Hermínio Ismael de Araújo-Júnior a,*
a
Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 274, 21.941-916, Cidade Universitária, Ilha do Fundão,
Rio de Janeiro, RJ, Brazil.
*Corresponding author, Tel.: +55 21 80153603, herminio.ismael@yahoo.com.br
Kleberson de Oliveira Porpino b
b
Departamento de Ciências Biológicas, Universidade do Estado do Rio Grande do Norte, Av. Professor Antônio Campos, s/n, 59.610-090, Costa e Silva, Mossoró, RN, Brazil.
kleporpino@yahoo.com.br
Celso Lira Ximenes c
c
Museu de Pré-história de Itapipoca, Av. Anastácio Braga, 349, 62.500-000, Centro, Itapipoca, CE, Brazil. clx.ximenes@gmail.com
Lílian Paglarelli Bergqvist a
a
Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 274, 21.941-916, Cidade Universitária, Ilha do Fundão,
Abstract
Fossil vertebrate assemblages of natural tanks occur only at the Northeastern Brazil and register a diversified fauna of Late Pleistocene Age in the South America. These deposits are very well known in relation to taxonomy, however, they still are unknown with regard to taphonomy. Recently, a natural tank, known as tank of the Jirau, was excavated at the Itapipoca municipality (Ceará State, Brazil) and taphonomic, taxonomic, sedimentological and stratigraphic data were collected. This work presents a detailed taphonomic analysis of the fossils from the Tank of the Jirau, aiming to understand the processes that influence in the deposition and preservation of vertebrates in natural tanks and to refine paleoecological and paleoenvironmental informations known about Late Pleistocene fauna from the Northeastern Brazil. Moreover, a comparative analysis with other tanks was developed to recognize taphonomic patterns for this singular type of deposit. The study revealed a predominance of megamammal Eremotherium laurillardi in the deposit, which is related to abundance of this species in the biocoenosis and to bone resistance. Ante-mortem bone alterations, also observed in this species, are linked to overload of the vertebral column and senility. Scarcity of “non- mammal” vertebrates is related to small amount of these individuals in the biocoenosis. Time- averaging is pronounced and it affects ontogenetic spectrum of the accumulation. Phenomenon of time-averaging is recognized based on weathering, petrographic and color patterns. Analyses of weathering stages, tooth and trample marks indicate wide exposition of the thanatocoenosis. Abrasion and petrographic patterns allow inferring reworking in the assemblage. Permineralization and substitution are fossilization processes observed. Biostratinomic, sedimentological and stratigraphic features indicate floods as depositional agents of clasts and bioclasts in the tank. Analyzed features suggest an arid or semiarid climate during Late Pleistocene in the Northeastern Brazil. Comparative analysis shows similarity between tank deposits known so far in the Brazil.
Keywords: Taphonomy, Natural tank, Pleistocene vertebrate, Itapipoca, Northeastern Brazil.
1 Introduction
Taphonomic analyses of Brazilian vertebrate fossil accumulations are scarce and most studies were carried out on Triassic taphocoenoses (Holz and Soares, 1995; Holz and Souto- Ribeiro, 2000; Bertoni-Machado and Holz, 2006; Bertoni-Machado, 2008; Bertoni-Machado et al., 2008). The taphonomy of fossil concentrations of Cenozoic vertebrates is still barely understood, despite some analyses on Paleocene (Bergqvist et al., 2011) and Pleistocene deposits (Bergqvist et al., 1997; Santos et al., 2002a; Auler et al., 2006; Alves et al., 2007; Dantas and Tasso, 2007; Araújo-Júnior and Porpino, 2011). Most of these taphonomic studies on Pleistocene vertebrates were based on superficial and poorly informative evidences.
Late Pleistocene outcrops are more common in the Northeastern Brazil, where the most abundant fossiliferous deposits are carstic (caves and fissures), alluvial, lakes and natural tanks (Cabral-de-Carvalho et al., 1966; Silva, 2001; Santos et al., 2002b; Bergqvist and Almeida, 2004; Porpino et al., 2004; Dantas et al., 2005; Silva et al., 2006; Ximenes, 2009; Dantas, 2009; Ribeiro and Carvalho, 2009; Araújo-Júnior and Porpino, 2011). Those assemblages are characterized by a well-known and diversified vertebrate fauna from late Pleistocene, which include mammals, reptiles, avians and amphibians (Paula-Couto, 1980; Cartelle, 1999; Bergqvist and Almeida, 2004; Dantas et al., 2005; Silva et al., 2006). Mammalian remains are the most common fossils in all aforementioned kinds of deposits and the species Eremotherium laurillardi (Megatheriidae; Pilosa), Stegomastodon waringi (Gomphotheriidae; Proboscidea), Toxodon platensis (Toxodontidae; Notoungulata),
Palaeolama major (Camelidae; Artiodactyla), Panochthus greslebini (Glyptodontidae;
(Macraucheniidae; Litopterna), and Smilodon populator (Felidae; Carnivora) are the most frequent taxa (Bergqvist and Almeida, 2004). Some of these species are likely endemics to the Brazilian Intertropical Region (sensu Cartelle, 1999).
