First record of Toxodontia for the Itaboraí Basin (Paleogene, Rio de Janeiro, Brazil) and an assessment on the evolution of isotemnid notoungulates (Pan-Perissodactyla, Mammalia)

doi:10.21203/rs.3.rs-4970056/v1

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First record of Toxodontia for the Itaboraí Basin (Paleogene, Rio de Janeiro, Brazil) and an assessment on the evolution of isotemnid notoungulates (Pan-Perissodactyla, Mammalia)

https://doi.org/10.21203/rs.3.rs-4970056/v1

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The traditional taxonomy of notoungulates divides this order in two main groups, Typotheria and Toxodontia. Within the latter, isotemnids encompass some of the earliest and more generalized species, radiating mostly during the Eocene and reaching also the Oligocene. Although somewhat conservative, the history of this group is marked by some morphological changes, and isotemnids presented a diversity of body sizes during the Paleogene. In Brazil, the Itaboraí Basin have yielded several paleontological records mainly represented by mammals. Its chronological context, in the first half of the Paleogene, places this basin as an important source of information on the evolution of South American vertebrates (particularly mammals). In this contribution we describe a new isotemnid species, based on a partial skull and isolated maxillary and dental material, recognized for levels of the Itaboraí Formation cropping out in São José de Itaboraí, Rio de Janeiro, Brazil. A phylogenetic analysis was performed in order to evaluate the relationships of this new taxon, which results indicate (in part) that Isotemnidae represent a natural group, with some of the characters being related to the development of canines. Considering the evolutionary history of the family in this context, we recognized a series of radiation pulses during the Paleogene, and a possible correlation with Eocene climatic events that apparently affected the evolutionary rates of these notoungulates. This new record enhances our knowledge on the fossil diversity within the context of the Itaboraí Basin, whose vertebrates stand as important exponents of the early evolution of the South American biota.

Phylogeny

Dentition

Canines

Paleoclimate

Fossil diversity

The Order Notoungulata was established by Roth in 1903, who recognized the monophyly of this group based on a unique configuration of the temporal region of the skull (Billet 2011). With more than 150 genera and occupying the most varied ecological niches, notoungulates are present from the Paleocene to early Holocene (Billet 2010; Braunn and Ribeiro 2017; García-López et al. 2019; Croft et al. 2020). The traditional taxonomy divides notoungulates in two suborders: Typotheria, with six families (although some of them regarded as paraphyletic assemblages: Oldfieldthomasiidae, Archaeopithecidae, Interatheriidae, Archaeohyracidae, Mesotheriidae, Hegetotheriidae) and Toxodontia, represented by five families (Isotemnidae, Homalodotheriidae, Leontiniidae, Notohippidae, and Toxodontidae). Additionally, two early-diverging families, Henricosborniidae and Notostylopidae, are widely recognized (Billet 2011; Croft et al. 2020). Nevertheless, successive revisions after the advent of cladistic methodology in paleontological surveys have challenged this arrangement, and several of these clades have been regarded as non-natural groupings (particularly Henricosborniidae, Oldfieldthomasiidae, Archaeohyracidae, Notohippidae, and Isotemnidae; see Cifelli 1993; Shockey 1997; Billet et al. 2009; Billet 2011).

Within Toxodontia, Isotemnidae was originally erected by Ameghino (1897) and later defined by Simpson (1967) as presenting, among other characters, a complete dentition with brachydont teeth and large canines. Due to the allegedly primitive nature of these characters, this family has been considered as the ancestral stock to the rest of Toxodontia (Simpson 1967; Vucetich and Bond 1982). Later phylogenetic studies have also supported the basal position of isotemnid genera regarding the remaining taxa of the suborder, although recovering the clade as a paraphyletic assemblage (Cifelli 1993; Billet 2011).

The Isotemnidae comprises eight genera (Simpson 1967; Vicetich and Bond 1982; Tejedor et al. 2009; Billet 2011; Babot et al. 2017; Carrillo and Asher 2017; Lorente 2019; Vera and Reguero 2023), most of them defined over specimens recovered from the Argentine Patagonia: Anisotemnus Ameghino, 1902, Distylophorus Ameghino, 1902, Pampatemnus Vucetich & Bond, 1982, Rhyphodon Roth, 1899, Thomashuxleya Ameghino, 1901, Periphragnis Roth 1899, Pleurostylodon Ameghino 1897, and Isotemnus Ameghino 1897. The life span of this family ranged from the Paleocene-Eocene limit within the Itaboraian South American Land Mammal Age (SALMA) to the Oligocene, within the Tinguirican SALMA (Simpson 1967; Vucetich and Bond 1982; Flynn et al. 2003; Shockey and Flynn 2007; Carrillo and Asher 2017). Although somewhat conservative, the history of the group is marked by some morphological changes, and isotemnids presented a diversity of body sizes during the Paleogene, going from relatively small to large forms (Croft et al. 2020; Solórzano and Núñez-Flores 2021).

In Brazil, the knowledge about native ungulates has been growing over the years. Currently, three sedimentary basins inserted in the context of the Continental Rift of Southeastern Brazil (Riccomini et al. 1996) constitute the bulk of the record of these groups of South America. Among these, notoungulates of Paleogene age can be found (from south to north) in the Curitiba Basin, Paraná state (with two indeterminate taxa; Sedor et al. 2017) and the Taubaté Basin, São Paulo state (Taubatherium Soria & Alvarenga, 1989). However, notougulate mammals are particularly represented in the Itaboraí Basin, Rio de Janeiro state (Bergqvist et al. 2005), this being the oldest basin formed in this tectonic context with the record of at least four species, some of them well-known (Castro et al. 2021).

