Bonnie F. Jacobs

Oligocene mammals from Ethiopia and faunal exchange between Afro-Arabia and Eurasia

Afro-Arabian mammalian communities underwent a marked transition near the Oligocene/Miocene boundary at approximately 24 million years (Myr) ago. Although it is well documented that the endemic paenungulate taxa were replaced by migrants from the Northern Hemisphere, the timing and evolutionary dynamics of this transition have long been a mystery because faunas from about 32 to 24 Myr ago are largely unknown. Here we report a late Oligocene fossil assemblage from Ethiopia, which constrains the migration to postdate 27 Myr ago, and yields new insight into the indigenous faunal dynamics that preceded this event. The fauna is composed of large paenungulate herbivores and reveals not only which earlier taxa persisted into the late Oligocene epoch but also demonstrates that one group, the Proboscidea, underwent a marked diversification. When Eurasian immigrants entered Afro-Arabia, a pattern of winners and losers among the endemics emerged: less diverse taxa such as arsinoitheres became extinct, moderately species-rich groups such as hyracoids continued into the Miocene with reduced diversity, whereas the proboscideans successfully carried their adaptive radiation out of Afro-Arabia and across the world.

A middle Miocene (12·2 my old) forest in the East African Rift Valley, Kenya

Abstract A 12·2 million year old leaf flora from the Rift Valley, Kenya, contains 57 taxa of which 25 have been identified to various taxonomic levels. Of these, 11 are found today in wet forests or rainforests. Study of foliar physiognomy, which is independent of taxonomy, indicates a moist to wet forest reconstruction. Of three grasses in the flora, Leptaspis and Humbertochloa are the first records of pre-Quaternary bambusoid (bamboo allies) fossils from Africa.

Eocene dry climate and woodland vegetation in tropical Africa reconstructed from fossil leaves from northern Tanzania

Eocene vegetation and climate data from tropical latitudes are sparse despite special interest in the Eocene as the warmest epoch of the Cenozoic and an often-cited analogue for greenhouse Earth conditions. Tropical Africa is noteworthy for its shortage of Eocene fossils, which could serve as proxies for climate and reveal community structural evolution during the continent’s geographic isolation. In this paper, we report paleobotanical remains from a middle Eocene crater lake at 128S paleolatitude in north central Tanzania, which provide a plant community reconstruction indicating wooded, rather than forest, vegetation and precipitation estimates near modern (660 mm/year). The plant community was dominated by caesalpinioid legumes and was physiognomically comparable to modern miombo woodland. Paleoprecipitation estimates, the first for the Paleogene of Africa, are calculated from fossil leaf morphology using regression equations derived from modern low-latitude leaves and climate. Mean annual precipitation estimates are 643F32 and 776F39 mm/year, and wet months precipitation estimates (all months averagingz50 mm) are 630F38 and 661F38 mm. A slightly larger proportion of annual precipitation occurred in the dry months compared with today, which may indicate greater equability of precipitation in the Eocene. D 2004 Elsevier B.V. All rights reserved.

Estimation of rainfall variables from leaf characters in tropical Africa

Abstract Regression models are defined for leaf physiognomic and climate variables from 30 modern equatorial African plant communities. Herbarium specimens provide physiognomic data for published species lists for the 30 sites; climate data are from a variety of published sources. By limiting the sampled region to equatorial Africa, where annual amount and seasonality of rainfall are critical determinants of vegetation, leaf physiognomy is found to be related linearly to moisture variables. Principal components analysis demonstrates that the pattern of variation among leaf variables is primarily a function of mean annual and wet months (total of all months with ≥50 mm) rainfall. Multiple regression models are derived for mean annual rainfall, wet months rainfall and the number of dry months. Accuracy of each model is tested by comparing the adjusted predicted values (the value for a case when it is not included in calculation of the regression line) to the actual climate values at the 30 sites. Leaf size is the strongest correlate of moisture; the mesophyll size class has the most significant positive relationship with annual and wet months rainfall. Entire-margined species increase with wet months rainfall and temperature. Acuminate tip and length/width ratio are also positively related to mean annual and wet months rainfall. Rounded base, emarginate tip, and acute tip are inversely related to rainfall variables. Regression models derived from modern plants and climate can be used to reconstruct seasonal and annual rainfall from Cenozoic African leaf fossils.