The taphonomy of natural tanks is the most elusive of all the Pleistocene vertebrate deposits from Brazil (see Porpino and Santos, 2002). Nonetheless, during last decade, new tanks have been carefully excavated in Northeastern Brazil (Alves et al., 2007; Ribeiro & Carvalho, 2009; Ximenes, 2009), allowing not only the recovery of fossil vertebrates, but the preliminary decoding of taphonomic information through field and laboratory investigations.
In this work, a taphonomic analysis of fossil accumulation of Pleistocene vertebrates from the Tank of Jirau, Itapipoca, Ceará State, Northeastern Brazil, is carried out. The aim of this work is to improve the knowledge on biostratinomic and fossildiagenetic process that acted during deposition and preservation of bones in natural tanks and shed light on the vertebrate paleoecology of the Late Pleistocene of northeastern Brazil. Additionally, we compare ours results with others tanks in order to highlight potential taphonomic patterns among these deposits.
2 Location, Geology and Stratigraphy
The natural tank of the Jirau (03º21’23.1” S 39º42’20.2” W; Fig. 1A) is inserted in the Jirau Paleontological Site, located at 35 Km northwest of Itapipoca city, north of Ceará State (Fig. 1B).
Fig. 1. General view and location of the Tank of the Jirau; A. view of the tank; B. location map of Itapipoca, Ceará State, Northeastern Brazil.
Tanks are natural depressions formed by chemical and physical weathering on basement rock outcrops that seasonally accumulate pluvial waters producing small lag deposits (Santos, 2001). According to Ximenes (2003), the tanks from Itapipoca show varied sizes and dominantly ellipsoid outline, though circular or irregular outlines are also observed. In several points of Itapipoca, they are associated to small inselbergs. Those depressions were filled by clastic sediments and bioclasts. The bioclasts are compound by late Pleistocene vertebrate remains chiefly large mammals collectively referred as “Pleistocene Megafauna” (Ximenes, 2003). The clastic sediments probably were not deposited by fluvial streams, but generated by the weathering of the basement rocks and transported by floods and wind action during raining and dry periods respectively (Santos, 2001).
The basement rock outcrop where the Tank of the Jirau lies is a neoprotherozoic granodiorite belonging to the Borborema Province geomorphological unity (Nascimento, 2006). This unity is an extensive region of Pre-Cambrian rock outcrops that encompasses an area of 405.000 Km² in the Northeastern Brazil. The tank has elliptical outline, with 22 m along its main axis, 3.5 m of width, 4 m of depth and 308.000 liters of minimum volume. It is classified by Ximenes (2003) as a semi-open tank (i.e., it has open edges).
The deposit that fill the Tank of Jirau is divided in three sedimentary layers (Fig. 2) from bottom to top: (i) a basal one (1.5 m thick) formed by quartz sands with rock fragments likely resulting from the weathering of basement rocks which comprise the tank walls; (ii) an intermediary layer (about 1 m thick), characterized by debris flow sedimentation, consisting in conglomerates supported by feldspatic clayey-sandy matrix, with fossils, quartz pebbles and rock fragments from tank walls; (iii) and an upper layer composed by clayey and silt sediments, with large amount of organic remains. The upper layer shows thickness variation along its bedding plane, being about 1 m thick in proximal part of the tank (open edge) and 1.5 m thick in the distal part.
Fig. 2. Stratigraphic section of the deposit of Jirau.
The presence of quartz sands associated with rock fragments and the absence of non- resistant minerals (e.g. feldspar) in the basal layer of the tank suggest reworking in the initial stage of deposition of clasts within tank. Probably, pluvial waters reworked the small amount of sediments deposited in the tank depression, during its initial expansion. Continuous sediment revolving eliminates the more instable minerals (Suguio, 2003). The fossiliferous layer shows poor granulometric sorting. This suggests that a high-energy agent acted in the sediment and thanatocoenosis deposition. However, the abundance of feldspar indicates a small reworking rate in comparison to the lower layer (Suguio, 2003). Rounded quartz pebbles were not generated from tank walls, but, probably from more distant areas. Decreasing granulometry toward the upper layer, which is composed by clayey and silt sediments, suggests decreasing influence of trative processes and increase of deposition by decantation (Suguio, 2003). Changes in the depositional process commonly result from changes in the climate or geomorphology (Bishop, 1980; Suguio, 2003). Cavalcante (2006) do not identified geomorphologic changes in the Ceará State area during Quaternary period, thus we attribute this modification to climate changes.