The Itaboraí Basin have yielded several paleontological records represented by mammals, reptiles, amphibians, birds, mollusks, pollen, and plants, and represents one of the oldest Cenozoic sedimentary deposits in South America (Bergqvist et al. 2005; Woodburne et al. 2014; Zanesco et al. 2019; Rangel et al. 2023). Hence, its chronological context, positioned in the first half of the Paleogene, places this basin as an important source of information to understand the evolution of the South American vertebrate fauna (particularly mammals) after the K/Pg extinction event. Indeed, the relevance of these deposits led to consider this basin as part of an environmental preservation area (Parque Natural Municipal Paleontológico de São José de Itaboraí) and the city of Itaboraí was declared as City of Paleontological Relevance by the government of the state of Rio de Janeiro (Law 10136, October 16th, 2023).

With the exception of Pyrotheria, the remaining traditionally recognized South American native ungulate (SANU) groups are present in the Itaboraí Basin (Bergqvist et al. 2005). Within notoungulates, early diverging clades (e.g., “Henricosborniidae”) are represented by the species Nanolophodon tutuca Castro, García-López & Bergqvist, 2021, and Typotheria (family “Oldfieldthomasiidae”) are represented by the species Colbertia magellanica (Price & Paula-Couto, 1950) and Itaboraitherium atavum (Paula-Couto, 1954). Additionally, this record is hitherto completed by the species Camargomendesia pristina Paula-Couto, 1978. Although referred to the “henricosborniid” genus Othnielmarshia by Paula-Couto (1979), recent studies have underlined the lack of morphological basis for this assignment, recommending the use of the original name (Castro et al. 2021; Vera and Mones 2023). Currently, the affinities of C. pristina remain uncertain.

Even though much is known about the paleontology of this sedimentary basin, the diversity of notoungulates is still low compared to other Paleogene South American localities (e.g., northwestern Argentina and Patagonia) hence, recent efforts have been initiated in order to review known collections and increase the number of specimens.

In this work, we present a new species of Isotemnidae (a clade previously unknown for any of the Paleogene sedimentary basins of Brazil) recovered from the Itaboraí Basin and based on several specimens including cranial remains, maxillary fragments with postcanine dentition, and isolated teeth. We also discuss the relevance of this record and present a phylogenetic study in order to test the phyletic nature of these notoungulates.

Geographical and geological settings

The Itaboraí Basin is located near the city of São José of the Itaboraí, Rio de Janeiro state, about 30 km East from the city of Rio de Janeiro (Fig. 1). It is a limestone sedimentary basin, located in the geological context of the Continental Rift of Southeastern Brazil, a result of late processes related to the breakup of Gondwana and the separation between Brazil and Africa (Riccomini et al., 1996; Sant’Anna et al., 2004). The same geologic process also formed the Resende, Taubaté, São Paulo, and Curitiba basins (Riccomini et al. 1996).

Being of elliptical shape, this small carbonate basin has a major length of 1400 m in NE-SW direction, 500 m in N-S direction, and 100 m deep (Bergqvist et al. 2005) (Fig. 2 − 1). It is delimited to the south by the São José Fault and intersected by a northwest-oriented Transversal Fault (Rodrigues-Francisco and Cunha 1978; Ferrari 2001; Bergqvist et al. 2005).

The diverse facies of continental carbonate rocks that fill this basin comprise banded and travertine (Fig. 2-3b), gray limestone (Fig. 2-3a, c, d, e), pisoid beds (Leinz 1938; Tibana et al. 1984; Medeiros and Bergqvist 1999; Sant’Anna et al. 2004). The travertine deposits were formed by physicochemical and microbial processes from hydrothermal sources rich in calcium carbonate (CaCo₃) (Fig. 2-3b) (Leinz 1938; Sant’Anna et al. 2004; Pereira et al. 2017; Adler et al. 2021). The fossiliferous calcrete facies that occur interbedded with travertine was formed from alluvial fan deposits containing detrital grains of silt to pebble (quartz, feldspar, mica, and lithic fragments), well-preserved terrestrial gastropods, and the clay minerals kaolinite and smectite (Sant’Anna et al. 2004) (Fig. 2–4).

A karstification process, which formed dissolution fissures, cut vertically the sedimentary facies of this basin, creating a zone of fossil accumulation (Leinz 1938; Tibana et al. 1984; Sant’Anna et al. 2004; Bergqvist et al. 2005). According to Bergqvist et al. (2005) most of the fossils in this basin were collected in sediments that filled the dissolution fissure (Fig. <link rid="fig2">2</link>–2).

The chronologic position of the Itaboraí Basin, and hence of the Itaboraian SALMA based on this deposits, is controversial (Paleocene-Eocene or only early Eocene). Bergqvist et al. (2005) proposed a Paleocene age for the Itaboraian, while Woodburne et al. (2014) and Rangel et al. (2023) indicated an early Eocene age, and del Papa et al. (2022) a late Paleocene to early Eocene age for this SALMA. However, there is a consensus that it is positioned at the early Paleogene (Bergqvist et al. 2005; Woodburne et al. 2014; del Papa et al. 2022). Also, it should be noted that recently, an analysis carried out by Mota et al. (2015) employing the ⁴⁰Ar/³⁹Ar radiometric method in ankaramite rocks located in the north side of the basin, above the carbonate sedimentary sequence, determined an approximate age of 54,89 ± 1,40 Ma. Nevertheless, it should be noted that this datum establishes a probable upper limit for the basin, as it is not possible to assure that the flow is posterior to all fissures (it is though definitely younger than the deepest fissures). This ankaramite deposit is represented by a tabular dike of subvertical direction, about 10 m thick and 150 m long, which cuts the basement and the limestone sequence (Klein and Valença 1984).

