A new protodidelphid (Mammalia, Marsupialia, Didelphimorphia) from the Itaboraí Basin and its implications for the evolution of the Protodidelphidae

the Protodidelphidae was a group of marsupials that lived in Gondwana from the early to middle Eocene. Among South American faunas, the Itaboraí Basin calls attention by the presence of four genera and six species. herein is described Bergqvistherium primigenia gen. et sp. nov., a new protodidelphid from the Itaboraí Basin lower Eocene. This taxon differs from other protodidelphids in the smaller size, developed entocristid, less brachyo-bunoid adaptations, and entoconid more mesial than the hypoconid. these characters are recovered as plesiomorphies of the Protodidelphidae, supporting Bergqvistherium as an early-divergent lineage of this group. the Protodidelphidae fauna of the Itaboraí Basin is represented by less specialized “basal” taxa, such as Bergqvistherium and Periprotodidelphis; and more specialized apical taxa, such as Guggenheimia, Protodidelphis, and Carolocoutoia. This result indicates that the diversification of apical protodidelphids probably was a result of a relatively short-time event, occurring during the early Eocene. this evolutionary event can be directly correlated to the increase in the temperatures and the extension of tropical forests resulted by the Paleocene-Eocene thermal Maximum during the Itaboraiense time span. the study supported a Late Cretaceous origin for the Protodidelphidae, which agrees with molecular studies for the Didelphimorphia.


