2.2 Machine learning
2.2.3 Data preprocessing methods
B. G. Baarli1, M. Cachão2, C. M.da Silva2, M. E.Johnson1, J. Ledesma-Vázquez3, E. Mayoral4 and A. Santos4
1 Department of Geosciences, Williams College, Williamstown, MA 01267. USA. [email protected] and
2 Departamento de Geologia, Centro de Geologia. Faculdade de Ciências da Universidade de Lisboa, Campo Grande. 1749-016 LISBOA. Portugal.
[email protected] and [email protected].
3 Faculdad de Ciencias Marinas, Universidad Autónoma de Baja California, Ensenada, BC 22800, México. [email protected].
4 Departamento de Geodinámica y Paleontologia, Universidad de Huelva Avd. 3 de Marzo, s/n, 21071 Huelva. Spain. [email protected] and [email protected].
Abstract: Nodules composed of free-rolling organisms living on level sea-bottoms – herein in general designated as rolloliths – are well known. Rhodoliths, accretions of red coralline algae are frequent. Also known are nodules of accreted corals (coralliths), barnacles (balanuliths) and serpulids. Herein are reported and discussed balanuliths and other rolling accretions with a highly diverse composition of encrusters and bioeroders from the Upper Pleistocene of Baja California Sur, México. Represented are several taxa of corals, bryozoans, serpulids, vermetid gastropods and bivalves in addition to Balanus sp.,
a brachiopod and a wealth of bioeroders. Possibly, this is the first record of fossil free-rolling vermetids: vermetuliths.
Key words: Fossil nodules, rolloliths, balanuliths, Vermetidae, Pleistocene, Baja California, Mexico.
Resumo: A existência de nódulos de organismos com esqueleto biomineralizado rolando livremente em
fundos marinhos nivelados, aqui genericamente designados como “rolótitos”, e já do conhecimento geral. Os rodólitos, concreções de algas rodofíceas coralinas, são os mais frequentes. Também são comuns os nódulos de corais, de cracas (balanulitos) e de serpulídeos (serpulitos). Neste trabalho são referidos e discutidos balanulitos e outras bioconcreções rolantes constituidas por associações muito biodiversificadas de organismos encrustantes e bioerosivos do Plistocénico Superior da Baja California Sur, México. Nestes rolólitos estão presentes vários taxa de corais, briozoários, serpulídeos, gastrópodes vermetídeos e de bivalves, bem como Balanus sp., um braquiópode e vários organismos bioerosivos. Este é, provavelmente, o primeiro registo fóssil de vermetídeos rolantes: vermetulitos.
Palavras-chave: Nódulos fósseis, rolólitos, balanulitos, Vermetidae, Plistocénico, Baja California, Mexico.
INTRODUCTION
Nodules composed of free-rolling organisms living on a level sea-bottom are well known from the literature (see overview by Moisette et al., 2010). A general
name for free-living, free-rolling, encrusting organisms of circumrotary growth has so far not been in common use, so we propose the word rolloliths here. Of these, rhodoliths consisting of red coralline algae occur frequently as fossils and the living representatives are so abundant that they are commercially mined from the sea-bottom off Brazil. Other rolling nodules formed by corals and bryozoa (called coralliths and bryoliths, respectively) are common; both groups known already from the Paleozoic. Ostreoliths consisting of free-rolling oysters and nodules formed mainly by serpulids are rarer. The latest addition under this topic are nodules of accreted barnacles described by Cadée (2007) as
balanuliths from the North Sea off The Netherlands. Cadée (2007) ended his article with a plea to paleontologists to look out for balanuliths in the fossil record. Nielsen (2009) answered that call by describing balanuliths from the Pliocene in northern Chile. These examples from Cadée and Nielsen both occur as encrusters around empty mollusk shells.
This study documents balanuliths and other rolling accretions with a highly diverse composition of encrusters and bioeroders from the upper Pleistocene of Baja California Sur, México (Fig. 1). These comparatively small nodules (1.5 to 6 cm in diameter) show several stages in development from initial encrustation on broken coral-fragments and small bivalve shells to fully encrusted nodules with a complex biota. The final step is the slow overgrowth by coralline red algae leading to the formation of rhodoliths.
GEOLOGICAL SETTING
The Upper Pleistocene Mulegé Formation at Playa La Palmita in Baja California Sur is described by Libby & Johnson (1997). Playa La Palmita is a coastal flat in the sheltered lee of the Punta Chivato promontory of Comondú Group andesite to the north and the related ridge of Mesa El Coloradito to the west (Fig. 2). The Mulegé Formation is up to 4 m thick in this area. It represents the sea-level highstand tagged as oxygen isotope 5e and constitutes a transgressive deposit against a former shore. Cintra-Buenrostro et al. (2002) described over 155 species of fossil bivalves and gastropods from these deposits. Thus, the study site was situated within a protected sandy bay and represents the accumulation of shallow subtidal deposits with very high productivity.
FIGURE 1. Geographic location of the study site at Playa La Palmita (Punta Chivato, Mulegé).
The described nodules come from a site 500m from the north end of the beach and about 150 m inland from Bahía Santa Inés (N 27º04’08.20” W111º58’30.70”). Near the south end of the beach, Libby & Johnson (1997) described an abandoned sea cliff with 2 m relief of dark andesite. The Mulegé Formation is about 3.5 m thick at the study site (Fig. 3). The section can be divided coarsely into two parts with about 2.0 m of mollusk-rich sands followed by finer sands rich in rhodolith debris and sand dollars. Towards the top of the mollusk-rich section, at 1.70 m, there occurs a firm- ground with the nodules resting above. The firm-ground level shows abundant fossilized thickets of the little encrusting coral Astrangia sp., small fingers of Porites panamensis, tiny oyster shells and Balanus sp. The size
of the encrusters and the friability of the sand on the firm-ground suggest that the hiatus in deposition was brief.