3 Material and methods
The tank of Jirau was excavated and its fossils were prepared from 2003 to 2008. The studied fossils are housed at the paleontological collection of the Museu de Pré-história de Itapipoca (MUPHI). A stratigraphic section of the deposit of Jirau was produced based on a trench, which allowed general view of the layers present within the tank and a partial view of the orientation of skeletal elements in the fossiliferous layer. Taxonomic identification was
based on comparable material of the paleomammalogy collection from the Pontifícia Universidade Católica de Minas Gerais (PUC-MG) and with pertinent literature.
The dataset used in the present study comprises 1.405 skeletal specimens collected in the tank of Jirau. Some types of skeletal elements, such as xenarthran osteoderms, horns and teeth, were excluded of our analysis because they are elements overrepresented in the skeleton of vertebrates and therefore they can introduce analytical bias to results.
The collecting of taphonomic data followed methods suggested by Dodson et al. (1980), Hill (1980), Shipman (1981), Badgley (1986), Behrensmeyer (1991), Lyman (1994, 2008), Holz and Simões (2002), Rogers (1994), Eberth et al. (2007a) and Simões et al. (2010a, 2010b), which have been widely employed in taphonomic analyses (e.g. Turnbull and Martill, 1988; Rogers, 1990; Holz and Barberena, 1994; Varricchio, 1995; Cassiliano, 1997; Coombs and Coombs-Jr., 1997; Ryan et al., 2001; Myers and Storrs, 2007; Britt et al., 2009; Fiorillo et al., 2010; Gangloff and Fiorillo, 2010; Araújo-Júnior and Porpino, 2011; Bergqvist et al., 2011). The following macroscopical features were considered: (A) taxonomic composition and ontogenetic stages; (B) articulation and fragmentation; (C) long bone orientation; (D) bone representation; (E) hydraulic equivalence; (F) breakage; (G) weathering; (H) abrasion; (I) trampling; (J) tooth marks; (K) invertebrate modifications; (L) rooting; and (M) color patterns and petrographic observations. For each registered color pattern, we made petrographic sections across transversal cuts in long bones in order to diagnose preservational stages of bone microstructure and fossilization stages and processes. In addition, ante-mortem bone alterations were identified and interpreted. Finally, a taphogram (Tab. 3) was constructed to show hypothetical sequence and relative duration of taphonomic process acting on the fossil accumulation from Jirau.
For counting of specimens, we used Number of Identifiable Skeletal Parts (NISP). For each taxon, Minimum Number of Individuals (MNI) was calculated following the method
described in Badgley (1986) and Lyman (1994, 2008). Ontogenetic stages (young or subadult and adult) of mammals were determinate based on interpretation of epiphysis-diaphysis fusion grade (to long bones) and centrum-disc fusion grade (to vertebrae) for mammals. We assume that totally fused elements pertain to adult individuals and incomplete fusion to subadults. Similarly, ontogenetic stages of non-mammalian vertebrates were defined based on the observation of fusion grade of sutures.
The terms “biocoenosis”, “thanatocoenosis” and “taphocoenosis” were used in this work based on definitions established by Lyman (1994): (A) biocoenosis, life assemblage; (B) thanatocoenosis, death assemblage, derived from the biocoenosis and subsequently modified by biostratinomic process; and (C) taphocoenosis, buried and preserved assemblage. Terms “megamammals”, “large mammals”, “midsized mammals” and small mammals” follow Araújo-Júnior and Porpino (2011): (A) megamammals >1000 Kg; (B) large mammals, between 100 and 1000 Kg; (C) midsized mammals, between 10 and 100 Kg; and (D) small mammals, <10 Kg.
Fossils from Jirau are here attributed to late Pleistocene based on previous datings of specimens from other tanks from the Northeastern Brazil which yielded ages compatible with this interval (see Kinoshita et al., 2005, 2008; Oliveira et al., 2009).
4 Taxonomic composition and ontogenetic stages
From the 1.405 specimens considered in this analysis, 1.303 were anatomically and taxonomically identified to species level. Some bones of more difficult identification, due to fragmentation or shortage of material, were identified to genus (19), sub-family (4), family (45) order (1) or class (33). The assemblage of Jirau is dominated by megamammals, but large and medium-sized mammals are also present in significant amounts. Crocodilians and turtles,
semi-aquatic vertebrates, are also part of the taphocoenosis. Faunal list, ontogenetic stages, NISP and MNI values are presented in Table 1.
