Australia: The Land Where Time Began

A biography of the Australian continent 

Pangaea in the Early Permian – Biological and Physical Evidence for Extreme Seasonality in Central Pangaea

It is indicated by climate models that during the Permian there was increased desertification in the continental interior of Pangaea, which would have affected the flora and fauna. In this paper Looy et al. present a multi-proxy palaeoenvironmental reconstruction of a terrestrial ecosystem in central Pangaea that dates to the Lopingian. The study based the reconstruction on biological and physical data recovered from the Moradi Formation, which is located in the Tim Mersoi sub-Basin, in northern Nigeria. It is indicated by palaeosols and sedimentological evidence that the prevailing climate was semiarid to very arid and with marked intervals during which there was a high availability of water. Histological analysis showed evidence of active metabolism and reveals distinct growth marks on pareiasaur bones. Growth rhythms as indicated by these interpretations of bone formation are considered to be markers for contrasting seasonality or episodic climate events. There is low diversity of the macrofossil floras which represent woods dominated by gymnosperms. Ovuliferous dwarf shoots of voltzian conifers are most notable, and tree trunks that are 25 m long that have branch scars that are placed irregularly. It is suggested by the combined biological and physical evidence that the deposition of the Moradi Formation occurred under a climate that was generally arid with recurring periods of water abundance which allowed for an ecosystem that was well established and that was dependent on ground water. With respect to its environment, this system is comparable to modern ecosystems such as the Namib Desert in southern Africa and the Lake Eyre Basin in Australia, which are discussed as modern analogues.

It has been shown by lithological and phytogeographic data that the region that is at present the interior of Africa experienced an increase in aridification from the Late Carboniferous – Early Permian to the Middle to Late Permian (Gibbs et al., 2002; Rees et al., 2002). It is estimated by climate models that in central Pangaea desert-like conditions prevailed during the Lopingian, with summertime high temperatures of around 40-50^oC being suggested by most recent models (Fluteau et al., 2005). Looy et al., suggest the change from a climate that was wet seasonally in the Cisuralian to a desert-like climate in the Lopingian must have altered considerably the composition of the flora and fauna in the local communities, as well as their potential pathways for dispersal (Sidor et al., 2005). It is difficult to assess exactly how the biota responded, however, the best conditions for the preservation of plants are not typical of arid climates (Gastaldo & Demko, 2010), which leaves a gap that is unsatisfactory in knowledge of ecosystems in central Pangaea.

Fortunately in the last 10 years rare fossil vertebrate and plant assemblages have been discovered in northern Niger that date to the Upper Permian (Damiani et al., 2006; Sidor et al., 2003, 2005; Smiley et al., 2008; Steyer et al., 2006; Steyer et al., 2006; Tsuji et al., 2013; Turner et al., 2015). Deposition of the Moradi Formation of the Tim Mersoi sub-basin during the Lopingian that occurred approximately 10^oS of the palaeoequator in central Pangaea. A community structure and a taxonomic community that is similar to that have been described for Morocco has been suggested by vertebrate fossil studies from the Moradi Formation (Germain, 2020; Jalil & Dutuit, 1996; Jalil & Janvier, 2005; Sidor, 2013; Steyer & Jalil, 2009). However, they differ distinctly from any of the assemblages of southern Pangaea that are better known (e.g. South Africa, Tanzania, Zambia, and Malawi), which were dominated by therapsids (e.g. Angielczyk et al., 2013; Sidor et al., 2005, 2013). Temnospondyls, pareiasaurs and captorhinids dominate the faunas of Niger and Morocco. There are 2 herbivorous reptiles that have been described from the Moradi Formation of Niger, Moradisaurus, a captorhinid, and Bunostegos, a pareiasaur, known only from this formation (de Ricqlès & Taquet, 1982; Sidor et al., 2003; Taquet, 1969; Tsuji et al., 2013; Turner et al., 2015). Both of the temnospondyls that have been described, Nigerpeton and Saharastega, similarly represent relicts, holdovers of lineages that are represented in faunas that are believed to have gone extinct near the boundary of the Pennsylvanian-Permian elsewhere (Damiani et al., 2006; Sidor et al., 2005, 2013; Steyer et al., 2006). Fragmentary gorgonopsid remains are the only evidence of a terrestrial predator to be found in these faunas, though there was not enough to make an identification at the genus level (Smiley et al., 2008). It has been hypothesised (Smiley et al., 2005) that as the result of the lack of any obvious physical barrier between the faunas of central and southern Pangaea then the climate was likely to be factor for biogeographic isolation and endemism in the Moradi assemblage.

Sedimentary, morphological, and mineralogical data from profiles of palaeosols in the Moradi Formation corroborate these climatic interpretations, which show evidence of arid conditions that prevailed at the time (Tabor et al., 2011; Smith et al., 2015). The palaeosols and the associated sedimentary strata are characteristic of those that formed a range of conditions that ranged from soils that were well drained to those that were poorly drained, and represent stable floodplains of ephemeral, anabranching fluvial depositional systems (Tabor et al., 2011). According to Looy et al. these soil types are indicative of areas that are semiarid and have water tables that are shallow, and include soils that are formed on plains surrounding ephemeral lakes or playa environments. At the present analogous soil types can be found in Namib Desert (Watson, 1992) in southwestern Africa and the Lake Eyre Basin in central and eastern Australia (Tabor et al. 2011; Fielding & Alexander, 2001; Tooth & Nanson, 2000). Zonobiome 3 of Heinrich Walter is represented by both fossil and extant landscapes, or hot and arid subtropical deserts that have very little rain (Breckle, 2002).

This paper presents new biological and physical evidence for extreme seasonality in the Moradi Formation that used a wide array of ecological proxies. The palaeosols and sedimentological data are briefly described, and the stable isotope compositions of palaeosols calcite and organic matter are presented and discussed in terms of their relevance to interpretations of palaeoclimate, biological productivity and the availability of water for floral and faunal communities across the palaeolandscapes. Data from the bone histology of tetrapods is presented here for evidence of seasonality, including cyclical periodic cessation of growth. The Moradi fossil floras that were dominated by gymnosperms, which include a large tree trunk, are described for the first time, and are compared to modern analogues in order to constrain the circumstances of the environment under which these plants grew. The implications of the combined data on the interpretation of the climate for central Pangaea in the Lopingian are discussed.

Conclusions

Description of the Moradi palaeoecosystem

The organisms recovered from this formation lack an amniotic egg so therefore depend on water for reproduction (temnospondyl amphibians), palaeosols that indicate semiarid conditions, carbon stable isotope data from palaeosols that indicate arid climatic conditions, and evidence of plant life that was tall and robust. When all of these facts are combined they suggest that in the Permian the Moradi Formation was deposited under a climate that was generally arid that experienced cyclical intermittent periods when there was abundant water, a place of extreme seasonality or episodicity, that supported what was probably an ecosystem that was well-established that depended on ground water. Looy et al. suggest that during the drier and probably longer intervals, the Moradi ecosystem was a large arid, relatively flat plain with sparse plant cover that had adapted to shallow ground water resources and temporary ponds of water in which rare temnospondyl amphibians survived. This system was cut and drained during the shorter, wet intervals, by temporary meandering systems that were filled occasionally by flash floods that were locally catastrophic (Smith et al., 2009, 2015).

Sources & Further reading

Looy, C. V., et al. (2016). "Biological and physical evidence for extreme seasonality in central Permian Pangea." Palaeogeography, Palaeoclimatology, Palaeoecology 451: 210-226.