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The Taxonomic Status of Notocaiman stromeri (Crocodylia, Alligatoroidea) and the Early Diversity of South American Caimanines
Abstract
The early diversification of caimanines is one of the least understood aspects of the evolutionary history of Crocodylia. The Palaeogene Argentinian caimanine record is extremely relevant because it provides key information on the early history of the clade. Most of the Palaeocene South American species are only or mainly known from partial lower jaws. Among the oldest Argentinian caimanines, Notocaiman stromeri (middle Palaeocene, Las Violetas Formation, Chubut Province) and Eocaiman palaeocenicus (lower Palaeocene, Salamanca Formation, Chubut Province) are represented by a partial left dentary and a fairly complete lower jaw, respectively. Notocaiman stromeri, has been phylogenetically interpreted from an alligatorid closely related to Eocaiman to an indeterminate eusuchian, but a modern anatomical revision of this species is lacking. Here, we redescribe in detail the only known specimen of Notocaiman stromeri (PVL 752) to revise its taxonomy and phylogenetic relationships. We concluded that Notocaiman and Notocaiman stromeri are nomina dubia and we refer PVL 752 to Eocaiman cf. E. palaeocenicus. Our phylogenetic analysis found Eocaiman palaeocenicus as the sister taxon to the Eocaiman cavernensis + Eocaiman itaboraiensis clade. Thus, here we reduce the alpha taxonomic diversity of alligatorids in the Palaeogene of South America but reinforce a geographically broad diversification of the genus Eocaiman, from the Palaeocene to the Eocene/Miocene in this continent.
La diversificación temprana de los caimaninos es uno de los aspectos menos comprendidos de la historia evolutiva de Crocodylia. El registro de caimaninos del Paleógeno de Argentina es extremadamente relevante porque proporciona información clave sobre la historia temprana de este clado. La mayoría de las especies sudamericanas del Paleoceno se conocen principalmente a partir de mandíbulas inferiores parciales. Entre los caimaninos argentinos más antiguos se encuentran Notocaiman stromeri (Paleoceno medio, Formación Las Violetas, Provincia del Chubut) y Eocaiman palaeocenicus (Paleoceno inferior, Formación Salamanca, Provincia del Chubut), cuyos holotipos están representados por un dentario izquierdo parcial y una mandíbula inferior bastante completa, respectivamente. Notocaiman stromeri, se ha interpretado filogenéticamente desde un alligatórido estrechamente relacionado con Eocaiman a un eusuquio indeterminado, pero aún falta una revisión anatómica actualizada de esta especie. Aquí, redescribimos en detalle el único espécimen conocido de Notocaiman stromeri (PVL 752) con el objetivo de revisar su taxonomía y relaciones filogenéticas. Concluimos que Notocaiman y Notocaiman stromeri son nomina dubia y referimos PVL 752 a Eocaiman cf. E. palaeocenicus. Nuestro análisis filogenético encontró a Eocaiman palaeocenicus como el taxón hermano del clado Eocaiman cavernensis + Eocaiman itaboraiensis. Entonces, aquí reducimos la diversidad taxonómica alfa de los alligatóridos paleógenos sudamericanos, pero reforzamos una diversificación geográficamente amplia del género Eocaiman, desde el Paleoceno al Eoceno/Mioceno en este continente.
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First appearing in the latest Cretaceous, Crocodylia is a clade of semi-aquatic, predatory reptiles, defined by the last common ancestor of extant alligators, caimans, crocodiles, and gharials. Despite large strides in resolving crocodylian interrelationships over the last three decades, several outstanding problems persist in crocodylian systematics. Most notably, there has been persistent discordance between morphological and molecular datasets surrounding the affinities of the extant gharials, Gavialis gangeticus and Tomistoma schlegelii . Whereas molecular data consistently support a sister taxon relationship, in which they are more closely related to crocodylids than to alligatorids, morphological data indicate that Gavialis is the sister taxon to all other extant crocodylians. Here we present a new morphological dataset for Crocodylia based on a critical reappraisal of published crocodylian character data matrices and extensive firsthand observations of a global sample of crocodylians. This comprises the most taxonomically comprehensive crocodylian dataset to date (144 OTUs scored for 330 characters) and includes a new, illustrated character list with modifications to the construction and scoring of characters, and 46 novel characters. Under a maximum parsimony framework, our analyses robustly recover Gavialis as more closely related to Tomistoma than to other extant crocodylians for the first time based on morphology alone. This result is recovered regardless of the weighting strategy and treatment of quantitative characters. However, analyses using continuous characters and extended implied weighting (with high k -values) produced the most resolved, well-supported, and stratigraphically congruent topologies overall. Resolution of the gharial problem reveals that: (1) several gavialoids lack plesiomorphic features that formerly drew them towards the stem of Crocodylia; and (2) more widespread similarities occur between species traditionally divided into tomistomines and gavialoids, with these interpreted here as homology rather than homoplasy. There remains significant temporal incongruence regarding the inferred divergence timing of the extant gharials, indicating that several putative gavialids (‘thoracosaurs’) are incorrectly placed and require future re-appraisal. New alligatoroid interrelationships include: (1) support for a North American origin of Caimaninae in the latest Cretaceous; (2) the recovery of the early Paleogene South American taxon Eocaiman as a ‘basal’ alligatoroid; and (3) the paraphyly of the Cenozoic European taxon Diplocynodon . Among crocodyloids, notable results include modifications to the taxonomic content of Mekosuchinae, including biogeographic affinities of this clade with latest Cretaceous–early Paleogene Asian crocodyloids. In light of our new results, we provide a comprehensive review of the evolutionary and biogeographic history of Crocodylia, which included multiple instances of transoceanic and continental dispersal.
