Nathan A. Jud

Holocene shifts in the assembly of plant and animal communities implicate human impacts.

Understanding how ecological communities are organized and how they change through time is critical to predicting the effects of climate change. Recent work documenting the co-occurrence structure of modern communities found that most significant species pairs co-occur less frequently than would be expected by chance. However, little is known about how co-occurrence structure changes through time. Here we evaluate changes in plant and animal community organization over geological time by quantifying the co-occurrence structure of 359,896 unique taxon pairs in 80 assemblages spanning the past 300 million years. Co-occurrences of most taxon pairs were statistically random, but a significant fraction were spatially aggregated or segregated. Aggregated pairs dominated from the Carboniferous period (307 million years ago) to the early Holocene epoch (11,700 years before present), when there was a pronounced shift to more segregated pairs, a trend that continues in modern assemblages. The shift began during the Holocene and coincided with increasing human population size and the spread of agriculture in North America. Before the shift, an average of 64% of significant pairs were aggregated; after the shift, the average dropped to 37%. The organization of modern and late Holocene plant and animal assemblages differs fundamentally from that of assemblages over the past 300 million years that predate the large-scale impacts of humans. Our results suggest that the rules governing the assembly of communities have recently been changed by human activity.Understanding how ecological communities are organized and how they change through time is critical to predicting the effects of climate change. Recent work documenting the co-occurrence structure of modern communities found that most significant species pairs co-occur less frequently than would be expected by chance. However, little is known about how co-occurrence structure changes through time. Here we evaluate changes in plant and animal community organization over geological time by quantifying the co-occurrence structure of 359,896 unique taxon pairs in 80 assemblages spanning the past 300 million years. Co-occurrences of most taxon pairs were statistically random, but a significant fraction were spatially aggregated or segregated. Aggregated pairs dominated from the Carboniferous period (307 million years ago) to the early Holocene epoch (11,700 years before present), when there was a pronounced shift to more segregated pairs, a trend that continues in modern assemblages. The shift began during the Holocene and coincided with increasing human population size and the spread of agriculture in North America. Before the shift, an average of 64% of significant pairs were aggregated; after the shift, the average dropped to 37%. The organization of modern and late Holocene plant and animal assemblages differs fundamentally from that of assemblages over the past 300 million years that predate the large-scale impacts of humans. Our results suggest that the rules governing the assembly of communities have recently been changed by human activity.

Pennsylvanian coniferopsid forests in sabkha facies reveal the nature of seasonal tropical biome

Pennsylvanian fossil forests are known from hundreds of sites across tropical Pangea, but nearly all comprise remains of humid Coal Forests. Here we report a unique occurrence of seasonally dry vegetation, preserved in growth position along >5 km of strike, in the Pennsylvanian (early Kasimovian, Missourian) of New Mexico (United States). Analyses of stump anatomy, diameter, and spatial density, coupled with observations of vascular traces and associated megaflora, show that this was a deciduous, mixed-age, coniferopsid woodland (∼100 trees per hectare) with an open canopy. The coniferopsids colonized coastal sabkha facies and show tree rings, confirming growth under seasonally dry conditions. Such woodlands probably served as the source of coniferopsids that replaced Coal Forests farther east in central Pangea during drier climate phases. Thus, the newly discovered woodland helps unravel biome-scale vegetation dynamics and allows calibration of climate models.

First North American fossil monkey and early Miocene tropical biotic interchange

New World monkeys (platyrrhines) are a diverse part of modern tropical ecosystems in North and South America, yet their early evolutionary history in the tropics is largely unknown. Molecular divergence estimates suggest that primates arrived in tropical Central America, the southern-most extent of the North American landmass, with several dispersals from South America starting with the emergence of the Isthmus of Panama 3–4 million years ago (Ma). The complete absence of primate fossils from Central America has, however, limited our understanding of their history in the New World. Here we present the first description of a fossil monkey recovered from the North American landmass, the oldest known crown platyrrhine, from a precisely dated 20.9-Ma layer in the Las Cascadas Formation in the Panama Canal Basin, Panama. This discovery suggests that family-level diversification of extant New World monkeys occurred in the tropics, with new divergence estimates for Cebidae between 22 and 25 Ma, and provides the oldest fossil evidence for mammalian interchange between South and North America. The timing is consistent with recent tectonic reconstructions of a relatively narrow Central American Seaway in the early Miocene epoch, coincident with over-water dispersals inferred for many other groups of animals and plants. Discovery of an early Miocene primate in Panama provides evidence for a circum-Caribbean tropical distribution of New World monkeys by this time, with ocean barriers not wholly restricting their northward movements, requiring a complex set of ecological factors to explain their absence in well-sampled similarly aged localities at higher latitudes of North America.