Table 1. Taxa found in the Jirau vertebrate assemblage with NISP, MNI and ontogenetic stages.
4.1 Mammals
NISP and MNI for mammals are 1.399 and 45, respectively. The fossil assemblage of Jirau is multitaxic and monodominant (sensu Eberth et al., 2007b). Megamammal
Eremotherium laurillardi (Megatheriidae; Fig. 3B and E) is the dominant species in the
accumulation (NISP = 1.143; MNI = 19), representing 81.35% of the total NISP and 42.2% of total MNI for mammals. Palaeolama major (Artiodactyla, Camelidae) a large mammal, is the second taxa with highest MNI value (= 5), but with NISP (= 47) less than Tayassu pecari (= 67), a midsized mammal (Artiodactyla, Tayassuidae). Taxa with smallest NISP are Equus (Amerhippus) neogaeus (large mammal; Perissodactyla, Equidae), Catonyx cuvieri (megamammal; Pilosa, Mylodontidae) and Holmesina paulacoutoi (megamammal; Cingulata, Pampatheriidae), all with NISP equal 1. Mazama sp. a small sized mammal, has NISP equal 5, overcoming some large mammals and megamammals recorded in Jirau (e.g. Stegomastodon
waringi – Gomphotheriidae, Proboscidea; Hippidion principale – Equidae, Perissodactyla).
Fig. 3. Mammal skeletal elements from the Jirau; A. incomplete tibia of Cervidae indet. (Artiodactyla); B. incomplete mandible of E. laurillardi (Megatheriidae, Pilosa); C. lumbar vertebra of Smilodon populator (Felidae, Carnivora); D. distal end of humerus of S.
Other mammal taxa are also represented in the fossiliferous deposit of Jirau besides those reported above: Tolypeutes tricinctus (small mammal; Cingulata, Dasypodidae),
Pachyarmatherium brasiliense (midsized mammal; Cingulata incertae sedis; see Porpino et
al., 2009), Ocnotherium giganteum (megamammal; Pilosa, Mylodontidae), Panochthus
greslebini and Glyptotherium sp. (megamammals; Cingulata, Glyptodontidae). However,
those animals were excluded of the taphonomic analysis because they are represented only by teeth and osteoderms (see above).
Among the tank deposits known so far, the tank of Jirau shows the greatest taxonomic diversity. For Peterson (1977) and Fürsich and Aberhan (1990), taxonomic diversity in fossil accumulations increases when time-averaging increases. Moreover, Behrensmeyer (1991) argued that vertebrate assemblages with high stage of fragmentation and high species richness reflect accumulations with high levels of temporal and spacial mixing (see also Bown and Kraus, 1981; Badgley, 1986; Wood et al., 1988; Schröder-Adams et al., 2001 for similar conclusion).
4.2 “Non-mammals”
Three “non-mammalian” taxa are recorded. Among those, Caimaninae (Alligatoridae, Crocodylia) is the taxa with greatest number of bones (NISP= 4; MNI= 1), which consist in three fragment of vertebral centrum and one small fragment of cranium. Only a single fragment of plastron was found and attributed to Testudines indet. Besides fossil reptiles, one fragment of long bone was attributed to large avian. In natural tanks, only Rhea sp. (Rheidae, Reiformes) was previously recorded (João Cativo tanks, Itapipoca; Paula-Couto, 1980). In the present case, the fragmentation of the avian material precluded a more detailed description; therefore, it was identified only as Aves incertae sedis.
Probably, crocodiles and turtles lived in nearby rivers and were transported to tanks during overflow events. We cannot dismiss the hypothesis that, due to their small size, turtles also could use the tank as habitat. Currently, it is possible to observe these animals living in small lakes formed inside tank depressions at Ceará State.
4.3 Ontogenetic stages
Among the skeletal elements identified, 52 specimens are attributable to sub-adults, and 1.353 to adults. Among the sub-adult specimens, 43 belong to E. laurillardi, four to S.
waringi, three to T. platensis, and two to T. pecari. With respect to the MNI, only seven
individuals were classified in the sub-adult stage and 41 to adult. E. laurillardi is represented by four sub-adult individuals while the others aforementioned species are represented by one individual each. All non-mammalian bones were assigned to adult because their sutures are closed.