Institutional abbreviations: MACN A, Museo de Ciencias Naturales “Bernardino Rivadavia”, Colección Ameghino, Ciudad Autónoma de Buenos Aires, Argentina, MCT-M, Museu de Ciências da Terra, Companhia de Pesquisa de Recursos Minerais, Fossil Mammal Collection, Rio de Janeiro, Brazil, MLP, Colección Paleontología de Vertebrados, Museo de La Plata, La Plata, Argentina. MCN, Museu de Ciências Naturais/SEMA- Porto Alegre, Rio Grande do Sul, Brazil. AMNH, American Museum of Natural History, Central Park West, New York, United States of America.

The specimens described here were collected in the Itaboraí Basin during the extraction process of limestone rocks in past decades (1933 to 1984). The specimen MCT.M.965 went through a mechanical preparation process with a pneumatic pen to remove the carbonate matrix attached. The right canine was trapped within this matrix by taphonomic processes, and after the mechanical preparation it was placed in its correct locus. Teeth images were captured under a stereomicroscope Zeiss Stemi 2000-C with a AxioCam ERC 5S camera attached and also with Can Nikon D7500 (AF-S Micro NIKKOR 40mm 1:2.8 G). The pictures were digitally treated in order to enhance the visualization of different structures. The measurements were made with ImageJ. Body mass was estimated based on the second upper molar length (SUML) using the regression formula of Solórzano and Nuñez-Flores (2021).

The phylogenetic analysis was performed based on a matrix of 87 characters (Ch. 53 inactive, see below) and 35 terminals. The matrix is a modified version of previously published datasets (Billet and de Muizon 2013; Deraco and García-López 2016; García-López et al. 2018; Castro et al. 2021) but focused in toxodont clades. The ingroup comprises 33 terminals corresponding to Toxodontia and includes all traditionally recognized clades (Isotemnidae, “Notohippidae”, Homalodotheriidae, Leontiinidae, and Toxodontidae) with the “henricosborniid” Simpsonotus and the typotherian Colbertia as outgroups. Most isotemnid genera are represented, but some of them were excluded given to the lack of information on upper dentition (e.g., Distylophorus). Different species of Isotemnus were included in a single terminal (Isotemnus spp.), as several specimens and species need revision in order to evaluate the actual diversity of the entity.

The matrix was analyzed with the TNT software (Goloboff et al. 2008), in two cases: in the first one, the matrix was analyzed under equally weighted characters. Once the result was obtained, a second analysis was made under implied weights, with different k values (results discussed correspond to K = 15). In both cases we use the New Technology Search function of TNT, as this represents a better option considering the relatively high number of missing entrances of our dataset. In all cases, the search strategy involved 10 replicates of three rounds of fusing and 10 cycles of drifting and ratchet, hitting at least 30 times the best score. Additionally, resulting optimal trees were subject to TBR branch-swapping, in order to find all the most parsimonious trees (MPT).

For body mass calculations, we use the equation proposed by Solórzano and Núñez-Flores (2021), considering the length of the second upper molar (SUML). This measure was selected given the availability of this tooth for most of the species considered and the fact that the holotype and most of the referred specimens correspond to upper dentition.

Mammalia Linnaeus, 1758

Pan-Perissodactyla Welker et al., 2015

Notoungulata Roth, 1903

Toxodontia Owen, 1853

Isotemnidae Ameghino, 1897

Itaboraitemnus gen. nov.

Type species: Itaboraitemnus macrodon.

Etymology: “Itaboraí”, after the city where the basin is located and “temnus”, from the greek Τεµγυς, meaning groove, a suffix commonly used by Ameghino and later authors to designate notoungulate genera, particularly isotemnids.

Diagnosis: As for the type and only species.

Geographic and stratigraphic range: São José de Itaboraí, Rio de Janeiro, Brazil, Itaboraian SALMA (Paleocene-Eoene).

Itaboraitemnus macrodon sp. nov.

(Figs. 3, 4)

Holotype: MCT.M.965, rostral half of cranium with right canine and P1-M3, and left P1-M2 (Fig. 3).

Referred material from this study: AMNH 49873, left maxillary fragment with P3-M3; MCN PV 1252; left maxillary fragment with P4-M2; MCT.M.3170, isolated left M2; MCT.M.3165, isolated left M3; MCT.M.3407, isolated left P3 or P4; MCT.M.3588, isolated left M3; MCT.M.3589, isolated right M1 (Fig. 4).

Etymology: macro, from the greek word µακρος, meaning large, and odon, from the greek word ὀδούς, meaning tooth, in reference to its large canine.

Diagnosis: Small notoungulate with brachydont dentition. The absence of the crista 1- mesiolabial fossette complex on molars and the presence of crista intermedia associated to crenulations on these teeth allow its inclusion within the Toxodontia. Among these forms, the generalized morphology of the postcanines and the presence of enlarged canines relates this new taxon to the species traditionally regarded as Isotemnidae and differentiate it from the rest of the basal stock of Toxodontia (i.e., “isotemnid-like notohippids” and basal Leontiniidae). Among isotemnids, it differs from Isotemnus by the absence of metacone column in the P3-4, cingulum less projected in labial side and absent in lingual premolars, metastyle slightly more convex and parastyle more conspicuous. In molars, metacone column is strong, with a wide flexus separating paracone and metacone folds on the labial wall. It differs from Anisotemnus, Pleurostylodon, Pampatemnus, Rhyphodon, Periphragnis, and Thomashuxleya for its smaller size, absence or barely evident metacone and paracone column and straight parastyle, leaving the ectoloph little winding. Although its morphology cannot be directly compared, it also differs from Distylophorus (only represented by lower dentition) by its smaller size.

