Richness of lichens growing on Eocene fossil penguin remains from Antarctica

Antarctica presents one of the most severe environmental conditions for life. Under these circumstances, cryptogams are the dominant photosynthetic organisms, among which we find a great richness of lichens. In Antarctic environments, lichens can grow on rocks or in this case on fossil remains, among the few available substrates. In the present contribution, we examined all fossil penguins of the Antarctic collection of the Museo de La Plata, as a significant sample of fossil vertebrates. The selected materials here described come from the Submeseta Formation (Eocene) on Seymour/Marambio Island, located northeast of the Antarctic Peninsula on the Weddell Sea. Given the scarcity of lichenological studies on this island, and the results presented here add significantly to our knowledge of the lichen species that occur there with the recognition of 11 taxa with a crustose morphology (epilithic and endolithic), the sampling of lichens growing on fossil bones acquired an evident importance.


Introduction
The Antarctic regions present some of the most severe environmental conditions endured by plant life, involving habitats characterized by intense cold, physiological drought, and seasonal limitation of insolation. Antarctica presents a series of terrestrial ecosystems characterized by a greatly reduced flora and fauna, which are isolated from other systems by extensive ocean barriers .
Terrestrial ecosystems are restricted to the 4% of icefree lands during the summer months (Rudolph 1977, see also Lee et al. 2017). The severe weather conditions make Antarctica one of the harshest environments on Earth for the development of higher plants (Green et al. 2007). These Antarctic ecosystems are unique since cryptogamic plants and invertebrate animals are the most abundant groups; and among them, lichens (lichenized fungi) have a dominant role (Lindsay 1978;Øvstedal and Smith 2001). Crustose, squamulose, foliose, and fruticose thalli forms, are common and widespread, constituting communities included in two subformations: the crustose lichen subformation and the fruticose lichen and moss cushion subformation (Smith and Gimingham 1976).
In Antarctica, and many other places, fossil remains act as a substrate and are susceptible to lichen colonization. As a consequence, lichen activity damages the structure of fossils that lose material and become more fragile. In this context of biodeterioration, some remarkable studies have been carried out (Fernández-Jalvo et al. 2002;Thackeray et al. 2005;Acosta Hospitaleche et al. 2011;Gouiric-Cavalli et al. 2018;Saitta et al. 2019). However, in a place, like Seymour Island, where only two lichen species are reported, Gyalolechia desertorum (Tomin) Søchting, Frödén & Arup and Myriolecis mons-nivis Darb. (Øvstedal and Smith 2001), a lot of work from a lichenological perspective is still necessary. In the present contribution, we examined all fossil penguins housed in the Antarctic collection of the Museo de La Plata, as a significant sample of fossil vertebrates of Seymour Island (Antarctica) looking for traces of lichens.

Materials
Lichen species described here were obtained from Eocene penguin bones permanently housed in the collections of the División Paleontología Vertebrados (DPV) of the Museo de La Plata (MLP), La Plata (Argentina). From the approximately 10,000 bones examined, a total of 120 specimens were selected to sample.

Methods
We examined 120 fossil remains looking for the better preserved lichen thalli. Lichens were observed under stereoscopic microscope Arcano Ztx and light Olympus BH2, and pictures were taken with a stereoscopic microscope ZEISS SteREO Discovery.V20.
Morphological and anatomical studies include freehand cuts of the reproductive structures, in order to observe ascus and spores. The usual histochemical reactions were performed: K (KOH 10%), C (NaClO), and I (KI), which indicates the amyloid reactions of the asci and hamathecium.

Study area
Materials come from Seymour/Marambio Island (Fig. 1), a small island located northeast of the Antarctic Peninsula on the Weddell Sea, West Antarctica. The entire island is a plateau of fourteen kilometers in length and eight kilometers in width, where the Argentinian Marambio Station has been established since 1969. The continued presence of this Station plus the weather conditions of the island, that is, ice free during the summer season, encouraged an intense scientific field work in the area. As a result, and after numerous and almost uninterrupted paleontological field works since 1977, the fossil vertebrate collection from Antarctica housed in the Museo de La Plata has become the largest in the whole world. It includes more than 20,000 fossil vertebrates; among them, bones were revised.
Penguin bones are very frequent among fossils from Seymour Island and constitute the most abundant vertebrates together with shark teeth. Bones examined here, all assigned to fossil penguins, come from different sites of the island located from 50 to 200 m.a.s.l. around the Marambio Station ( Fig. 1). Most of the materials were collected in the locality (DPV) 13/84, a famous fossil site of Seymour Island initially explored by Otto Nordenskjöld (Wiman 1905), and subsequently by numerous paleontologists (see Acosta Hospitaleche et al. 2017Hospitaleche et al. , 2019. Bartonian levels of the Submeseta II Allomember of the Submeseta Formation crop out on this site and also in the equivalent locality DPV 14/84. In DPV 16/84, sediments are younger and correspond to the Priabonian Submeseta Allomember III of the Submeseta Formation.