An incomplete fossil record and unstable phylogenies of extinct taxa hamper reconstructing the early evolution of Caimaninae. We describe previously unpublished articulated fossils of a key species, Tsoabichi greenriverensis from the early Eocene Green River Formation of North America, exhibiting further character evidence for the caimanine affinities of this taxon. Parsimony analysis of modified morphological taxon-character datasets coupled with a critical review of character evolution and published phylogenies reveals that fossil evidence for Palaeogene crown group and Late Cretaceous total-group representatives is unreliable due to uncertain character evolution in early Alligatoridae. The earliest unambiguous fossil age for total and crown-group Caimaninae are 63.5 Ma and 18.06 Ma, respectively. These calibration points follow best practices and are vital for better constrained estimates of time calibrated analyses. Phylogeny continues to imply two separate Caimaninae dispersals between North and South America, but instead of a northward back-dispersal, we find two Palaeogene dispersals to South America an equally likely hypothesis. Miocene taxa of Central America previously assigned to the stem lineage ancestral to South American Caimaninae are reinterpreted as part of a Neogene northward expansion of the crown group.
Dramatic early Cenozoic climatic shifts resulted in faunal reorganization on a global scale. Among vertebrates, multiple groups of mammals (e.g., adapiform and omomyiform primates, mesonychids, taeniodonts, dichobunid artiodactyls) are well known from the Western Interior of North America in the warm, greenhouse conditions of the early Eocene, but a dramatic drop in the diversity of these groups, along with the introduction of more dry-tolerant taxa, occurred near the Eocene–Oligocene boundary. Crocodyliforms underwent a striking loss of diversity at this time as well. Pre-Uintan crocodyliform assemblages in the central Western Interior are characterized by multiple taxa, whereas Chadronian assemblages are depauperate with only Alligator prenasalis previously known. Crocodyliform diversity through the intervening Uintan and Duchesnean is not well understood. The middle Eocene Devil’s Graveyard Formation (DGF) of southwest Texas provides new data from southern latitudes during that crucial period. A new specimen from the middle member of the DGF (late Uintan–Duchesnean) is the most complete cranial material of an alligatorid known from Paleogene deposits outside the Western Interior. We identify this specimen as a caimanine based on notched descending laminae of the pterygoids posterior to the choanae and long descending processes of the exoccipitals that are in contact with the basioccipital tubera. Unlike Eocaiman cavernensis , the anterior palatine process is rounded rather than quadrangular. The relationships and age of this new taxon support the hypothesis that the modern distribution of caimanines represents a contraction of a more expansive early Cenozoic distribution. We hypothesize that the range of caimanines tracked shifting warm, humid climatic conditions that contracted latitudinally toward the hothouse-icehouse transition later in the Eocene.
Morphological and molecular data suggest a close relationship for alligators and caimans. The first fossil appearances combined with phylogenetic hypotheses suggest a divergence of the groups near the Cretaceous–Palaeogene boundary, but the early fossil record of Caimaninae is incomplete, and large gaps exist between the earliest representatives of the group. A new caimanine from lower Palaeocene (Tiffanian) deposits in the Black Peaks Formation of Brewster County, Texas is established upon two specimens of different size that bear similarities to Bottosaurus harlani from the uppermost Cretaceous and lowermost Palaeogene of New Jersey. The larger individual consists of a partial skull and lower jaw in addition to postcranial material. The smaller individual preserves a snout and posterior portions of the skull. Both specimens suggest an animal with a comparatively short, flat, broad snout. Species of Bottosaurus share diagnostic morphological character states but are differentiated in meaningful ways. Phylogenetic analysis shows that the new species is sister to B. harlani, indicates an early radiation of North American caimanines and elucidates a more complicated biogeographical history than previously hypothesized. A growing body of evidence suggests that Caimaninae may be diagnosed by ancestral characters, potentially drawing basal alligatoroids crownwards in phylogenetic trees.