Geographic and stratigraphic range: 22° 50’ 23.58” S, 42° 52’ 31.97” W, São José de Itaboraí, Rio de Janeiro, Brazil. Itaboraí Formation, Itaboraian SALMA (Paleocene-Eocene, sensu del Papa et al. 2022)

Description: Cranial features are mostly visible on the specimen MCT.M.965. Its preserves a fragment of right premaxilla, both left and right maxilla, part of the right jugal, and fragments of palatine, nasal, and frontal. Among cranial structures, the anterior root of the zygomatic arch is preserved on the right side (Fig. 3a-c). This is mostly formed by a transversely flattened zygomatic process of the maxilla (as usual in several notoungulates), which bears a lateroventral flattened surface (Fig. 3c) corresponding to the insertion area for masseter muscles (Turnbull 1971). The dorsal part of the zygomatic root is formed by the jugal, which contacts the maxilla by means of an irregular suture, visible laterally and running posteroventrally – rostrodorsally (Fig. 3c). On the infraorbital surface, this suture runs parallel to the ventral part of the orbital rim, that extends rostrally to the level of P3-4. The orbital floor is preserved at both sides (although incomplete of the left) and formed by the maxillary tuberosity (Fig. 3a, b). The floor is smooth and the tuberosity is well defined and rounded (as it is an adult individual with the M3 fully erupted). The maxillary foramen is not visible, as it is covered by sedimentary matrix on both sides. Laterally, on the facial exposure of the maxilla, the infraorbital foramen is visible on the right side, and is located at the level of the P3/P4 and near the anterior margin of the orbit (Fig. 3c). It is a simple aperture facing rostrally. Ventrally, the palate is poorly preserved. Even though, a relatively wide postpalatine notch is visible on the left side of the cranium (Fig. 3a). Other cranial features are obscured by diagenetic deformation.

As for the dentition, all postcanine pieces are brachydont. Measurements are given in Table 1.

Anterior dentition. The canine is enlarged (although not modified as a tusk-like tooth) and hence, its crown is clearly higher than the P1 (roughly two times the P1 crown height). The crown has an acute apex and, although partially broken, it is still possible to observe its asymmetry. It is slightly projected distally, making the angle of the mesial face more inclined than the opposite side. The tooth bears relatively sharp mesial and distal vertical crests, which run over the entire height of the crown. The labial wall of the tooth is markedly convex, while the lingual wall is relatively more flat, bearing mesial and distal concave areas. There are faint labial and lingual cingula, extending from the mesial to the distal side.

Postcanine teeth. The P1 is markedly smaller than the rest of the postcanine series. This is apparently single-rotted and has an oval occlusal outline (long axis mesiodistally arranged). Although simple regarding other premolars, the protocone, paracone, parastyle, and metastyle are easily individualized. The protocone is incipient and much lower than the paracone. It is visible as a very small cusp, mostly evident in lingual view (Fig. 3a). The paracone is the higher cusp of the tooth (although it is blunt in labial view). The area of the parastyle in occlusal view is smaller than that of the metastyle; however, the former is more defined and evident. There are no fossae on the central area of the occlusal surface but there is a faint enamel covered surface on the mesiolabial area, in a position similar to that of the mesiolabial fossette in other postcanines. There is an incipient labial cingulum and faint flexi at both sides of the paracone, separating this cusp from the parastyle and metastyle.

The occlusal outline is sub-triangular in the case of the P2, with a straighter and more defined ectoloph. Furthermore, the complexity of the crown increases with the development of some structures absent in the anterior locus. The protocone, although small, is fully developed in this case. This cusp is connected mesially to a slightly sigmoid and faint protoloph, which runs mesiolabially until it reaches the parastyle. The protocone is mesially limited by a weak cingulum, mostly evident on the mesiolingual wall of the tooth. In turn, the distal cingulum is stronger and is clearly differentiated from the distal wall of the protocone. On the opposite side towards the labial wall, the paracone is higher than the protocone and more defined mesially, and is separated from the parastyle by a well-developed flexus and an enamel occlusal area, connected to an enamel surface similar to a mesiolabial fossette. The parastyle is mesially prominent; it is lower than the paracone, although the occlusal surface of both cusps is subequal. Protocone and paracone are aligned in an oblique buccolingual axis (with the paracone slightly more mesial). There is an incipient central fossa located at the middle length of that axis, faintly connected to the enamel occlusal area mentioned above. The area of the ectoloph distal to the paracone is straight, without trace of metacone column and ending in a sharp and small metastyle (well-defined distally). There is a moderately developed labial cingulum, almost continuous on the cervical area of the ectoloph (it is nearly interrupted at the level of the paracone base).