Estimation of low-latitude paleoclimates using fossil angiosperm leaves: examples from the Miocene Tugen Hills, Kenya

Abstract In the last decade, several statistical models have been proposed to quantify the relationships among leaf morphological characters and climate parameters. The models, based on modern plants and climate from varying geographic areas, and derived using varied statistical analyses, were intended for paleoclimatic reconstruction based on the morphological characters of fossil leaves. The goal of the research presented here is to evaluate these and newly constructed models in order to estimate past climate in tropical Africa from fossil leaves. Models found to estimate current climate most accurately using modern African leaf assemblages are used to estimate past climate from fossil leaves at three middle and late Miocene paleobotanical sites in the Tugen Hills, Kenya. Regression models derived from predictor data having a majority of sites from higher than 25°N-S latitude consistently overestimate mean annual precipitation at modern African sites by an average of 990 mm. A pronounced cold season, as...—In the last decade, several statistical models have been proposed to quantify the relationships among leaf morphological characters and climate parameters. The models, based on modern plants and climate from varying geographic areas, and derived using varied statistical analyses, were intended for paleoclimatic reconstruction based on the morphological characters of fossil leaves. The goal of the research presented here is to evaluate these and newly constructed models in order to estimate past climate in tropical Africa from fossil leaves. Models found to estimate current climate most accurately using modern African leaf assemblages are used to estimate past climate from fossil leaves at three middle and late Miocene paleobotanical sites in the Tugen Hills, Kenya. Regression models derived from predictor data having a majority of sites from higher than 258NS latitude consistently overestimate mean annual precipitation at modern African sites by an average of 990 mm. A pronounced cold season, as at high latitudes, has an inhibitory effect on leaf size, the primary correlate of rainfall, and may negatively affect the accuracy with which models derived from high latitudes estimate rainfall in the Tropics, which lack a cold season. Models derived from data sets consisting of samples from ,258N-S latitude yield similar and more accurate estimates for mean annual precipitation at modern African sites, expressing the predominant relationship between yearly or seasonal rainfall and leaf size at lower latitudes. Models that estimate temperature parameters, whether derived from high or low latitudes, were found to be inaccurate with modern tropical African samples. The hypothesis is proposed that non-entire margins, the primary correlate with temperature, are more likely to be present on the leaves of deciduous plants, whether they lose their leaves because of cold (at high latitudes) or seasonal drought (at low latitudes). Generally, this study indicates that modern predictor data sets from which models are drawn should be representative of the predominant climate parameters expected among fossil sites. Four regression models are approximately equal in their ability to estimate accurately mean annual precipitation at the modern African sites. They are derived from African data or combinations of African plus other low-latitude data and provide consistent rainfall reconstructions at the three fossil sites. Estimates are 955 6 29 to 1185 6 96 mm/yr for Kabarsero (12.6 Ma), 490 6 46 to 693 6 32 mm/yr for Waril (9–10 Ma), and 730 6 30 to 1019 6 32 mm/yr for Kapturo. Wet-months precipitation estimates are 857 6 30 mm/yr for Kabarsero, 437 6 30 for Waril, and 627 630 for Kapturo. These are the first quantitative estimates of climate for the Miocene of East Africa. The seasonally dry climate inferred for Waril may indicate that the Asian monsoon was established by about 9–10 Ma. Alternatively, the seasonally dry climate may reflect local topographic changes caused by rift valley development. However, the plant localities suggest that, although progressive drying may have been a trend during the Tertiary, there was not a unidirectional change from forested to open environments in the Kenya rift between 12.6 and 6.8 Ma, the time interval just prior to the origin of hominids. Bonnie Fine Jacobs. Environmental Science Program, Post Office Box 750395, Southern Methodist University, Dallas, Texas 75275-0395. E-mail: bjacobs@mail.smu.edu Accepted: 11 February 2002

Fossil legumes from the Middle Eocene (46.0 Ma) Mahenge flora of Singida, Tanzania.