Caimaninae is one of the few crocodylian lineages that still has living representatives. Today, most of its six extant species are restricted to South and Central America. However, recent discoveries have revealed a more complex evolutionary history, with a fossil record richer than previously thought and a possible North American origin. Among the oldest caimanines is Eocaiman cavernensis, from the Eocene of Patagonia, Argentina. It was described by George G. Simpson in the 1930s, representing the first caimanine reported for the Palaeogene. Since then, E. cavernensis has been ubiquitous in phylogenetic studies on the group, but a more detailed morphological description and revision of the taxon were lacking. Here, we present a reassessment of E. cavernensis, based on first-hand examination and micro-computed tomography of the holotype, and reinterpret different aspects of its morphology. We explore the phylogenetic affinities of E. cavernensis and other caimanines using parsimony and Bayesian inference approaches. Our results provide evidence for a monophyletic Eocaiman genus within Caimaninae, even though some highly incomplete taxa (including the congeneric Eocaiman itaboraiensis) represent significant sources of phylogenetic instability. We also found Culebrasuchus mesoamericanus as sister to all other caimanines and the North American globidontans (i.e. Brachychampsa and closer relatives) outside Caimaninae. A time-calibrated tree, obtained using a fossilized birth-death model, shows a possible Campanian origin for the group (76.97 ± 6.7 Ma), which is older than the age estimated using molecular data, and suggests that the earliest cladogenetic events of caimanines took place rapidly and across the K-Pg boundary.
Alligatoroidea is the most species-rich crocodylomorph clade of the Cenozoic of South America, with nearly all species belonging to the Caimaninae clade. However, the earliest records of Caimaninae in South America, which are from the Palaeocene, are based mostly on incomplete specimens, which increases the importance of detailed taxonomic and phylogenetic studies on these taxa. This paper offers a taxonomic and phylogenetic review of Necrosuchus ionensis, a caimanine species from the Salamanca Formation of the Palaeocene of Argentina. Necrosuchus ionensis is considered a valid species, albeit with a different diagnosis from that proposed by previous authors. The phylogenetic analysis shows, for the first time, that N. ionensis belongs to the derived Caimaninae clade Jacarea. However, a better understanding of the Jacarea clade is needed, and alternative placements for N. ionensis might be considered. Nevertheless, the placement of N. ionensis as a derived caimanine raises interesting perspectives on the early evolution and radiation of caimanines, which are thoroughly discussed in this paper together with other results obtained in this study, such as the recovery of the North American caimanines Bottosaurus and Tsoabichi as a clade.
Cranial and postcranial elements attributed to the alligatoroid Bottosaurus harlani Meyer, 1832 Meyer, H. von. 1832. Paleologica zur Geshichte der Erde und irher Geschöpfe. S. Schmerber, Frankurt-am-Main, 560 pp. [Google Scholar], are described from the Rowan Fossil Quarry, a Cretaceous–Paleogene locality in Mantua Township, New Jersey, U.S.A. This specimen (NJSM 11265) represents the most complete individual attributable to the species and includes significant postcranial elements not found in other specimens. Posterior elements of the skull are described for the first time, and the species is placed into a phylogenetic context. Aspects of the skull table, including constricted supratemporal fenestrae, a linear frontoparietal suture, and a large trapezoidal dorsal supraoccipital exposure, are similar to those of caimans. Bottosaurus harlani is included within Caimaninae due to its possession of a linear frontoparietal suture between the supratemporal fenestrae. Optimal trees from phylogenetic analysis recover B. harlani in three different positions, as a sister either to the modern dwarf caimans (Paleosuchus), to living species of Paleosuchus, or to both. That a substantial stratigraphic gap separates B. harlani from both species of Paleosuchus, which first appears in the Miocene, along with low character and nodal support, raises questions about this relationship. The phylogenetic placement of B. harlani may reflect incomplete knowledge of the taxon, and further study might support placement outside of Caimaninae. More complete analyses of B. harlani and other Cretaceous–Paleogene alligatoroids will help to illuminate the relationships among these forms and their living relatives.
Citation for this article: Cossette, A. P., and C. A. Brochu. 2018. A new specimen of the alligatoroid Bottosaurus harlani and the early history of character evolution in alligatorids. Journal of Vertebrate Paleontology. DOI:10.1080/02724634.2018.1486321.
Caimanines are crocodylians currently restricted to South and Central America and the oldest members are from lower Palaeocene localities of the Salamanca Formation (Chubut Province, Argentina). We report here a new caimanine from this same unit represented by a skull roof and partial braincase. Its phylogenetic relationships were explored in a cladistic analysis using standard characters and a morphogeometric two-dimensional configuration of the skull roof. The phylogenetic results were used for an event-based supermodel quantitative palaeobiogeographic analysis. The new species is recovered as the most basal member of the South American caimanines, and the Cretaceous North American lineage 'Brachychampsa and related forms' as the most basal Caimaninae. The biogeographic results estimated north-central North America as the ancestral area of Caimaninae, showing that the Cretaceous and Palaeocene species of the group were more widespread than thought and became regionally extinct in North America around the Cretaceous-Palaeocene boundary. A dispersal event from north-central North America during the middle Late Cretaceous explains the arrival of the group to South America. The Palaeogene assemblage of Patagonian crocodylians is composed of three lineages of caimanines as a consequence of independent dispersal events that occurred between North and South America and within South America around the Cretaceous-Palaeogene boundary.
