competitors).
This is borne out by the trace fossils including Nereites and Zoophycos, traces which are typical of poorly oxygenated, stressful conditions. These ichnogenera are typical of deeper marine deposits in post-Palaeozoic strata (ca. 250 million years ago and younger), whereas in the Palaeozoic such traces were common in the intertidal zone, as observed on Rabbit Ridge.
We hypothesise that the dominance of lingulids at this horizon, compared with bivalve molluscs at other Witpoort Formation localities (Coombs Hill and Waterloo Farm) reflects salinity gradients and other typical shallow water ecological stresses amongst a spectrum of coastal estuaries and lagoons developed along the Agulhas Sea shoreline during the Famennian.
References
1) Harris C, and R. W. Gess, 2022, Insights from a monospecific lingulid brachiopod bed in the Late Devonian of South Africa, Palaios, 37(9), 471-485.
2) Darwin C., 1859, On the origin of species by means of natural selection, or, The preservation of favoured races in the struggle for life. J. Murray: London.