STUDIED MATERIAL
From a sample of 45 specimens, the nodules are small and vary from 1.5 to 6 cm in diameter with most 3-5 cm in diameter. They tend to be oblong, possibly echoing encrustation around a branch of coral. All, with the exception of three rhodoliths where the core is not visible, consist of remains of organisms of several taxa. The coral Astrangia sp. is the dominant taxon in 18 (40%), of the nodules. There are 10 balanuliths (22%), 6 rhodoliths (13%), 4 each with dominant bryozoa or serpulids (8%), one with the vermitid Serpulorbus sp. as dominant, and two that are evenly diverse with no dominant taxa.
DISCUSSION
The presence of Astrangia sp. is not necessarily
indicative of coralliths, because it is difficult to distinguish if their remains corresponded to a broken branch or a free living colony. This coral genus is not known to occur unattached in the modern sea. However, a worn specimen of A. poculata at the Smithsonian Institution from S. Carolina, USA (USNM 80195) appears to have encrusted a bivalve shell entirely, thus having corallites on all sides – “a roller“ (Stephen D. Chairns, pers. comm., 2010). Only one of the specimens (Fig. 3a) was certainly free living. It shows corallites growing on all sides, even facing the cavity within. Dissecting the other nodules might reveal their true nature. Fossils of the coral Porites panamensis are common in situ on the firm ground and living Porites have been found as coralliths in rhodolith beds just offshore the study site (Reyes-Bonilla et al., 1997).
Surprisingly, it is rare in the nodules occurring only as a few, small broken fragments.
Coralline red algae are present on all the nodules in the sample collection ranging from the smallest patch to heavy encrustations that leave no original encrusters visible (Fig. 3b). Clearly, the non-algal components constitute the core of well developed rhodoliths in the area. The fine sands with echinoderms above the
mollusk-rich sands that enclose the firm-ground consist of abundant rhodolith debris indicating the presence of rhodolith banks just offshore.
FIGURE 2: Photo taken from airplane showing the study site. The earlier shoreline follows approximately the road to the west and it is below the airstrip to the north.
The balanuliths display a clear succession from a few individuals encrusting on a fragment of Porites, to a scallop valve encrusted on both sides, to nodules that are spherical and fully encrusted around a core (Fig. 3c). The balanuliths often are infested with serpulids, something that might reflect the relative fast growth-rate of both types of organisms (Cadée, 2007; Bosence, 1979). Cadée (2007) noted that the balanuliths he studied were generated in a matter of months.
FIGURE 3. The Playa La Palmita Pleistocene sediments. Note the firm-ground on the upper part of the sequence.
Several kinds of bryozoa are found encrusting on the nodules, but only one taxon of cheilostomate bryozoans occurs in clumps capable of dominating a nodule. Modern bryoliths also are found in rhodolith beds outside Punta Chivato (James et al. 2006), as are large Piocene bryoliths from the nearby Loreto Basin (Cuffey & Johnson, 1997). However, the fossil colonies described herein are mostly irregular in shape and
would not have constituted a bryolith alone. Combined with encrusting corals and barnacles, they undoubtedly rolled loose on the sea bottom (Fig. 3d).
The serpulids dominating the free-rolling biota are always associated with other organisms. Aguierre et al. (1993) reports on serpulid nodules from the Upper Pliocene in Spain. Most are encrusted by red algae. It is not quite clear if these nodules represent active growth by serpulids. In our examples, balanids are entwined by serpulids on all sides and they undoubtedly lived together (Fig. 3e).
Loose aggregations of vermetids living free on the sea bottom, Vermetus and Dendropoma, are described from the modern East Atlantic and Mediterranean (Calvo & Templado, 2004). Specimens of Dendropoma
were cemented by red algae. The nodule with vermetids in this study are abraded and encrusted with red algae so it is hard to know for sure if the gastropods were alive during the time when the nodule rolled free on the sea- bottom. Possibly, this is the first example of free-rolling vermetids from the fossil record (Fig. 3f).
CONCLUSION
Overall, the nodules show a very high diversity. Represented are several genera and species of corals, bryozoa, serpulids, vermetids and bivalves in addition to
Balanus sp., a brachiopod and a wealth of bioeroders.
Thus, the nodules provide the opportunity to investigate community development constrained to a very limited space. Sectioning also will reveal how common the coralliths and possibly vermetuliths are and the succession from a bioclast, acting as hard substrate, to a free-rolling nodule composed of living organisms.
ACKNOWLEGMENTS
Johnson’s participation was supported by a travel grant from the Class of 1945 Faculty World Fellowship during sabbatical leave. A. Santos received financial support from the Ministry of Science and Technology of Span in the form of a Juan de la Cierva contract (Refª JCI-2008-2431). Financial support was also given by the Junta de Andalucía (Spanish government) to the Research Group RNM316 (Tectonics and Palaeonto- logy).
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FIGURE 3. Different types of rolloliths: A. Corallith; B. Nodule with partial red algae cover; C. Balanulith; D. Nodule with bryozoan, Astrangia sp.
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