Results
In general terms, we observed that taphonomic damage reaches all the remains collected there. As a consequence, most of them are disarticulated and fragmented, with evidence of fish predation and scavenging, invertebrate feeding activity, encrusters, and bioerosion occasioned by lichen activity, among other traces.
After the examination of nearly 10,000 penguin bones, and given that our main objective was the systematic analysis of the lichens associated with these fossils, we selected the more complete and fertile lichen specimens, discarding those badly preserved and already sampled. A total of 10 species and a specimen that could only be identified at genus level were found. All species belong to the Division Ascomycota and Subdivision Pezizomycotina. A description of each species, the site of collection, distribution, and ecological data are provided below in the systematic section. Observed material MLP 08-XI-30-28, MLP 13-XI-28-140, and MLP 13-XI-28-410.
Ecology and distribution bipolar species. In Antarctica: distributed in Southern Antarctic Peninsula and continental Antarctica. On rocks.
Ecology and distribution Cosmopolitan species. In Antarctica: distributed in South Orkney Islands, South Shetland Islands, and Antarctic Peninsula. Grows mainly on various enriched siliceous rocks, boulders, and pebbles, but occurs also on lime-rich stone, slate, mortar, and concrete.
Ecology and distribution Cosmopolitan species. In Antarctica: distributed in South Orkney Islands, South Shetland Islands, Antarctic Peninsula, and continental Antarctica. On rocks in inland hills.
Ecology and distribution Cosmopolitan species. Distribution in Antarctica: due to taxonomic confusion, the distribution of this taxon is uncertain. On boulders and stones of recent glacier moraines or on introduced wood. Main component of epilithic communities in newly deglaciated moraines and one of the most important pioneer taxa of lichens. On a wide range of substrata, including calcareous and siliceous rock, concrete and mortar, dusty bark, and many man-made substrates, including metal, also commensally on other lichens.
Ecology and distribution Endemic from Antarctica, distributed in Antarctic Peninsula and South Shetland Islands. In snow bed communities.
Ecology and distribution Endemic from Antarctica, distributed in South Shetland Islands, Antarctic Peninsula, and continental Antarctica. Collected on wood, rocks, detritus, and whale bones, and seems to be not rare, and widespread.