Middle Eocene age caesalpinioid and mimosoid legume leaves are reported from the Mahenge site in north-central Tanzania. The Mahenge flora complements a sparse Paleogene tropical African fossil plant record, which until now consisted of a single macrobotanical assemblage, limited palynological studies in West Africa and Egypt, and fossil wood studies primarily from poorly dated deposits. Mahenge leaf macrofossils have the potential to add significantly to what is known of the evolutionary history of extant African plant groups and to expand our currently limited knowledge of African Paleogene environments. The site is associated with a kimberlite eruption and demonstrates the potential value of kimberlite-associated lake deposits as much-needed resources for African Paleogene floras. In this report we document a relatively diverse component of the flora consisting of the leaves of at least five species of Leguminosae. A new species of the extant genus Acacia (Mimosoideae), described herein, is represented by a bipinnate leaf. Another taxon is described as a new species of the extant genus Aphanocalyx (Caesalpinioideae), and a third leaf type may be related to the extant genus Cynometra (Caesalpinioideae). Two additional leaf types are less well understood: one appears to be referable to the Caesalpinioideae and subfamily affinities of the other taxon are unknown.

Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils

Significance Melanin is a widespread pigment that provides black to reddish brown hues to organisms. Recent evidence has shown that melanin is retained in exceptionally preserved fossils, including feathered dinosaurs, allowing the reconstruction of ancient color patterns. However, little is known about the chemical preservation of melanin or its distribution in the fossil record. Here, we show that melanin is preserved in a number of soft-bodied fossils, but its burial under high pressure and temperature for millions of years alters its original chemistry. The widespread occurrence of melanin substantiates the applicability of reconstructing aspects of original color patterns and allows us to dismiss the alternative suggestion that these structures are microbial in origin. In living organisms, color patterns, behavior, and ecology are closely linked. Thus, detection of fossil pigments may permit inferences about important aspects of ancient animal ecology and evolution. Melanin-bearing melanosomes were suggested to preserve as organic residues in exceptionally preserved fossils, retaining distinct morphology that is associated with aspects of original color patterns. Nevertheless, these oblong and spherical structures have also been identified as fossilized bacteria. To date, chemical studies have not directly considered the effects of diagenesis on melanin preservation, and how this may influence its identification. Here we use time-of-flight secondary ion mass spectrometry to identify and chemically characterize melanin in a diverse sample of previously unstudied extant and fossil taxa, including fossils with notably different diagenetic histories and geologic ages. We document signatures consistent with melanin preservation in fossils ranging from feathers, to mammals, to amphibians. Using principal component analyses, we characterize putative mixtures of eumelanin and phaeomelanin in both fossil and extant samples. Surprisingly, both extant and fossil amphibians generally exhibit melanosomes with a mixed eumelanin/phaeomelanin composition rather than pure eumelanin, as assumed previously. We argue that experimental maturation of modern melanin samples replicates diagenetic chemical alteration of melanin observed in fossils. This refutes the hypothesis that such fossil microbodies could be bacteria, and demonstrates that melanin is widely responsible for the organic soft tissue outlines in vertebrates found at exceptional fossil localities, thus allowing for the reconstruction of certain aspects of original pigment patterns.