The fossil record of Alligatoroidea in Argentina is mainly represented by Caimaninae alligatorids. This lineage recorded two important
moments in its natural history, one at the beginning of the Paleogene (Late Paleocene–Middle Eocene) and the other in the Neogene (Late
Miocene). The most ancient record of alligatoroids in South America comes from the Early Paleocene of Patagonia. It includes basal forms of
caimanines such as Necrosuchus ionensis, Eocaiman palaeocenicus, Eocaiman cavernensis, and probably a new species of alligatoroid, which
provide key morphological information on the evolutionary and biogeographic history of these crocodylians. Another important moment of the
evolutionary history of caimanines is the diversification of the lineage observed during the Miocene. Although there is some isolated cranial
material of caimanines from the Late Miocene of northwestern Argentina, the most abundant and diverse fossil Miocene material comes from
the northeast, from a level informally called “Conglomerado Osífero”. Two genera of caimanines are known from this stratigraphic level
(Caiman and Mourasuchus) with at least five valid species. Here we present an update of the knowledge of Cenozoic alligatorids of Argentina,
as a tribute to Zulma Gasparini for her invaluable contribution to the understanding of the crocodilian evolution in South America.
Amazonia contains one of the world's richest biotas, but origins of this diversity remain obscure. Onset of the Amazon River drainage at approximately 10.5 Ma represented a major shift in Neotropical ecosystems, and proto-Amazonian biotas just prior to this pivotal episode are integral to understanding origins of Amazonian biodiversity, yet vertebrate fossil evidence is extraordinarily rare. Two new species-rich bonebeds from late Middle Miocene proto-Amazonian deposits of northeastern Peru document the same hyperdiverse assemblage of seven co-occurring crocodylian species. Besides the large-bodied Purussaurus and Mourasuchus, all other crocodylians are new taxa, including a stem caiman-Gnatusuchus pebasensis-bearing a massive shovel-shaped mandible, procumbent anterior and globular posterior teeth, and a mammal-like diastema. This unusual species is an extreme exemplar of a radiation of small caimans with crushing dentitions recording peculiar feeding strategies correlated with a peak in proto-Amazonian molluscan diversity and abundance. These faunas evolved within dysoxic marshes and swamps of the long-lived Pebas Mega-Wetland System and declined with inception of the transcontinental Amazon drainage, favouring diversification of longirostrine crocodylians and more modern generalist-feeding caimans. The rise and demise of distinctive, highly productive aquatic ecosystems substantially influenced evolution of Amazonian biodiversity hotspots of crocodylians and other organisms throughout the Neogene.
© 2015 The Author(s) Published by the Royal Society. All rights reserved.
An appraisal of Paleogene floral and land mammal faunal dynamics in South America suggests that both biotic elements responded at rate and extent generally comparable to that portrayed by the global climate pattern of the interval. A major difference in the South American record is the initial as well as subsequent much greater diversity of both Neotropical and Austral floras relative to North American counterparts. Conversely, the concurrent mammal faunas in South America did not match, much less exceed, the diversity seen to the north. It appears unlikely that this difference is solely due to the virtual absence of immigrants subsequent to the initial dispersal of mammals to South America, and cannot be explained solely by the different collecting histories of the two regions. Possible roles played by non-mammalian vertebrates in niche exploitation remain to be explored.
The Paleogene floras of Patagonia and Chile show a climatic pattern that approximates that of North America, with an increase in both Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) from the Paleocene into the Early Eocene Climatic Optimum (EECO), although the Paleocene-Eocene Thermal Maximum (PETM) is not recognized in the available data set. Post-EECO temperatures declined in both regions, but more so in the north than the south, which also retained a higher rate of precipitation.
The South American Paleogene mammal faunas developed gradual, but distinct, changes in composition and diversity as the EECO was approached, but actually declined somewhat during its peak, contrary to the record in North America. At about 40 Ma, a post-EECO decline was recovered in both hemispheres, but the South American record achieved its greatest diversity then, rather than at the peak of the EECO as in the north. This post-EECO faunal turnover apparently was a response to the changing conditions when global climate was deteriorating toward the Oligocene. Under the progressively more temperate to seasonally arid conditions in South America, this turnover reflected a major change from the more archaic, and more tropical to subtropical-adapted mammals, to the beginning of the ultimately modern South American fauna, achieved completely by the Eocene-Oligocene transition. Interestingly, hypsodonty was achieved by South American cursorial mammals about 15–20 m.y. earlier than in North America. In addition to being composed of essentially different groups of mammals, those of the South American continent seem to have responded to the climatic changes associated with the ECCO and subsequent conditions in a pattern that was initially comparable to, but subsequently different from, their North American counterparts.