The dominance of adults in relation to sub-adults in the accumulation of Jirau is compatible with a catastrophic death scenario for the biocoenosis existing around this deposit during Late Pleistocene (Voorhies, 1969; Shipman, 1981; Holz and Simões, 2002). However, senility signs observed in some fossils (see item 6) suggest an attritional assemblage. The likely occurrence of time-averaging in this accumulation (see above), prevents more accurate conclusions, as this phenomenon affects dramatically the ontogenetic frequency distribution that would be expected for fossiliferous associations generated by catastrophic death, due to the collapse of several generations in a single accumulation (Behrensmeyer, 1982; Bertoni- Machado, 2008). Agenbroad (1984) found that some fossil-bearing assemblages dominated by individuals and species with high number of adults in relation to sub-adults are more indicative of temporal-mixed accumulations than assemblages generated by catastrophic death. Peterson (1977) and Fürsich and Aberhan (1990) concur with Agenbroad (1984) and
point to the fact that taxonomic composition and diversity in skeletal accumulations tend to increase as the degree of time-averaging increases. Thus, it is possible that the ontogenetic pattern found for the tank of Jirau have been the result of time-averaging.
5 Taphonomy
5.1 Articulation and physical integrity
All fossils found in the fossil accumulation are disarticulated. According to Holz and Simões (2002), disarticulated elements would indicate extensive exposure time before the burial of carcasses. In addition, factors such as transport, trampling and scavenging can hasten disarticulation (Toots, 1965; Voorhies, 1969; Hill, 1979; Shipman, 1981; Weigelt, 1989; Behrensmeyer and Hook, 1992; Holz and Simões, 2002; Pawłowska, 2010).
With respect to physical integrity, 82% of the analyzed specimens correspond to bone fragments (<50% of the bone), but complete (100% of the bone preserved) and partial (between 50 and 95 % of the bone) elements also occur representing 4% and 14% of skeletal elements, respectively (Fig. 4A). Among the specimens found (fragments, partial and complete) those with area up to 15 x 15 cm (Fig. 4B) are better represented. Specimens with greater length between 5 and 10 cm are the most abundant (Fig. 4C). According to Shipman (1981) and Holz and Simões (2002), bone fragments can be generated by various processes, ranging from physical (e.g. transport, reworking and lithostatic pressure) to biological (e.g. trampling and scavenging). This situation renders difficult to attribute the high degree of fragmentation observed to one of the processes mentioned. In the case of the studied accumulation, other features (e.g. fossildiagenetic patterns, tooth marks, trample marks) might be more diagnostic for the processes operating in the taphonomic history of the Jirau assemblage.
Fig. 4. Physical integrity and size patterns observed in the Jirau fossil accumulation; A. bone integrity; B. measures of bones (greatest length versus greatest width); C. bioclast length frequency.
5.2 Long bone orientation
The pattern of long bone orientation in fossil vertebrate accumulations can be indicative of energy and direction of the stream responsible for the deposition of bones in sedimentary strata (Shipman, 1981; Eberth et al., 2007a). During the collection of the fossils here analyzed, preferential orientation of the long bones was not mapped. However, in the open trench in the tank (see Material and Methods), we could observe that the elements still preserved in the fossiliferous layer do not show preferred orientation, which may indicate that the material was deposited by a high energy agent (Holz and Simões, 2002).
Bergqvist (1993) and Bergqvist et al. (1997) hypothesized that seasonal floods were the main agent of deposition of vertebrate remains in natural tanks. This is supported by the absence of orientation, associated to the presence of skeletal elements with low dispersal potential embedded in a sedimentary layer exhibiting poorly selected clasts (see Location, Geology and Stratigraphy).
5.3 Bone representation
The most abundant skeletal elements in the Jirau fossil assemblage are ribs (37.39%), vertebrae (17.72%) and humeri (15.30%).Turtle shells (0.07%), calcaneum (0.21%), scapulae (0.28%), clavicles (0.8%) and astragalus (0.49%) are poorly represented. Ribs and vertebrae are the most abundant elements in the skeleton of vertebrates (Moore, 1994) and probably this should have been the reason for the large amount of these elements in Jirau. By the other hand, the abundance of humerus is unusual, because in these deposits, humeri are often found
in small amounts. This pattern can be related to: (i) the greater resistance of this type of bone to destruction, (ii) or biogenic sorting during biostratinomy. Most humeri (79.53%) belongs to
E. laurillardi, a megamammal which had very dense humerus and femur that might had
hindered biological transport of these elements. Thus, it is likely that the abundance of humerus is related to the resistance of this type of bone to destructive processes. The small amount of turtle shells can be linked to the rarity of these animals in the vicinity of the tank,