Global climatic drivers of leaf size

Leaf size, climate, and energy balance Why does plant leaf size increase at lower latitudes, as exemplified by the evolutionary success of species with very large leaves in the tropics? Wright et al. analyzed leaf data for 7670 plant species, along with climatic data, from 682 sites worldwide. Their findings reveal consistent patterns and explain why earlier predictions from energy balance theory had only limited success. The authors provide a fully quantitative explanation for the latitudinal gradient in leaf size, with implications for plant ecology and physiology, vegetation modeling, and paleobotany. Science, this issue p. 917 Day- and nighttime leaf-to-air temperature differences drive global gradients in leaf size. Leaf size varies by over a 100,000-fold among species worldwide. Although 19th-century plant geographers noted that the wet tropics harbor plants with exceptionally large leaves, the latitudinal gradient of leaf size has not been well quantified nor the key climatic drivers convincingly identified. Here, we characterize worldwide patterns in leaf size. Large-leaved species predominate in wet, hot, sunny environments; small-leaved species typify hot, sunny environments only in arid conditions; small leaves are also found in high latitudes and elevations. By modeling the balance of leaf energy inputs and outputs, we show that daytime and nighttime leaf-to-air temperature differences are key to geographic gradients in leaf size. This knowledge can enrich “next-generation” vegetation models in which leaf temperature and water use during photosynthesis play key roles.

The Conifer Frenelopsis ramosissima (Cheirolepidiaceae) in the Lower Cretaceous of Texas: Systematic, Biogeographical, and Paleoecological Implications

Until now, our knowledge of the Lower Cretaceous conifer Frenelopsis ramosissima was based exclusively on branches from a few sites in the Potomac Group of eastern Virginia and Maryland. Affinities with the important Mesozoic family Cheirolepidiaceae have been assumed despite the historical absence of diagnostic attached or associated Classopollis‐producing pollen cones. This plant has been reconstructed as a small, stem‐succulent shrub that inhabited diverse, mesic plant communities. Here, we present a reconsideration of F. ramosissima based on new fossils from the Jones Ranch sauropod dinosaur quarry site (Twin Mountains Formation) near Glen Rose, Texas, which represents a ca. 2100‐km range extension for this conifer. Compelling support for assignment to the Cheirolepidiaceae is provided by the first account of associated pollen cones with in situ Classopollis‐type pollen. Features of associated remains are consistent with this determination, including ovulate cones with persistent bracts, cone scales with a probable epimatium, and wood with mixed pitting on the tracheid radial walls. Our analysis of the new Texas fossils refutes or seriously challenges many widely accepted hypotheses regarding the biogeography, structure, and paleoecology of F. ramosissima. For example, this plant can no longer be considered a Potomac Group–endemic taxon. Furthermore, the taphonomy and sedimentology of the Jones Ranch locality indicate a monospecific stand of F. ramosissima plants from a semiarid climate rather than a diverse mesic community, as indicated by the Potomac Group occurrences. Finally, associated logs indicate that F. ramosissima was a large tree (at least 22.4 m tall, extrapolated from maximum trunk diameter) with abundant wood in the trunk and lower branch orders.

Paleontological Investigations at the Eocene Locality of Mahenge in North-Central Tanzania, East Africa

Since the early 1930s, a number of geologists prospecting for diamonds in the region of Singida in north-central Tanzania have reported the occurrence of superbly preserved fossils in lake beds overlying kimberlite pipes (Teale, 1931, 1932; Eades and Reeve, 1938; Williams, 1939; Greenwood, 1960; Mannard, 1962; Greenwood and Patterson, 1967). The sediments containing the fossils consist of shales and mudstones deposited in small crater lakes. These fossil localities have never been the subject of detailed paleontological investigation. As a consequence, in 1996 the senior author initiated excavations at the middle Eocene locality of Mahenge, the most productive fossil-bearing locality in the region (Greenwood, 1960; Mannard, 1962). The results of this expedition confirm the exceptional productivity and scientific importance of this site, and serve to highlight the paleontological potential of the region in general. A sizeable collection of fossil fishes, frogs, insects and plant remains was recovered. The fossils are remarkably well-preserved and, in the case of vertebrates, are generally represented by entire and articulated skeletons, occasionally preserving details of their soft anatomy (Greenwood, 1960; this report).