Coombs Hill: A Late Devonian fossil locality in the Witpoort Formation (Witteberg Group, South Africa)

FUNDING: DSI/NRF Centre of Excellence in Palaeosciences, DSI/NRF African Origins Platform, National Research Foundation (South Africa), the Palaeontological Scientific Trust (PAST), the Millennium Trust Coombs Hill, a new fossil locality in the Witpoort Formation (Witteberg Group) of South Africa, preserves a record of Famennian (Late Devonian) life in Gondwana. Fossil plants collected at Coombs Hill are preliminarily assigned to several classes. Shelly invertebrates include a variety of bivalve mollusc forms, some of which appear to be preserved in life position. Biodiversity at Coombs Hill is comparable to that of the well-known Waterloo Farm lagerstätte in ordinal diversity, but exhibits differences in species composition. Ongoing taxonomic analysis will provide a rare window into the ecology of high-latitude environments during this pivotal stage of Earth history, which immediately preceded the end-Devonian extinction. Sandstone dominated sedimentary facies at Coombs Hill suggest a high-energy coastal marine setting, with brackish back-barrier estuarine/lagoonally derived fossiliferous mudstones. Exact stratigraphic placement within the Witpoort Formation is hampered by structural deformation, and precise age comparisons with Waterloo Farm are currently tenuous.


FUNDING:
DSI/NRF Centre of Excellence in Palaeosciences, DSI/NRF African Origins Platform, National Research Foundation (South Africa), the Palaeontological Scientific Trust (PAST), the Millennium Trust Coombs Hill, a new fossil locality in the Witpoort Formation (Witteberg Group) of South Africa, preserves a record of Famennian (Late Devonian) life in Gondwana. Fossil plants collected at Coombs Hill are preliminarily assigned to several classes. Shelly invertebrates include a variety of bivalve mollusc forms, some of which appear to be preserved in life position. Biodiversity at Coombs Hill is comparable to that of the well-known Waterloo Farm lagerstätte in ordinal diversity, but exhibits differences in species composition. Ongoing taxonomic analysis will provide a rare window into the ecology of high-latitude environments during this pivotal stage of Earth history, which immediately preceded the end-Devonian extinction. Sandstone dominated sedimentary facies at Coombs Hill suggest a high-energy coastal marine setting, with brackish back-barrier estuarine/lagoonally derived fossiliferous mudstones. Exact stratigraphic placement within the Witpoort Formation is hampered by structural deformation, and precise age comparisons with Waterloo Farm are currently tenuous.

Significance:
• A new fossil locality at Coombs Hill comprises the second known site with a suite of well-preserved continental and marginal marine fossils from the Witpoort Formation, providing an exceptionally rare example of high-latitude life during the critical latest Devonian Famennian age.

Introduction
Coombs Hill (33°17'51.86"S 26°45'28.70"E) is situated within the predominantly quartzitic Witpoort Formation (mid Witteberg Group, Cape Supergroup) east of Makhanda ( Figure 1). The Witteberg Group accumulated in a shallow marine setting along the southern coastline of Gondwana, between the Givetian (Middle Devonian) and Visean (Early Carboniferous), with the Witpoort Formation being of Famennian age 1,2 -a time in which the region was within the Antarctic Circle 3 ( Figure 1).
Strata of the Witpoort Formation, along with the rest of the Cape Supergroup, crop out for more than 1000 km along South Africa's southern and western coastlines, having been tectonically deformed during the formation of the late Palaeozoic Cape Fold Belt. 4 The Witpoort Formation has, in recent decades, provided evidence for high-latitude latest Devonian life, although exclusively from a single lagerstätte at Waterloo Farm ( Figure 1), which comprises the only known high-latitude marginal marine ecosystem from the Famennian. The 23 plant and animal taxa as yet diagnosed include the only known high-latitude Devonian tetrapods, the earliest known lampreys and Gondwana's only Devonian terrestrial invertebrates. 3,5,6 Here, we report the discovery of a new locality at Coombs Hill, 21 km to the east of Waterloo Farm.
The Late Devonian records the end of the Siluro-Devonian hothouse climate, with the appearance of continental glaciers and increasingly dramatic sea-level (eustatic) fluctuations 7 associated with a series of major biotic crises 8 . Cooling is considered by many authors to have resulted from drawdown of CO 2 associated with widespread vegetation of land. 9,10 Abundant vegetation at Coombs Hill and Waterloo Farm provides the only evidence for highlatitude forestation during the Late Devonian biocrisis.