The P3 and P4 will be described together since these teeth are very similar to each other (although the P4 is larger). The outline of these teeth is more oval than in the case of the P2, due to the much more rounded lingual wall of the protocone and lingual part of the mesial and distal cingula (Fig. 3a, 4a, b, c). The protocone is well defined in occlusal view and is connected to a sigmoid protoloph, stronger than in the P2, and connected to the parastyle. This pattern is mostly evident in the holotype (Fig. 3), and the specimens MCN-PV 1252 and MCT.M.3407 (an isolated premolar), but shows a variation in AMNH 49873 (Fig. 4). In this latter case, the protoloph is interrupted on the mesiolabial slope of the protocone. There is a low area on the P3, that marks such interruption; as for the P4, an unworn enamel crest reaching the apex of the protocone indicates that the interruption of the protoloph is due to a slight difference in the wear pattern. Overall, this potential source of variation should be considered in future studies. The parastyle is relatively large and points mesiolabially in occlusal view. The paracone is higher than the protocone and is also separated from the parastyle by an enamel wall and enamel occlusal area. This area, unlike the case of the P2, is concave, similar to a fossette, and more connected to the central fossa. The central fossa is roughly triangular (more developed and deep in the P4) and bears small cristae attached to the ectoloph in both the P3 and P4. In the specimen MCN-PV 1252, which shows lesser wear, these cristae are developed as a crochet-like structure (Fig. 4c). The metastyle is well developed in these premolars and much stronger than in the P2. The labial wall of these teeth is dominated by the paracone column, which is preceded by a well-marked vertical flexus (separating this column from the parastyle). Distally on this wall there are concave areas in both premolars, on the zone of the metacone (not differentiated occlusally) and a conspicuous vertical flexus preceding the metastyle column. There is a wide loph connecting the protocone and the zone of the metastyle. Mesial and distal cingula are also well developed (the distal cingulum is wider than the mesial one in occlusal view). There are labial cingula on these premolars that display some variation, being more developed in MCT.M.956 (holotype) but weak in AMNH 49873, MCN-PV 1252, and MCT.M.3407 (Fig. 3a, 4a, b, c).

Both the M1 and M2 are very similar in morphology; however, the M1 is clearly smaller in occlusal view and the crown is usually lower than the M2. The lingual cusps of these molars (protocone and hypocone) are subequal in occlusal view, although the base of the protocone is far more extended towards the central fossa. Both cusps are separated by a depression continuous with a wide lingual flexus; hence, there is not a well-developed entocrista. The protocone shows an acute mesiolingual corner and is continuous with a straight and well-developed protoloph, which connects with the distolingual area of the parastyle. The hypocone is continuous with a transverse metaloph. In the M1 the metaloph is straighter than in the M2. In the latter, this structure is distally deviated at the level of the crochet and distolabial fossette. Also, at the M1, the lingual extension of the hypocone is more pronounced than in the case of the protocone, while in the M2, both cusps are more symmetrical.

The parastyle is strong (larger than the paracone in occlusal view) on both molars, and points mesiolabially. It is separated from the paracone by a well-developed but shallow sulcus. On the ectoloph, paracone and metacone are subequal in height on the M1; these cusps are aligned in a straight mesiodistal line and the labial columns show a similar development in low wear stages. In turn, the metacone is lower and its column is slightly less defined than that of the paracone in the M2. The ectoflexus segment between these is also more oblique in this case, regarding the M1. In turn, in both molars there is a short labial cingulum at the base of the ectoloph, in the area between paracone and metacone. The metastyle is almost as developed as the parastyle but it is projected distally.

The central fossa of M1-2 has the typical configuration for pre-Oligocene notoungulates, being oblique (shallower on the M1). There is a short crochet connected to the crista 2 and other cristae that run from the lingual side of the ectoloph. Given the lower wear degree of the specimen MCN-PV 1252, two cristae (or crenulations) can be identified both in M1 and M2. These are located between the proper crochet and a mesial crista intermedia. Another slight variation is present in AMNH 49873, were the crochet is slightly shorter and points to (but not reaches) the crista 2 on the M2. This complex of crochet and cristae becomes a straight oblique crenulated loph with more advanced wear (as in the holotype, MCT.M.3170, and MCT.M.3589). The distolabial fossette is in general well developed in all molars, although this structure is weaker on the M1. As for the M2, this fossette is more persistent and shows a subtriangular outline, particularly with low wear stages (e.g., MCN-PV 1252).

Cingula are present on the mesial and distal walls of M1-2. Both cingula have a similar development and relatively narrow considering forms such as Pampatemnus. The mesial cingulum does not reach the lingual wall of the protocone in any specimen (although it is particularly prominent in MCT.M.3170, an isolated M2). In turn, there is a subtle variation related to the distal cingulum, as it shows a vestigial extension on the lingual wall of the hypocone of the M2, only visible on MCN-PV 1252.

Although most structures are developed in a similar way to previous premolars in the M3, main differences are related to the usual reduction of the hypocone, common to several brachydont Paleogene species (Fig. 3a, 4b, f, g). Hence, in occlusal view, the M3 is roughly triangular, showing also a more oblique ectoloph. The protocone is the larger cusp in this view (although shorter than the paracone). Paracone and metacone folds are well-developed and subequal, with only subtle variations through the sample of specimens. The metacone fold is less developed in MCT.M.3165 but seems more prominent in MCT.M.3588. However, in this latter case, the apex of the metacone (and also that of the paracone) show a singular wear pattern, possibly due to chemical digestion, that obscures the true morphology. Several cristae form an intricate pattern in a crista intermedia position, mesial to the crochet, which is straight and short (although relatively longer in MCT.M.3165). Although present, hypocone, metaloph, and distal cingulum are smaller than in the previous molars. Parastyle is also reduced, being small and distally projected. Other structures, such as mesial cingulum and distolabial fossette are similar to those of the anterior molars. And additional variation is present on MCT.M.3165 (Fig. 4g), which shows a short lingual cingulum developed between the lingual slopes of protocone and hypocone. This cingulum is rather strong and bears a cusp like apex. Consequently, this isolated M3 shows a more trapezoidal outline, differing from the rest of the sample.