A new small species of Eocaiman is described on the basis of three anterior left mandibular rami and one isolated tooth. The specimens came from the middle-upper Paleocene Itaboraí Basin (Rio de Janeiro State, Brazil; Itaboraian South American Land Mammal Age). The new taxon differs from the other two Eocaiman species, such as its small size, likely participation of the splenial in the mandibular symphysis, a reduced angle between the longitudinal axis of the symphysis and the mandibular ramus, and enlarged ninth and tenth dentary teeth (in addition to the large first and fourth dentary teeth). The participation of the splenial in the mandibular symphysis is a unique character among caimanines (with the only possible exception being Tsoabichi greenriverensis). The new taxon provides new information on the taxonomic and anatomical diversity of the genus Eocaiman, a taxon of prime importance to understand the evolutionary origins of caimans given its position as the basalmost member of Caimaninae. Furthermore, the new taxon has a relatively small body size in comparison with other species of Eocaiman, a case paralleled by other Itaboraian reptilian groups (e.g. snakes), suggesting that this ecosystem provides critical data to test the relationship between reptilian body size and climate.
Few South American macrofloras of Paleocene age are known, and this
limits our knowledge of diversity and composition between the
end-Cretaceous event and the Eocene appearance of high floral diversity.
We report new, unbiased collections of 2516 compression specimens from
the Paleocene Salamanca Formation (ca. 61.7 Ma) from two localities in
the Palacio de los Loros exposures in southern Chubut, Patagonia,
Argentina. Our samples reveal considerably greater richness than was
previously known from the Paleocene of Patagonia, including 36 species
of angiosperm leaves as well as angiosperm fruits, flowers, and seeds;
ferns; and conifer leaves, cones, and seeds. The floras, which are from
siltstone and sandstone channel-fills deposited on low-relief floodplain
landscapes in a humid, warm temperate climate, are climatically and
paleoenvironmentally comparable to many quantitatively collected
Paleocene floras from the Western Interior of North America. Adjusted
for sample size, there are >50% more species at each Palacio de los
Loros quarry than in any comparable U.S. Paleocene sample. These results
indicate more vibrant terrestrial ecosystems in Patagonian than in North
American floodplain environments ˜4 m.y. after the end-Cretaceous
extinction, and they push back the time line 10 m.y. for the evolution
of high floral diversity in South America. The cause of the dis parity
is unknown but could involve reduced impact effects because of greater
distance from the Chicxulub site, higher latest Cretaceous diversity, or
faster recovery or immigration rates.
Three new specimens of the rare alligatoroid Brachychampsa montana from the upper Campanian Kirtland Formation (De-na-zin Member), San Juan Basin, New Mexico, are the first bona fide examples of this taxon from this stratigraphic interval. The most complete specimen (SMP VP-1312) consists of much of the skull table and snout and a nearly complete mandible. SMP VP-1264 consists solely of an incomplete partial skull table (parietal, right squamosal), partial basicranium (basioccipital, otoccipital), and incomplete right quadrate, whereas NMMNH P-7988 is an isolated frontal. Together, these new specimens supplement and allow for further comparison to other globidontans and to previously documented specimens of Brachychampsa.?Brachychampsa sealeyi, from the Menefee Formation of the San Juan Basin, New Mexico, is based on a juvenile specimen that cannot be distinguished from B. montana, so it is a subjective junior synonym. The crocodylian assemblage of the Kirtland Formation consists of Brachychampsa montana, Leidyosuchus sp. and Denazinosuchus kirtlandicus. It is doubtful that tooth morphology alone can be correlated with “cheloniphagous” diet within the globidontans.
Tsoabichi greenriverensis, tax. nov., is based on cranial and postcranial remains from the lower Eocene (Wasatchian) Green River Formation of Wyoming. Casts of additional material in private collections include the complete skeleton of an immature specimen. It has abrupt supratemporal fenestral rims, extensive supraoccipital exposure on the skull table, and bipartite ventral osteoderms. The nasals are reduced to a thin anterior process as they approach the external naris, and the rim of the naris extends beyond the dorsal surface of the snout as a thin crest. There was no ‘spectacle’ between the orbits, but there are three distinct craniocaudally elongate ridges between the orbits. The parietal extends to the posterior margin of the skull table on either side of the large V-shaped dorsal exposure of the supraoccipital. Some of the dorsal osteoderms are mediolaterally elongate and bear two dorsal keels. A close relationship with caimans is supported by a parsimony analysis, although its precise position is unclear and some of the characters supporting a caimanine relationship also occur in poorly known Late Cretaceous and Early Tertiary alligatoroids. A possible close relationship with Paleosuchus (the living dwarf caimans) would imply a very complex biogeographic scenario with multiple dispersals between the Americas during the Tertiary.