Structural geology
The

Stratigraphy and sedimentology
A stratigraphic log of exposures along the recent road cutting at Coombs Hill records roughly 80 m of stratigraphic sequence (Figure 3). The majority of the strata comprise clean blue-grey (when fresh; 5B 6/2-5B 7/1 (Munsell colour system)), medium-grained quartz arenites, which are stacked in a succession displaying only subtle lithological changes, and punctuated by discrete mudstone horizons, comprising ~10% of the succession. Many of the quartzitic strata superficially appear red to purple due to precipitation of minerals along joints. Weathered quartzites usually alter to light brown (5YR 5/6) or light grey (N8). Palaeocurrent trends derived from cross-strata (n=47) are polymodal, with modes towards the east, south-southwest, and a minor component to the northwest (Figure 3). These trends, relative to a roughly east-west trending shoreline 12,13 , indicate both offshore and longshore sediment transport with a minor landward component, consistent with the fluctuations of current, wave and storm energy in shallow marine rather than fluvial settings 14,15 .
Many of the facies associations formerly described in the Witpoort Formation around Makhanda 13 are recognised at Coombs Hill, and three broad depositional environments are provisionally identified: (1) shoreface, (2) foreshore and (3) backshore (as illustrated in Figure 3). The new locality is similarly interpreted as representing a shallow to marginal-marine succession characterised by high wave energy and continual reworking of sediments.  The alternation between shoreface and backshore is the result of transgressive-regressive cycles, the chronology and driving mechanisms of which are not clearly understood. Further detailed facies studies are required to test the postulated depositional environments.

Fossil biota
Excavations by R.W.G. and C.H. in 2015 and 2016 produced a representative sample of the biotic remains preserved in the mudstones, including more than 200 specimens of relatively well-preserved plants and shelly invertebrates which are curated at the Albany Museum.
Mudstones preserve an association of putative marine algae, abundant terrestrial plant matter and bivalve remains. This evidence is in line with existing hypotheses which propose stagnant brackish-water lagoonal 13 or estuarine 16  Archaeopteris notosaria, hitherto only known from Waterloo Farm, provides the earliest evidence for trees at high palaeolatitudes. 18 Exquisite vegetative and fertile remains from Coombs Hill contribute significantly to taxonomic understanding of this species (Figure 5a  Algal thalli, similar to Hungerfordia fionae Hiller and Gess 1995, are relatively common at Coombs Hill (Figure 5c), although they are more elongate and less bilaterally symmetrical, probably representing a new species.
Invertebrates are solely represented by abundant articulated bivalves consistently preserved with the umbo upwards (in apparent life position), usually in mudstones but sometimes in underlying sandstones (Figure 5d). These likely comprise several euryhaline genera, and their dominance of the muddy substrate suggests a brackish setting. 19

Future research
The palaeontology of Coombs Hill is the subject of a series of taxonomic papers allowing palaeoecological comparisons with Waterloo Farm. Structural analysis will facilitate finer stratigraphic resolution of these localities and improved understanding of their relative ages. Detailed reconstructions of the depositional environments form part of a broader study on the Witpoort Formation and are important in understanding the relative influence of autocylic (i.e. subsidence and sediment supply) and allocyclic (eustatic) processes on environmental change that could elucidate major bioevents in the Late Devonian.