The first analysis, performed under equally weighted characters, resulted in 30 most parsimonious trees (MPT) and the strict consensus is shown in Fig. 4. These results presented show some resolution, although a large polytomy encompasses most Toxodontia, except isotemnids and the specimen IBIGEO-P 12 (Toxodontia indet.). However, the most important result is the recovery of Isotemnidae as a natural group, supported by four synapomorphies (see below). Relationships are quite unresolved within Isotemnidae but two monophiletic groups were recovered within the family: (Anisotemnus, Pampatemnus) and (Periphragnis, Thomashuxleya). The mentioned polytomy, supported by seven synapomorphies (Appendix 1) is integrated by the sum of genera traditionally referred as “notohippids” (both following the definition of Simpson [1967] and Bond and López [1993]) arranged as a polyphyletic assemblage, plus Homalodotheriidae, Leontiniidae, and Toxodontidae as natural groups.

The second analysis was performed under implied weights, exploring different K-values. This resulted in five MPT, and the consensus is also shown in Fig. 4 (corresponding to K = 15). Isotemnidae is, once again, recovered as a natural group, with the node bearing the same synapomorphies as in the analysis under equally weighted characters. In the same way, the inner monophyletic groups previously detailed are also recovered within the family. Regarding the remaining Toxodontia, they have shown an interesting arrangement, with Puelia, Pampahippus, and Plexotemnus as successive stem lineages leading to the rest of Toxodontia. These genera, regarded as “isotemnid-like notohippids” in some contributions (Shockey 1997; Deraco and García-López 2016; García-López et al. 2018) where recovered together in a natural group by Martínez et al. (2021) and mentioned as “paranotohippids”. Although this group was not recovered here, in this publication and in Martínez et al. (2021) they are well-separated from the rest of “notohippids”, which are more closely related to leontiniids and toxodontids (see below).

The remaining Toxodontia include the family Homalodotheriidae as sister group of a node composed by Leontiniidae, the stem of post Eocene “notohippids” (e.g., Eomorphippus, Rynchippus, Pascualihippus), and Toxodontidae. The composition and general arrangement of this node is here quite similar to the clade named Eutoxodontia by Martínez et al. (2021), even considering some differences in the position of “notohippid” genera. Hence, the use of that name is supported here.

During most of the 20th century, almost all studies focusing on Paleogene notoungulates were based on the fossil record from the Argentine Patagonia. This is quite evident by the several contributions made by Ameghino in the late 19th century and in the early part of the 20th century, and later contributions such as those of Patterson (1932, 1934a, 1934b, 1935, 1936, 1937) and Simpson (e.g., 1932, 1936, 1948, 1967), which established the basis for the studies performed in later decades. This bias led to a general scenario, not necessarily erroneous, but with several aspects that have been challenged in recent decades, as more extra-Patagonian localities yielded new materials and new fossil communities were discovered. In this sense, contributions documenting records from Central Chile, Peru, Bolivia, Brazil, and northwestern Argentina had come to expand the original framework, bringing into light a spectrum of basal forms, functional specializations, and representatives of singular radiations (e.g., Paula-Couto 1978; Pascual et al. 1981; Vucetich and Bond 1982; Soria and Alvarenga1989; Reguero et al. 2003; Hitz et al. 2006, 2008; Deraco et al. 2008; Babot et al. 2017; Sedor et al. 2017; García-López et al. 2018, 2020; Castro et al. 2021). Within this set of extra-Patagonian records, several correspond to the strip of middle latitudes of South America and, particularly, northwestern Argentina. This is worth noting, as for some years several authors have considered that the climatic and environmental conditions that affected this region were somewhat different from those under which Patagonian fossil communities developed at the same time (e.g., Bond 1981; Pascual et al. 1996; Ortiz Jaureguizar and Cladera 2006; García-López and Powell 2009; Woodburne et al. 2014). This particular scenario would have caused some degree of endemicity for the fossil assemblages recorded in northwestern Argentina and, despite of the large number of new records that can be traced back to the middle third of the 20th century, only recently a common record of a South American ungulate species was documented between this area and Patagonian localities (García-López et al. 2020; also see Pascual et al. [1981] for mentions of Patagonian genera in northwestern outcrops; nevertheless, in most cases these taxonomic assignments have been changed or need revision).

In turn, the São José de Itaboraí Basin also represents a mid-latitude, fossil-bearing locality for the continent. In this context, it stands as one of the most important of these fossil deposits, not only for its diversity and chronological background, but also because this locality is one of the first non-Patagonian levels to have yielded a significant Paleogene fossil fauna, which was early studied and compared with its southern counterparts (Price and Paula-Couto 1946, 1950; Paula-Couto 1950, 1952).

Notoungulate remains are significantly abundant in the Itaboraí Basin. Nevertheless, despite the number of described species (see introduction), the diversity of the order in this locality remains relatively low compared to other Paleogene assemblages (Castro et al. 2021), and no record of the suborder Toxodontia had been documented for this locality until the present contribution, a significant fact considering the number of isotemnids and “notohippids'' specimens exhumed in the Lower Lumbrera Formation (i.e., Pampatemnus and Pampahippus), which is, following recent authors, chronologically coetaneous with at least the latter half of the time span in which the Itaboraí Basin fauna might have developed (see Fernícola et al. 2021; del Papa et al. 2022).