Discussion and conclusions
The substrate where lichen grew corresponds to fossil penguin bones that lived in Antarctica between the Ypresian and Bartonian ages (Eocene). After transportation and unburial, these fossils suffer a series of taphonomic processes, among which the lichen attack is included. The unconsolidated sediments carrying these fossils are mostly sands and lutites, while vertebrate remains, such as the penguin bones examined here, are commonly preserved as calcium phosphate. All reported lichens in this contribution presented a crustose growth both (epilithic and endolithic), which are expected for this area (Øvstedal and Smith 2001). Excepting Myriolecis mons-nivis, which was previously reported for Seymour Island (Øvstedal and Smith 2001), the other species here described were previously found only in the Antarctic Peninsula (Øvstedal and Smith 2001Smith , 2004Ertz and Diederich 2004;Olech 2004) and constitute new reports for this island.
The lichens assemblage examined here comprises two species endemic from Antarctica, two with a bipolar distribution and others cosmopolitan. The Antarctic species Oevstedalia antarctica and Myriolecis mons-nivis have been cited growing on different kinds of surfaces, probing their generalist condition regarding the substrate preferences. On the other hand, Lecidea andersonii, the bipolar species, is also found in Norway and Iceland (Ruprecht et al. 2010), and Staurothele aff. frustulenta, previously considered as a bipolar taxon, and cited for Antarctica and Scandinavia (Øvstedal Oevstedalia antarctica, and a vertical fissure on the bone surface filled by small apothecia. Variations in tone and colour correspond to differences in the maturation or dehydration degree of the apothecia. Scale bars represent 1 mm and Smith 2001), has been recently found in Italy (Nimis and Martellos 2017), South Korea, and Argentina, showing a wider distribution than previously thought. In both last countries, Staurothele aff. frustulenta was growing in concrete (Rosato 2006;Kondratyuk et al. 2017), a substrate completely different than fossil bones. The cosmopolitan species, Myriolecis dispersa and Candelariella aurella do not seem to have a preference in the substrate on which they grow, since they have been found growing on concrete in urban environments (Sliwa 2007;García 2018) and also on fossil bones, like the cases presented here.
All the species described here prefer well-lit places (Øvstedal and Smith 2001;Arup 2009), which is expected since in the absence of vascular plants, lichens must be adapted to receive direct solar radiation. Their endolithic condition could be an important advantageous to withstand the extreme cold, since most of the thallus would be protected inside the rocks (Pointing and Belnap 2012;Wierzchos et al. 2013;de los Ríos et al. 2014;Wierzchos et al 2015), or bones in these cases.
In the summer season, the rock surface prevents the thallus from suffering sudden changes in temperature due to the action of the wind, which can be very windy in Antarctica (Friedmann 1982). However, endolithic growth can also be interpreted as a successful strategy to avoid high light intensities which may damage photosynthesis (Tretiach 1995), or as a strategy to avoid competition. Endolithic growth provides access to an otherwise sparsely inhabited habitat, not sharing the space with epilithic species (Bungartz et al. 2004).
Although bones do not provide a continuous and stable substrate for lichens, they frequently grow on the surface of fossil bones, and many species are particularly found within them. Independently of fossil size, we found lichen in small bone fragments (e.g. broken phalanges) and also in larger elements, such as femora and coracoids that belonged to giant penguins. However, we noticed that weathered surfaces with exposed trabecular tissues, surficial fissures, and fractures (see for example Fig. 2b) are the preferred substrate, providing probably a more protected place against the extreme environmental conditions dominated by snow and strong winds.
Crustaceous morphologies (epilithic and endolithic) could be more resistant to mechanical action, contrary to morphologies with greater thallus development (foliose and fruticose), which are more affected. The predominance of endolithic over epilithic lichen species is also explained by the mechanical damage produced by the action of the wind and snow plus the transport. However, it could be also a strategy to avoid competition in reduced substrate surfaces. Endolithic lichens are characterized by lower biomass than epilithic ones, and maximum hydration of the thallus is reached at levels five times lower (Tretiach 1995). Therefore, the optimal content of thallus water for photosynthesis and respiration is lower in endolithic than that in epilithic lichens (Tretiach and Pecchiari 1995), and also the endolithic growth structure increases porosity and probably allows more moisture to accumulate (Bungartz et al. 2004).
Whereas weather conditions are similar in the whole James Ross Basin, it means, in the Seymour/Marambio, Cockburn, Snow Hill, Vega, and James Ross islands, the topography and presence of glaciers in some areas determine strong differences in the substrate. Some of the lichen species were described from bones collected in Seymour Island, also grow in the nearby Cockburn Island, an even smaller island also without glaciers and snow during summer. However, given the presence of a more consolidated substrate in the surficial deposits (rocks, not bones) of the latter, we observed that lichens are extremely more abundant in Cockburn Island. The unconsolidated condition of the sediments exposed in Seymour Island attempts against the settlement of extended lichen communities.
In this context, fossil bones are an important source for the study of lichens richness, providing a suitable substrate for growing and protecting them from extreme conditions. Besides, considering that most of the biological studies, including lichenological sampling, are developed on the west side on the Antarctic Peninsula (e.g. Pannewitz et al. 2003;Olech 2004;Rodriguez et al. 2018), fossils already housed in collections became even more important, providing a unique possibility to study them in their original substrate.
Acknowledgements Antarctic field trip was completely funded by Dirección Nacional del Antártico and the Instituto Antártico Argentino, and logistic support in the field was provided by Fuerza Aérea Argentina. Partial help was provided by the Consejo Nacional de Investigaciones Científicas y Tecnológicas, Agencia Nacional de Promoción Científica y Tecnológica (PICT 2017-0607) and Universidad Nacional de La Plata (N838). CAH is particularly grateful to Oceanwide Expeditions, Vlissingen (NL) for financial support. Helpful comments of the reviewers Helen Peat, Paula Casanovas, and Leandro Pérez improved the manuscript.
Funding Partial help was provided by the Consejo Nacional de Investigaciones Científicas y Tecnológicas, Agencia Nacional de Promoción Científica y Tecnológica (PICT 2017-0607) and Universidad Nacional de La Plata (N838). CAH is particularly grateful to Oceanwide Expeditions, Vlissingen (NL) for financial support.