Todea from the Lower Cretaceous of western North America: implications for the phylogeny, systematics, and evolution of modern Osmundaceae

The first fossil evidence for the fern genus Todea has been recovered from the Lower Cretaceous of British Columbia, Canada, providing paleontological data to strengthen hypotheses regarding patterns of evolution and phylogeny within Osmundaceae. The fossil consists of a branching rhizome, adventitious roots, and leaf bases. The dictyoxylic stem has up to eight xylem bundles around a sclerenchymatous pith. Leaf traces diverge from cauline bundles in a typical osmundaceous pattern and leaf bases display a sheath of sclerenchyma around a C-shaped xylem trace with 2-8 protoxylem strands. Within the adaxial concavity of each leaf trace, a single sclerenchyma bundle becomes C-shaped as it enters the cortex. The sclerotic cortex is heterogeneous with an indistinct outer margin. The discovery of Todea tidwellii sp. nov. reveals that the genus Todea evolved by the Lower Cretaceous. A phylogenetic analysis combining morphological characters of living and extinct species with a previously published nucleotide sequence matrix confirms the taxonomic placement of T. tidwellii. Results also support the hypothesis that Osmunda s.l. represents a paraphyletic assemblage and that living species be segregated into two genera, Osmunda and Osmundastrum. Fossil evidence confirms that Osmundaceae originated in the Southern Hemisphere during the Permian, underwent rapid diversification, and species extended around the world during the Triassic. Crown group Osmundaceae originated by the Late Triassic, with living species appearing by the Late Cretaceous.

Potomacapnos apeleutheron gen. et sp. nov., a new Early Cretaceous angiosperm from the Potomac Group and its implications for the evolution of eudicot leaf architecture

PREMISE OF THE STUDY Eudicots diverged early in the evolution of flowering plants and now comprise more than 70% of angiosperm species. In spite of the importance of eudicots, our understanding of the early evolution of this clade is limited by a poor fossil record and uncertainty about the order of early phylogenetic branching. The study of Lower Cretaceous fossils can reveal much about the evolution, morphology, and ecology of the eudicots. METHODS Fossils described here were collected from Aptian sediments of the Potomac Group exposed at the Dutch Gap locality in Virginia, USA. Specimens were prepared by degaging, then described and compared with leaves of relevant extant and fossil plants. We conducted a phylogenetic analysis of morphological characters using parsimony while constraining the tree search with the topology found through molecular phylogenetic analyses. KEY RESULTS The new species is closely related to ranunculalean eudicots and has leaf architecture remarkably similar to some living Fumarioideae (Papaveraceae). CONCLUSIONS These are the oldest eudicot megafossils from North America, and they show complex leaf architecture reflecting developmental pathways unique to extant eudicots. The morphology and small size of the fossils suggest that they were herbaceous plants, as is seen in other putative early eudicots. The absence of co-occurring tricolpate pollen at Dutch Gap either (1) reflects low preservation probability for pollen of entomophilous herbs or (2) indicates that some leaf features of extant eudicots appeared before the origin of tricolpate pollen.

Paleoecological and Phylogenetic Implications of Saxicaulis meckertii gen. et sp. nov.: A Bennettitalean Stem from the Upper Cretaceous of Western North America

A new anatomically preserved bennettitalean stem has been recovered from the Upper Cretaceous (Coniacian) Eden Main locality on Vancouver Island, British Columbia, Canada. The fossil, described as Saxicaulis meckertii gen. et sp. nov., is permineralized and consists of a eustelic stem with diverging nongirdling leaf traces, a narrow zone of dense wood, primary cortex, and adventitious roots. Important vegetative characters that differentiate bennettitalean stems from cycad stems are reviewed, and while the anatomy of the stem conforms to Bennettitales, it is not consistent with either the Williamsoniaceae or the Cycadeoidaceae as they are currently understood. This fossil documents greater structural diversity among Cretaceous bennettitaleans than previously known and tentatively documents an additional growth habit (underground stem). Co‐occurring fossils and plant structure suggest this plant may have inhabited an environment prone to fires. Additionally, there is evidence for two forms of plant‐animal interaction in the fossil. The first is a large gallery in the pith lined with wound reaction tissue and accessed through a 3‐mm hole on the surface of the stem that represents a new type of herbivore damage for Bennettitales. Reaction tissue occurs in rings and plates throughout the plant body. Smaller galleries containing coprolites of oribatid mites and lacking wound reaction tissue are present in the cortex. The complexity of Upper Cretaceous bennettitalean plant‐animal interactions through herbivory and detritivory is evaluated.

Integrated Chronology, Flora and Faunas, and Paleoecology of the Alajuela Formation, Late Miocene of Panama.