Hence, the new isotemnid reported here fills a notable “geographical gap” within the South American middle-latitude Paleogene record. Patagonian representatives of this notoungulates are relatively diverse, including a wide body-size range (Solórzano and Núñez-Flores 2021), and some of the structurally less specialized taxa among Toxodontia (e.g., Isotemnus; Cifelli 1993). Most species and most complete specimens correspond to the SALMAs Casamayoran (Vacan and Barranca) and Mustersan beds (Woodburne et al. 2014) and hence, illustrate the middle and later phase of the Eocene record. The Itaboraian species here described then represents part of the first pulse of radiation for these notoungulates, along with Isotemnus and Pampatemnus (Fig. 6).

Simpson (1967) stated that isotemnids represented the structurally most basal toxodonts, and this status was reflected in characterizations of the clade made by later authors (e.g., Cifelli 1993). Perhaps the most noteworthy aspect of the present phylogenetic hypothesis is the recovery of Isotemnidae as a natural group. This result, among other traits, rests on the presence of a canine morphologically differentiated from the remaining anterior dentition in all species traditionally considered within this group. Different definitions and phylogenetic hypotheses through several decades have considered size and morphology of toxodont canines. Simpson (1967 p.118) included the “usually enlarged canines” as one of the traits defining Isotemnidae. Other contributions have included characters related to canine morphology (e.g., Billet 2011), although usually the size has been compared to taxa with much enlarged and tusk-like canines, without considering a moderate enlargement, which we consider to truly differentiate isotemnids from all other notoungulate clades. Hence, until now, no phylogeny has recovered isotemnids as monophyletic; although support in our analysis is low, this underlines the importance of considering carefully the morphology of the anterior dentition and its variants among different toxodont clades.

Recent phylogenetic proposals on Toxodontia, have reached results somewhat similar to those obtained here, being based on different matrices. One of the most interesting approaches is that of Martínez et al. (2021), focused on “notohippid” notoungulates. The results of these authors showed some parallelism with our hypothesis regarding the major clades involved (see below). Additionally, some of the early diverging forms within Toxodontia were grouped in that analysis, in a manner that can be compared with the scheme obtained here. In fact, most of the isotemnid genera included by those authors integrate a monophyletic group, but this excludes Pampatemnus infernalis, hence precluding the consideration of traditional isotemnids as a natural group. In our analysis, Isotemnidae is supported by four synapomorphies: Ch. 21 (1→0), disto-labial fossette on upper molars coexisting with the central fossa; Ch. 59 (0→1), zygomatic arches semicircular and well-expanded in dorsal view; Ch. 63 (0→1), dorsal edge of the posterior root of the zygomatic arch with strong relief (homoplasty with Eutoxodontia); and Ch. 87 (1→0), canines non incisor-like. Of these, only the strong relief of the dorsal edge of the zygoma (or zygomatic crest) is also recovered by Martínez et al. (2021) for their group of isotemnid genera. In turn, those authors recovered a node that they informally named “Paranotohippids”, including -among others- Puelia, Plexotemnus, and Pampahippus. These taxa are also included in our study, but they are arranged as a paraphyletic group occupying the stem leading to remaining toxodonts (Homalodotheriidae plus Eutoxodontia; see below), and maintaining a similar arrangement than that obtained by Deraco and García-López (2016).

Although the aim of the present contribution is not to discuss the relationships among more derived and Neogene/Quaternary clades among Toxodontia, it is noteworthy that the remaining taxa in our analysis result in a similar arrangement of that of Martínez et al. (2021). In both contributions, the remaining toxodonts include the monophyletic homalodotheriids as sister group of the node leading to “notohippids” (considering the definition by Simpson 1967), plus leontiniids, and toxodontids. This clade was named as Eutoxodontia by Martínez et al. (2021). However, in our analysis the same clade (also with Leontiniidae in a basal position regarding all other eutoxodonts) is supported by only one synapomorphy: Ch. 63 (0→1), dorsal edge of the posterior root of the zygomatic arch with strong relief (representing a homoplastyc trait regarding Isotemnidae). This contrasts with the analysis of Martínez et al. (2021) in which Eutoxodontia is supported by several characters, including postcranial traits. In any case, both results may reflect a similar evolutionary pattern for this suborder.

Considering most genera of basal toxodonts and our phylogenetic hypothesis of the isotemnids as a natural group, some comments can be made on the evolutionary history and adaptive trends among the representatives of the latter group. The chronological distribution of this family reflects two main pulses during the Eocene and matches well with the environmental events and faunal turnovers documented during the second half of the Eocene and the early Oligocene (Braunn and Ribeiro 2017). The first pulse of isotemnid radiation took place during the Itaboraian SALMA and would have developed under the conditions imposed by the Early Eocene Climatic Optimum (EECO; Woodburne et al. 2014). It was represented by at least three genera (Isotemnus, Itaboraitemnus, and Pampatemnus) distributed between Patagonia and the middle-latitude strip of Brazil and Argentina. Considering the diversity of species included within Isotemnus and Pampatemnus, as well as some mentions that require further studies (e.g., Pascual et al. 1981), the record of the first isotemnid radiation is remarkable, as it reflects a significant number of taxa and might be a glimpse of a much earlier and yet to be discovered presence of these forms in the Paleogene.

The second pulse by isotemnids is recorded during the Casamayoran SALMA (and particularly during the Barrancan subage) coinciding with the middle Eocene Climatic Optimum (MECO; Woodburne et al. 2014; Braunn and Riberiro 2017). In this case, the isotemnid radiation is almost exclusively a Patagonic event, with the family so far absent in the record of rich fossil-bearing mid-latitude units such as the Upper Lumbrera Formation (Babot et al. 2017) and a single mention of an indeterminate isotemnid for the Barrancan levels of the Quebrada de los Colorados Formation (Payrola Bosio et al. 2009). This pulse would also imply an early diversification of other toxodont taxa, including advanced “notohippids” and leontiniids, and marking the initial records among eutoxodonts (Martínez et al. 2021).