INTRODUCTION
the Protodidelphidae Marshall, 1987, is an extinct marsupial lineage that existed during the early and middle Eocene in South America and Antarctic Peninsula (Paula Couto 1952, 1962, 1970, Marshall 1987, Goin et al. 1999, tejedor et al. 2009, Oliveira and Goin 2011, 2012) and (Oliveira É.V., unpublished data).this taxon is represented by Protodidelphis Paula Couto 1952;Guggenheimia Paula Couto, 1952;Carolocoutoia Goin et al. 1998;and Periprotodidelphis Oliveira and Goin, 2011. the oldest occurrences of this lineage are known for the Itaboraí Basin, rio de Janeiro, Brazil, and Las Flores, Argentina; both considered as lower Eocene (Itaboraian SALMA -around 55-52 Ma - Woodburne et al. 2014a) in age.All members of the Protodidelphidae are known to have existed during this time span.the youngest fossil records are known for the La Meseta Formation, Antarctica, and Paso del Sapo, Argentina; both date from the lower-middle Eocene ("Sapoan" SALMA) (Goin et al. 1999(Goin et al. , tejedor et al. 2009)), which identifies the Protodidelphidae as a short time span surviving lineage during the Paleogene of South America.Notwithstanding, the evolution of other metatherian lineages can be traced back to the Paleocene of South America or even to the Late Cretaceous of North America (Case et al. 2005, Forasiepi et al. 2009, Goin et al. 2012, 2016, Oliveira and Goin 2012, Carneiro and Oliveira 2017a, b, Carneiro 2018), which is also expected for the Protodidelphidae.
the Paleocene metatherian fossil bearing localities in South America are restricted to tiupampa, Bolivia (lower Paleocene -tiupampian SALMA -Muizon 1992); Laguna Umayo, Peru (upper Paleocene or Itaboraian SALMA -Sigé et al. 2004, Gelfo andSigé 2011); Grenier section of the Lefipán Formation, Argentina (lower Paleocene - Goin et al. 2006), and Punta Peligro, Argentina (lower Paleocene -Peligran SALMA - Woodburne et al. 2014b); however, none of these localities have a protodidelphid among the identified metatherians.The Paleocene diversification of the Protodidelphidae can be inferred based on the great diversity of this group during the Itaboraian SALMA (early Eocene), as four genera are recorded, as commented.Among them, Guggenheimia is considered by Oliveira and Goin (2011) as an early-divergent lineage of the Protodidelphidae based on the relatively smaller size and less bunoid molars.Nevertheless, this genus shows several morphological traits that support an earlier evolutionary history for the Protodidelphidae.
During a survey of the paleontological collection in the Departamento Nacional de Produção Mineral (DNPM), an isolated m3 of a 'protodidelphid-like' taxon was identified in the fossils collected in 1968, which differs in several ways from other genera from the Itaboraí Basin.Interestingly, this material is similar in some morphological traces to Guggenheimia; however, it bears some plesiomorphic characters for a brachyobunoid taxon, supporting a probable earlier divergence.
here, it is described a new genus of protodidelphid from the Itaboraí Basin, discussing its implications for the evolution of the Protodidelphidae and for the fauna of the Itaboraí Basin.
the Brazilian taxa were directly analyzed (i.e.fossil specimens and casts), in addition, literature descriptions, casts, digital and SEM pictures were also analyzed.the North American taxa were studied based on literature descriptions, casts, digital and SEM pictures.the casts of Naturita Formation taxa and several specimens of Varalphadon were sent by richard L. Cifelli and Joshua E. Cohen from the Sam Noble Oklahoma Museum of Natural history, USA.A list of casts is given in the supplementary material, that can be accessed here https://www.researchgate.net/publication/327248822_Supplementary_Material.
the SEM pictures of the new taxon were made with the scanning electron microscope JEOL JSM-6390LV at the Centro de Microscopia Eletrônica de Varredura do Departamento de Invertebrados do Museu Nacional, Museu Nacional, rio de Janeiro, Brazil.
the morphological matrix was submitted to a new technology search with TNT 1.5 (Goloboff and Catalano 2016) using the sectorial, ratchet, drift and tree fusing strategies with 500 replications.the morphological matrix is available as supplementary material, also at https://www.researchgate.net/publication/327248822_Supplementary_Material.Bremer supports and tree scores were calculated with tNt 1.5.the phylogeny presents 642 unordered characters, including cranial, dentary, dental and postcranial characters, and 184 therian taxa, including more than 170 metatherians and closely related taxa, from the Cretaceous and Cenozoic of North America, Asia, South America, Antarctic, Europe and Australia.(Fig. 2) ZooBank Life Science Identifier (LSID)urn:lsid:zoobank.org:act:0DDF46FB-B2DF-467C-A8D5-96F03CB989E6 Etymology: 'Bergqvist', in honor of Lílian Paglarelli Bergqvist for her contributions for the knowledge of the Itaboraí Basin; 'therium', from the Greek, beast; a common designation of fossil mammals.Gender is feminine.
Included species: the type only.
Diagnosis: As for the genus.
Hypodigm: the type only.
Locality and horizon: As for the genus.
Description: the m3 is 2,16 mm in mesiodistal length, 1,29 mm in the labiolingual width of the trigonid, and 1,26 mm in the labiolingual width of the talonid.the paraconid is moderately developed and slightly more labially positioned than the metaconid, and its apex is mesially projected.the paracristid is short and does not show a "curved" shape.the trigonid is mesiodistally and labiolingually compressed.the metaconid is slightly distal (i.e.nearly aligned) to the protoconid.the entoconid is conical, and welldeveloped, but not massive (i.e.does not occupy most of or the entire lingual half of the talonid basin).the entocristid and the distolabial crest of the metaconid are developed.the hypoconid is distal to the entoconid.the cristid obliqua contacts the distal wall of the trigonid somewhat more labial than the carnassial notch.the hypoconulid is moderately developed and is twinned to the entoconid, but is not distolingual in position.the anterobasal cingulid is moderately developed and the posterobasal cingulid is well-developed.