Several aspects of current resampling methods to assess group support are reviewed. When the characters have different prior weights or some state transformation costs are different, the frequencies under either bootstrapping or jackknifing can be distorted, producing either under- or overestimations of the actual group support. This is avoided by symmetric resampling, where the probability p of increasing the weight of a character equals the probability of decreasing it. Problems with interpreting absolute group frequencies as a measure of the support are discussed; group support does not necessarily vary with the frequency itself, since in some cases groups with positive support may have much lower frequencies than groups with no support at all. Three possible solutions for this problem are suggested. The first is measuring the support as the difference in frequency between the group and its most frequent contradictory group. The second is calculating frequencies for values of p below the threshold u
INTRODUCCIÓN Desde principios del siglo pasado, las unidades del Terciario inferior de la Pa-tagonia Central fueron ampliamente es-tudiadas debido a su importante conteni-do en fósiles. En especial, el actual Gru-po Río Chico (Paleoceno Superior-Eo-ceno Medio), cuyos depósitos de origen continental afloran en el ámbito de la cuenca del Golfo San Jorge (Figs. 1a y 1b), contó con numerosas investigacio-nes paleontológicas dada su abundancia en mamíferos fósiles (Ameghino 1906,, entre otros). Sin embargo, los aspectos sedimentológi-cos y estratigráficos del Grupo Río Chi-co fueron sólo someramente abordados por Andreis et al. (1975), Andreis (1977) y Legarreta y Uliana (1994), quienes plan-tearon dos esquemas estratigráficos dife-rentes. Ambos esquemas muestran una amplia variación en la definición de sus límites, tanto internos como inferior y su-perior (Fig. 2). Al mismo tiempo, la au-sencia de dataciones radio-isotópicas pa-ra el Grupo Río Chico conduce a que su edad sea poco precisa, sustentándose su asignación cronológica sólo en la rela-ción con las unidades colindantes y en la información de paleovertebrados. A partir de esta situación surge la necesi-dad de uniformar los esquemas
A common problem in phylogenetic analysis is the presence of unstable taxa that are depicted in multiple positions in optimal topologies. These uncertainties are reflected in strict consensus trees with polytomies that hamper the interpretation of the phylogenetic results. We propose a protocol for detecting unstable branches (either terminal taxa or clades) and identifying particular characters related to their instability in cladistic analysis. This procedure is based on an iterative evaluation of the agreement of triplets among the optimal topologies (i.e. most-parsimonious trees, MPTs) and examination of character optimizations on these trees. Different types of characters underlying the unstable behaviour of taxa are detected: those with conflicting scorings that support alternative positions of problematic taxa and those with missing data in the unstable taxa that could reduce their instability if they are scored. The entire process is automated through a TNT script that provides a list of characters related to the instability of each unstable taxon. The outcome of this procedure can be used as a guide for further research efforts focused on the revision or addition of (morphological or molecular) phylogenetic data for elucidating the affinities of unstable taxa.
© The Willi Hennig Society 2009.
posed the problem abstractly, a single character on one tree, addressed only one horn of the dilemma,(branches ambiguously,of zero or greater length were always assigned zero length), considered solutions only within the philosophical frame- work offered by PAUP, and offered no advice regarding what practicing system- atists could do to detect the problem. In addition, the treatment of the zero-length branch problem by programmers is already more diverse than reported by Wilkinson (1995), and users of the most common packages should be aware of the different interpretations adopted by programmers. Wilkinson (1995) offered some algorithmic suggestions for a solution, but his solution, although helpful, strikes us as less than ideal. We also focus here on what practicing systemadsts can do now to detect and avoid the problem. Finally, the solution is not necessarily as simple as eliminadng "arbitrary resolutions". Most importandy, Wilkinson (1995) did not consider what happens if branches of ambiguous,length are permitted to be non-zero rather than zero-length. A more fundamental issue is the meaning,of "acceptable support" for phylogenedc,hypoth- eses, and therefore which hypotheses are worth considering, as Platnick et al. (1991) discussed. That debate is just beginning, but should involve the user com- munity in addidon to programmers and methodologists. Table 1 conveys the problem of zero-length branches more concretely than the
Alligatoroidea is the most species-rich crocodylomorph clade of the Cenozoic of South America, with nearly all species belonging to the Caimaninae clade. However, the earliest records of Caimaninae in South America, which are from the Palaeocene, are based mostly on incomplete specimens, which increases the importance of detailed taxonomic and phylogenetic studies on these taxa. This paper offers a taxonomic and phylogenetic review of Necrosuchus ionensis, a caimanine species from the Salamanca Formation of the Palaeocene of Argentina. Necrosuchus ionensis is considered a valid species, albeit with a different diagnosis from that proposed by previous authors. The phylogenetic analysis shows, for the first time, that N. ionensis belongs to the derived Caimaninae clade Jacarea. However, a better understanding of the Jacarea clade is needed, and alternative placements for N. ionensis might be considered. Nevertheless, the placement of N. ionensis as a derived caimanine raises interesting perspectives on the early evolution and radiation of caimanines, which are thoroughly discussed in this paper together with other results obtained in this study, such as the recovery of the North American caimanines Bottosaurus and Tsoabichi as a clade.