A third event in the history of isotemnids is apparently related to the profound climatic changes recorded in the context of the last span of the Eocene and early Oligocene, when cooler and drier conditions were established following the isolation of the Antarctic continent favoring the diversification and dominance of hypsodont clades among notoungulates (Ortiz Jaureguizar and Cladera 2006; Woodburne et al. 2014; Braunn and Ribeiro 2017). In relation to these events, toward the end of the Barrancan subage and beginning of late Eocene, isotemnids underwent a significant turnover, with the diminish of typical Eocene taxa (e.g., Isotemnus, Pleurostylodon) and their replacement by forms such as Rhyphodon and Pariphragnis (Fig. 6).

These successive radiation pulses observed among isotemnids not only seem to show parallelism with the different environmental events recorded for the Eocene, but they also fit, to some extent, into the evolutionary models established for certain phenotypic traits among notoungulates. Particularly, the changes in body mass of this order seem to show a clear tendency towards the selection of larger forms, leastways among toxodonts (Solórzano and Nuñez-Flores 2021). These authors establish that the evolution of this characteristic among toxodonts did not occur gradually, but rather responded to pulses of increase, of which the first would have occurred during the late middle Eocene (Bartonian, Casamayoran). In our scheme, the second radiation of isotemnids, after their initial diversification, would coincide with this first pulse of increase in average body mass. When we take the estimated body mass for different isotemnids, it stands out that one of the smallest corresponds to Isotemnus talego, from the Itaboraian SALMA. Itaboraitemnus also corresponds to a small form with an estimate based of 5.02 to 6.28 kg (Fig. 7). In this way, the first records of I. talego e Itaboraitemnus fall into this scheme, as representatives of early radiation and anticipated the first pulse of size increase in the family. Once a new pulse of diversity was established during the Barrancan, larger forms began to predominate, such as Thomashuxleya (a mean of 315 kg, considering the SUML equation and measurements given in Solórzano and Núñez-Flores 2021). The change of forms towards the upper part of the Eocene, again reflects the mentioned trend, given that only relatively large taxa would survive (e.g., Rhyphodon, mean body mass of 112 kg, and Periphragnis, mean body mass of 282 kg; both considering the SUML equation and measurements provided by Solórzano and Núñez-Flores 2021), with medium-sized forms disappearing. This would also respond to some factors, such as lower extinction rates for large forms (Solórzano and Nuñez-Flores 2021). Finally, towards the end of the family's history, in the Oligocene, only some of the representatives with the greatest body mass in the lineage would prevail, such as Periphragnis.

In summary, this succession of faunistic events among isotemnids and other Paleogene toxodonts matches well with the environmental framework currently established for the Paleogene. The response of these taxa to this series of changes serves as a compelling example of their physiological plasticity and their role as reliable indicators of the major evolutionary trends of the South American Cenozoic. Additionally, the number of early Eocene genera, now increased with the new record of Itaboraitemnus, suggests that the previous history of Isotemnidae (and toxodonts in general) might be better known as new Paleogene sequences are explored and new discoveries are reported. In any case, morphological surveys and phylogenetical contextualization must be conducted on these taxa, particularly on those represented by more complete cranial and postcranial material (e.g., Pampatemnus; see Vucetich and Bond, 1982).

Itaboraitemnus macrodon represents a new genus and species of Isotemnidae (Toxodontia, Notoungulata) for the Itaboraí Basin (late Paleocene - early Eocene). This species is characterized mainly by its small size in the context of toxodonts and dental features that differentiate it from all other isotemnids. The presence of Itaboraitemnus in this middle-latitude locality fills a gap in the record of Paleogene clades, as Toxodontia has no previous mentions among the South American native ungulates from the Itaboraí Basin.

Despite the low support, Isotemnidae is recovered in part of our results as a monophyletic group, an arrangement not previously recovered nor commented in any other contribution based on cladistic methodology. This novel point of view is based mainly in the singularities of the anterior dentition (i.e., enlarged canines) and reflects the case also observed in other toxodonts, where variation in the arrangement and morphology of incisors and canines may also be used to define groups (e.g., Leontiinidae, Toxodontidae).

Isotemnids evolution matches with a series of climatic events during the Eocene, as apparently at least two diversification events took place during climatic optima, and a final turnover within the family is related to the Eocene-Oligocene transition event. Moreover, their history also shows how these events affected the general history of toxodonts, also marked by the apparition of other important suprageneric clades during the Eocene, as Eutoxodontia. Also, regarding evolutionary trends, changes observed in body-size among isotemnid species through time also parallel general tendencies among toxodonts (i.e., overall increase in size through time).

FUNDING INFORMATION

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) to LORC.

Author Contribution

All authors contributed to and reviewed the manuscript as well as the figures editions.

Acknowledgement

We are very grateful to Dr. Rafael Costa and his team (MCT), Dr. Ana Maria Ribeiro (MCN), Dr. Marcelo Reguero and Dr. M. Susana Bargo (MLP), Dr. Algustin Martinelli and Dr. Maria Belén von Baczko (MACN), for allowing us to access and study the Notoungulata from this collection. My sincere thanks also to MSc. Tabata Zanesco for all her comments on the Itaboraiana SANUs and to Dr. Guillaume Billet for allowing me to use the photograph of the AMNH morphotype.

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First record of Toxodontia for the Itaboraí Basin (Paleogene, Rio de Janeiro, Brazil) and an assessment on the evolution of isotemnid notoungulates (Pan-Perissodactyla, Mammalia)

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