Measurements: See table I.
Remarks: Bergqvistherium primigenia is considered to be a protodidelphid based on the presence of a conical and well-developed entoconid, brachyodont molar, developed cingulids, metaconid distal to protoconid, paraconid slightly more lingual than the metaconid, and paraconid with some degree of mesiodistal compression.
An Acad Bras Cienc (2019) 91(Suppl.2) e20180440 6 | 13 It should not be considered as belonging to the Chulpasiinae as it lacks a talonid somewhat wider than trigonid, the entoconid is relatively less developed (i.e.not massive developed), the hypoconulid is relatively smaller, and the paraconid is relatively narrower and more mesiodistally compressed.
the Protodidelphidae is the sister taxon of the Glasbiidae Clemens, 1966; which is represented by Palangania, Periakros and Glasbius.Protodidelphidae and Glasbiidae are the sister taxa of Reigia.the Protodidelphidae, Glasbiidae and Reigia are recovered as the sister taxon of the Didelphoidea, which is represented by the Didelphidae, Sparassocynidae and Monodelphopsis.these lineages represent the Didelphimorphia and can be considered as belonging to the Marsupialia (Fig. 4). the inclusion of the Protodidelphidae among the Didelphimorphia was supported by the analyses of Ladevèze and Muizon (2010), Carneiro and Oliveira (2017a, b) and Carneiro (2018).
the pylogenetic analysis support the sister relation between Periprotodidelphis and Anatoliadelphys, recovering a South American ancestral area for the lineage of the last taxon.this result differs from the one of Maga and Beck (2017), who proposed a North American ancestor area for Anatoliadelphys.this result can be considered as preliminary evidence supporting the hypothesis of the Atlantogea, as proposed by Ezcurra and Agnolín (2012).2015).this event is correlated with the increase in abundance and diversity of metatherians in South America, especially bachyo-bunoid taxa, such as the Protodidelphidae and the Polydolopimorphia, as recorded in the localities dating back to the Itaboraian SALMA (Woodburne et al. 2014a, b, Goin et al. 2016).
The presence of five different genera with seven species among the metatherian assemblage of the Itaboraí Basin (i.e.Bergqvistherium primigenia, Guggenheimia brasiliensis, G. crocheti, Periprotodidelphis bergqvistae, Protodidelphis vanzolinii, P. mastodontoides and Carolocoutoia ferigoloi) supports the diversification of the Protodidelphidae beginning earlier than the Itaboraian SALMA time span (i.e.early Eocenesensu Woodburne et al. 2014a).the phylogenetic analysis recovered Glasbius as an apical glasbiid, which indicates that the lineages of the other glasbiid (i.e.Palangania and Periakros), Reigia and the Protodidelphidae existed by the time of the oldest fossil records of the former taxon in the USA (i.e.around 66 Ma, latest Maastrichtian -Clemens 1966, Davis 2007, Williamson et al. 2012, 2014, Boyd et al. 2017).If correct, these results support a Late Cretaceous origin of the Protodidelphidae, with this group surviving as a rare lineage in the faunas of South America before the PEtM.
this evidence should not be treated as surprising, as molecular studies date the origin of the Didelphimorphia to the Santonian-early Campanian time span during the Late Cretaceous, around 80-79 Ma (Kirsch et al. 1997, Bininda-Emonds et al. 2007), which is a compatible time span for the results recovered by the phylogenetic analysis.
the apical position of Glasbius among the glasbiid and the early Campanian (Late Cretaceous) origin of the Didelphimorphia indicate that Marsupialia did not evolve in North America during the Late Cretaceous.this result supports an 'Aves ridge' or Gondwanan origin for the Didelphimorphia and Marsupialia.2011).the protodidelphids recovered in most of the collections at this locality show a greater number of apomorphies than Bergqvistherium and Periprotodidelphis (e.g. more brachyo-bunoid molars, larger size, relatively smaller paraconids and more inflated entoconids), which supports a more specialized frugivorous diet for them (Goin et al. 1998, Oliveira and Goin 2011, 2012, Zimicz A.N., unpublished data).Woodburne et al. (2014a) placed the Itaboraí Basin during the lower Eocene (Itaboraian SALMA), being correlated with the Early Eocene thermal Optimum (EEtO); and Woodburne et al. (2014b) discussed the increase in extension of the tropical forests in South America after the PEtM, which can be associated to the increase in the abundance and diversity of more bunoid mammals, like the protodidelphids Guggenheimia, Protodidelphis, and Carolocoutoia.
the phylogenetic analysis recovered Bergqvistherium and Periprotodidelphis as earlier divergent lineages of the Protodidelphidae, which supports an older origin for these two taxa than Guggenheimia, Protodidelphis, and Carolocoutoia. the existence of "basal" and apical faunal components in the Itaboraí Basin indicates that the diversification and increase in abundance of more apical and specialized frugivorous protodidelphids probably took place during the Itaboraian SALMA. the lower Eocene age dated for the Itaboraí Basin indicates that the diversification of Guggenheimia, Protodidelphis, and Carolocoutoia occurred in a relatively short time span, with the increase in the temperatures as the main environmental event correlated to the increase in size, brachyobunodonty and other frugivorous adaptations for these three genera.
the dental adaptations of Guggenheimia indicate the ingestion of hard food items, judging by the robust dentary and greater mesiodistal compression of the upper molars; Protodidelphis was more likely to be a frugivorous-generalist, and Carolocoutoia a strict frugivorous (Zimicz A.N., unpublished data).this indicates an apparently lower niche overlapping between these three genera and a wide range of occupied trophic levels for this group, as Bergqvistherium is more likely to be a small frugivorous-generalist taxon.
the great taxonomical and ecological diversity of the Protodidelphidae from the Itaboraí Basin support the PEtM as the main environmental event responsible for the evolution of the group.Some previous studies have proposed the hypothesis that the fossil bearing deposits of the Itaboraí Basin span a considerable age range (i.e. from the late Paleocene to early Eocene) (Gayet et al. 1991, Marshall et al. 1997, rage 1998, Bergqvist et al. 2009, Pinheiro et al. 2012).this could be tentatively followed, as the protodidelphids of the Itaboraí Basin show a relative degree of morphological and ecological specialization, with "basal" and apical taxa in the same locality; however, the lower Eocene time span of the Itaboraí Basin, as proposed by Woodburne et al. (2014a, b), is here followed based on the bunoid adaptations present in the apical protodidelphids of this fauna, which are compatible with the increase of the extension of the tropical forests, as proposed by Woodburne et al. (2014b).
these evidences follow the results of Goin et al. (2012Goin et al. ( , 2016) ) and Woodburne et al. (2014a, b), who considered the Itaboraí Basin as lower Eocene in age, and attested the importance of the PEtM as one of the most important environmental events during the Paleogene for the evolution of the South American metatherians.
the extinction of the Protodidelphidae could be related to the global cooling that happened during the Eocene, as no member of this lineage is known to have lived after the middle Eocene of South America and Antarctic (i.e."Sapoan" SALMA -Woodburne et al. 2014a) (Fig. 5).