Melanosuchus niger (Crocodylia, Alligatoroidea) is one of the six living caimanine species widely distributed throughout the Amazon River basin today. Although there is only one extant species of Melanosuchus, fossil material assigned to this genus, represented by M. fisheri, has been reported from the late Miocene in South America. However, the validity of this taxon has been questioned and a recent investigation indicates that the referred specimen of M. fisheri (MCZ 4336) actually belongs to Globidentosuchus brachyrostris, while those diagnostic characters present in the holotype (MCNC 243) fall into the spectrum of intraspecific variation of M. niger. Here, we compare the skull shape of the holotype of M. fisheri with the ontogenetic series of the four jacarean species (M. niger, Caiman yacare, Caiman crocodilus, and Caiman latirostris) using 2D-geometric morphometric analyses in two different views. The analyses indicate that MCNC 243 falls into the morphospace of M. niger and C. latirostris. Despite strong shape similarities between juveniles of C. latirostris and MCNC 243, further anatomical comparisons reveal notable differences between them. In contrast, no concrete anatomical differences can be found between MCNC 243 and M. niger, although shape analyses indicate that MCNC 243 is relatively robust for its size. Thus, this study is able to confirm that the genus Melanosuchus was present in the late Miocene, but it still remains unclear if MCNC 243 should be treated as a junior synonym or probably a sister species of M. niger. Its Miocene age favors the second option, but as the shape analyses were also not able to extract any diagnostic characters, it should be retained as Melanosuchus cf. niger.
Melanosuchus Niger Spix is distributed throughout the Amazon River basin today. The extinct Melanosuchus fisheri Medina from the late Miocene of Venezuela was erected based on two almost complete, but heavily deformed skulls (the holotype MCNC 243 and the referred specimen MCZ 4336), which show morphological differences from each other. The comparison indicates that only the holotype can be referred to Melanosuchus Gray. We propose MCZ 4336 is a representative of the caimanine Globidentosuchus brachyrostris Scheyer, Aguilera, Delfino, Fortier, Carlini, Sánchez, Carrillo-Briceño, Quiroz and Sãnchez-Villagra. Although the taxonomy of M. fisheri is taken into question herein, the classification of the holotype still sustains the hypothesis that the genus is registered in South America since the late Miocene.
Version 1.5 of the computer program TNT completely integrates landmark data into phylogenetic analysis. Landmark data consist of coordinates (in two or three dimensions) for the terminal taxa; TNT reconstructs shapes for the internal nodes such that the difference between ancestor and descendant shapes for all tree branches sums up to a minimum; this sum is used as tree score. Landmark data can be analysed alone or in combination with standard characters; all the applicable commands and options in TNT can be used transparently after reading a landmark data set. The program continues implementing all the types of analyses in former versions, including discrete and continuous characters (which can now be read at any scale, and automatically rescaled by TNT). Using algorithms described in this paper, searches for landmark data can be made tens to hundreds of times faster than it was possible before (from T to 3T times faster, where T is the number of taxa), thus making phylogenetic analysis of landmarks feasible even on standard personal computers.
The several recorded foraminiferal assemblages from the South American and African continents and those of the South Atlantic Ocean are summarized in the present work.
A new alligatorine eusuchian, Stangerochampsa mccabei gen. et sp. nov., is described on the basis of a partial skeleton from the Horseshoe Canyon Formation (Early Maastrichtian) of southern Alberta. It is unique in possessing an ectopterygoid/palatine contact, a ventrolateral process of the quadrate, a groove-like recess for nerves and blood vessels in the upper jaw, a rectangular palatine with a lateral process at its midpoint, and a basioccipital with a ventral exposure longer than that of the pterygoid. Several derived characters indicate a close relationship of S. mccabei with two Late Cretaceous alligatorines, Brachychampsa montana and Albertochampsa langstoni. A preliminary phylogenetic analysis, based on 46 characters of selected taxa, leads to the hypothesis that Leidyosuchus, rather than Hylaeochampsa, is the most primitive eusuchian, supports the monophyly of the Alligatorinae (with the exclusion of Prodiplocynodon), and suggests that the Alligatorinae may consist of at least two distinct assemblages.
Cranial fragments associated with the holotype of Necrosuchus ionensis reveal a dorsally shifted foramen aereum on the quadrate and a long, slender descending process of the exoccipital lateral to the basioccipital and approaching the basioccipital tubera. The former suggests that Necrosuchus is an alligatoroid and not a crocodylid, as first suggested; and the latter that it is a caiman. The scapulocoracoid shows evidence of early closure of the synchondrosis, further supporting a caiman affinity. Although we cannot yet pinpoint the phylogenetic placement of Necrosuchus amongst caimans, it nevertheless establishes a caimanine presence in South America by the Early Palaeocene. A review of other Palaeocene–Eocene caimans reveals a complex biogeographical history suggesting multiple dispersal events between North and South America, even if the modern caiman assemblage is monophyletic.
© 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163, S228–S256.