The late Miocene was an important time to understand the geological, climatic, and biotic evolution of the ancient New World tropics and the context for the Great American Biotic Interchange (GABI). Despite this importance, upper Miocene deposits containing diverse faunas and floras and their associated geological context are rare in Central America. We present an integrated study of the geological and paleontological context and age of a new locality from Lago Alajuela in northern Panama (Caribbean side) containing late Miocene marine and terrestrial fossils (plants, invertebrates, and vertebrates) from the Alajuela Formation. These taxa indicate predominantly estuarine and shallow marine paleoenvironments, along with terrestrial influences based on the occurrence of land mammals. Sr-isotope ratio analyses of in situ scallop shells indicate an age for the Alajuela Formation of 9.77 ± 0.22 Ma, which also equates to a latest Clarendonian (Cl3) North American Land Mammal Age. Along with the roughly contemporaneous late Miocene Gatun and Lago Bayano faunas in Panama, we now have the opportunity to reconstruct the dynamics of the Central America seaway that existed before final closure coincident with formation of the Isthmus of Panama.

Morphotype Catalog of a Zone I (Aptian—Earliest Albian) Flora from Fairlington, Virginia, USA

Abstract Plant fossil collections from Lower Cretaceous floodplain deposits sometimes provide snapshots of local communities that included early herbaceous angiosperms. Here I describe the plant megafossils in a collection made from the lower part of the Potomac Group near Fairlington, Virginia, USA. The collection includes 123 identifiable plant fossils on 42 hand specimens, comprising nine plant morphotypes: three ferns, five gymnosperms and one angiosperm. All but one of the described morphotypes also occur in other collections from the lower Potomac Group. There is one moderately abundant angiosperm species in this collection that has been found at other Potomac Group sites but was incorrectly identified as a fern. Well-preserved specimens of this morphotype clearly show axillary branching, complex leaves and glandular teeth at the apices of lobes on the leaves; together, these characteristics suggest it was an herbaceous eudicot angiosperm. The one new morphotype in this collection is an enigmatic plant of intermediate abundance and unknown affinity. On the basis of the simple, pinnate stenophyllous leaves with two vein orders, reticulate secondary veins and the attachment of one leaf to a carbonized axis roughly 1 cm across with long internodes, I hypothesize that it was a shrubby riparian gymnosperm. This work represents the first step toward a complete morphotype catalog for the flora of the lower Potomac Group.

Panascleroticoxylon crystallosa gen. et sp. nov.: a new Miocene malpighialean tree from Panama

We report fossil wood specimens from two Miocene sites in Panama, Central America: Hodges Hill (Cucaracha Formation; Burdigalian, c.19 Ma) and Lago Alajuela (Alajuela Formation; Tortonian, c.10 Ma), where material is preserved as calcic and silicic permineralizations, respectively. The fossils show an unusual combination of features: diffuse porous vessel arrangement, simple perforation plates, alternate intervessel pitting, vessel–ray parenchyma pits either with much reduced borders or similar to the intervessel pits, abundant sclerotic tyloses, rays markedly heterocellular with long uniseriate tails, and rare to absent axial parenchyma. This combination of features allows assignment of the fossils to Malpighiales, and we note similarities with four predominantly tropical families: Salicaceae, Achariaceae, and especially, Phyllanthaceae, and Euphorbiaceae. These findings improve our knowledge of Miocene neotropical diversity and highlight the importance of Malpighiales in the forests of Panama prior to the collision of the Americas.

Biogeographic Implications of Mammea paramericana sp. nov. from the Lower Miocene of Panama and the Evolution of Calophyllaceae

Premise of research. Mammea (Calophyllaceae) is a pantropical genus of 70 tree species, most of which are native to Madagascar. We describe a new species of Mammea on the basis of a fossil stem from the lower Miocene of Panama, discuss its implications for the biogeography of the genus, and assemble a set of morphological characters to examine the phylogeny of Calophyllaceae and confirm our identification. Methodology. We prepared the fossil stem using the cellulose acetate peel technique and compared its anatomy with that of extant plants, including a specimen of Mammea americana obtained from Puerto Rico, and with that of other fossils. For our phylogenetic analysis, we modified a previously published morphological matrix of reproductive and vegetative characters and added 14 wood characters, for a total of 66 characters. Pivotal results. Distinctive anatomical features of Mammea paramericana sp. nov. include predominantly solitary vessels, vasicentric tracheids, fibers with distinctly bordered pits in both radial and tangential walls, diffuse-in-aggregates axial parenchyma, narrow rays with upright marginal cells, and radial canals. The fossil stem has shorter pith cells, wider vessels, and fewer rows of upright marginal ray cells, and it lacks scalariform perforation plates compared with the sample of extant M. americana from Puerto Rico. The close relationship between the fossil and modern Mammea is supported by all of the most parsimonious trees from the phylogenetic analysis, as is the monophyly of Calophyllaceae. Conclusions. This fossil is the earliest evidence of Mammea in the Neotropics and is roughly coeval with previously recognized fossils of the genus from Africa. The fossil demonstrates that the diversification of Mammea in the Americas was underway by 19 Ma. Mammea likely reached the Neotropics from Africa via long-distance dispersal. While the morphological phylogenetic analysis recovers some of the relationships found in molecular topologies, it remains unresolved with respect to the immediate relatives of Mammea.