CONCLUSIONS
the description of Bergqvistherium primigenia from the Itaboraí Basin increases the diversity of the Protodidelphidae in this locality.the presence of several plesiomorphic traits in this taxon supported its recovering in the phylogenetic analysis as an early-divergent protodidelphid.the phylogenetic analysis supported a Late Cretaceous origin for the Protodidelphidae.Following the results, the increase in the abundance, trophic diversity and diversification of apical taxa of this group can be directly correlated to the environmental changes that occurred after the PEtM. the presence of "basal" and apical genera of the Protodidelphidae in the Itaboraí Basin indicates that the diversification of more apical taxa was probably a result of a relatively shorttime evolutionary event during the time span of the Itaboraiense (early Eocene), as a response to the increase in temperatures and extension of the tropical forests in South America.

DISCUSSION
thE LAtE CrEtACEOUS-EArLY PALEOGENE OrIGIN OF thE PrOtODIDELPhIDAE the PEtM (Paleocene-Eocene thermal Maximum) probably occurred around 55,2 Ma, with the increase of the global temperatures and the extension of tropical forests (Bowen et al.
thE PrOtODIDELPhIDAE FAUNA OF thE ItABOrAI BASIN: EVIDENCE OF A ShOrt-tIME DIVErSIFICAtION DUrING thE ItABOrAIENSE Among the protodidelphids found in the Itaboraí Basin, Periprotodidelphis and Bergqvistherium are represented just by a single tooth; while a far more abundant Protodidelphis (30 teeth for both species) and a moderately more abundant Guggenheimia (seven teeth) are also recorded(Oliveira and Goin

Figure 4 -
Figure4-the result of the phylogenetic analysis.Consensus tree of the 24 most parsimonious trees found in the analysis.Bergqvistherium is highlighted in bold.Numbers below the branches indicate the Bremer Support.

Figure 5 -
Figure 5 -temporal cladogram of the Protodidelphidae.Phylogeny of the Protodidelphidae calibrated in time, based on the phylogeny present in the figure 4. The wide bars indicate the recorded temporal range of the Protodidelphidae genera.The SALMAs that present protodidelphids are identified.SALMAs: Itaboraian; "Sapoan".