Alligatoroidea, a stem-based group including living alligators and caimans and all taxa closer to them than to Crocodylus or Gavialis, includes seven living species and a large diversity of extinct taxa extending back to the Campanian (Late Cretaceous). Parsimony analysis of 164 discrete morphological characters supports several previous hypotheses based on morphology and molecules: Diplocynodon, from the European Tertiary, is a monophyletic alligatoroid lineage; Brachychampsa and Stangerochampsa are derived alligatoroid taxa but not demonstrably within the crown-group Alligatoridae; and, within the crown-group, caimans form a robust clade. However, this study indicates the paraphyly of “Allognathosuchus;” the alligatoroid affinities of Leidyosuchus canadensL but not of most other “Leidyosuchus;” the very distant relationship between Hylaeochampsa and any extant crocodylian lineage, including Alligatoroidea; and the fact that Diplocynodon must extend at least as far as the Campanian, even though the oldest well-known Diplocynodon are from the Eocene. This work supports a close relationship between Diplocynodon and Baryphracta deponiae from the Eocene of Germany; monophyly of Alligator and the caiman assemblage as currently applied in the literature; the caiman affinities of the bizarre nettosuchids and Purussaurus; and a closer relationship between Caiman latirostris and Melanosuchus niger than between C. latirostris and other Caiman.
Phylogenetic taxonomic principles have been applied to Crocodylia by several workers, and this study expands upon this with a discussion of phylogeny-based name definitions for Crocodylia and nested subgroups. Leidyosuchus is the basalmost alligatoroid, but within its sister taxon we can recognize a pair of stem-based groups—Diplocynodontinae (Diplocynodon rateili and all taxa closer to it than to Alligator mississippiensis) and Globidonta (Alligator mississippiensis and all taxa closer to it than to Diplocynodon rateili). The crown-group Alligatoridae (last common ancestor of Alligator, Caiman, Melanosuchus, and Paleosuchus and all of its descendents) can likewise be split into a pair of stem-based daughter lineages: Alligatorinae (Alligator and taxa closer to it than to Caiman) and Caimaninae (Caiman and taxa closer to it than to Alligator).
Salt intolerance has been used by previous authors to force alligatorid dispersal along land bridges. Despite the problem of inappropriate climate, a land crossing across Beringia during the Late Tertiary best explains the current existence of an Asiatic alligatorid (Alligator sinensis), but based on known fossil occurrences, caimans and alligators diverged from each other in North America sometime during the Late Cretaceous and spread to South America by the Paleocene, when North and South America were separated. A single dispersal event at or near the Cretaceous-Tertiary Boundary is sufficient to explain the presence of caimans in South America, with a reinvasion from South America explaining the presence of a nettosuchid in the North American Eocene (Orthogenysuchus).
The main features of the phylogeny program TNT are discussed. Windows versions have a menu interface, while Macintosh and Linux versions are command-driven. The program can analyze data sets with discrete (additive, non-additive, step-matrix) as well as continuous characters (evaluated with Farris optimization). Effective analysis of large data sets can be carried out in reasonable times, and a number of methods to help identifying wildcard taxa in the case of ambiguous data sets are implemented. A variety of methods for diagnosing trees and exploring character evolution is available in TNT, and publication-quality tree-diagrams can be saved as metafiles. Through the use of a number of native commands and a simple but powerful scripting language, TNT allows the user an enormous flexibility in phylogenetic analyses or simulations.
© The Willi Hennig Society 2008.
Caroli a Linné, Systema Naturae per Regna Tria Naturae, Secudum Classes, Ordines, Genera, Species, cum characteribus, differentiis, synonymis, locis
- J Gmelin
Gmelin, J. (1789). Caroli a Linné, Systema Naturae per Regna Tria
Naturae, Secudum Classes, Ordines, Genera, Species, cum characteribus, differentiis, synonymis, locis. G. E. Beer.
Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America. University of California Publications in Geological Sciences
- W Langston
- Jr
Langston, W., Jr. (1965). Fossil crocodilians from Colombia and the
Cenozoic history of the Crocodilia in South America. University
of California Publications in Geological Sciences, 52, 1-157.
Nuevo aligatorino del Paleoceno argentino
- C Rusconi
Rusconi, C. (1937). Nuevo aligatorino del Paleoceno argentino.
Boletín Paleontológico de Buenos Aires, 8, 1-5.
A new crocodilian from the Notostylops beds of Patagonia
- G G Simpson
Simpson, G. G. (1933). A new crocodilian from the Notostylops beds
of Patagonia. American Museum Novitates, 623, 1-9.
PAUP: Phylogenetic analysis using parsimony, Version 3.1
- D L Swofford
- D P Begle
Swofford, D. L., & Begle, D. P. (1993). PAUP: Phylogenetic analysis
using parsimony, Version 3.1. Illinois Natural History Survey,
Champaign, Illinois.
Obra Centenario del Museo de La Plata
- R R Andreis
Revisión de los Crocodilia (Reptilia) fósiles del Territorio Argentino: su evolución, sus relaciones filogenéticas, su clasificación y sus implicancias estratigráficas
- Z Gasparini