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Geotimes
 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 July 2000


Highlights
Vertebrates

Maureen A. O'Leary

Fossil vertebrate discoveries of 1999 belied Natural History Museum in London paleontologist Andrew Smith’s prediction that we have a “substantially biased fossil record that cannot be bettered no matter where we go in the world.”

Smith’s statement, in the Dec. 2 Nature supplement, sounded strange coming a month after D-G. Shu of Northwest University in Xi’an, China, and colleagues tracked soft-bodied vertebrates into the lower Cambrian. Who would have thought that before the millennium’s close we would find fishes with intestines and blood vessels preserved that were our closest relatives from the “‘Big Bang’ of life,” as Philippe Janvier of the National Museum of Natural History in France described the Cambrian explosion?

We often formulate ideas about how characters evolve and about taxa we do not have, but history shows repeatedly that such ideas are often wrong once evidence emerges. There is no substitute in science for empiricism. Hypotheses of evolutionary transition and adaptation, whether molecular or paleontological, are always in need of further corroboration — or rejection.

The year of the fish

Conodonts rank among the oldest vertebrates in the fossil record and were possibly the sister taxon to fishes. They remain important for biostratigraphy but their tooth-like structures continue to perplex functional morphologists. Even more intriguing, these taxa now appear to have been capable of mammal-like closure of the teeth, or occlusion, say Mark Purnell and Phil Donoghue of the University of Leicester in the UK. The sophistication of conodont occlusion is striking because they lack jaws. The discovery of this convergence not only implies complex feeding in this basal clade but also challenges how we interpret the uniqueness of mammalian mastication.

The abrupt appearance of animal phyla known as the Cambrian explosion has not, until now, been particularly informative about the origins of vertebrates. Shu and colleagues recently described a new Lagerstätten of the oldest vertebrates from the Early Cambrian Chengjiang beds of China: delicate little non-mineralized animals that feature vertebrate structures like a cartilaginous skull, gills (indirect evidence for neural crest cells), and complex muscle blocks.

Enigmatic taxa representing basal bony fishes have changed how we identify ray-finned or lobe-finned fish. Bony fishes have the most species of any vertebrate clade and to understand how and when these fish emerged, what the earliest forms looked like, and how they split into ray-finned and lobe-finned forms is to clarify an extremely significant radiation in vertebrate history. Min Zhu of the Institute of Vertebrate Paleontology in Beijing and colleagues extended the record of bony fishes to the Latest Silurian/Earliest Devonian period. Their specimen forces a reevaluation of our diagnoses of the clades of bony fishes. It contains a mixture of characters previously thought to belong uniquely to either the lobe-finned fishes Sarcopterygii or to the ray-finned Actinopterygii fishes.

Finally, new fish fossils from the Paleozoic of Canada exhibit a variety of morphological structures interpretable as pelvic and pectoral fins. These structures are critical to our understanding of fish locomotion, adaptation and, ultimately, the origin of tetrapods. Mark Wilson of the University of Alberta in Edmonton argues that these fossils have fin-like structures located in unanticipated positions and challenge our assessment of the origin and evolution of fins.
 
Amniote Lagerstätten
Exquisite preservation became the rule for many new amniote discoveries (birds, reptiles, mammals or any other group of vertebrates that undergoes embryonic development within a thin membrane forming a closed sac, or amnion). The dromaeosaurid theropod Sinornithosaurus from the Cretaceous of China described by Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology of Beijing, and colleagues, preserved a covering of filaments, demonstrating the presence of feather-like structures similar to birds. Due to the position of this fossil on the theropod phylogenetic tree, we have a working hypothesis that a number of theropod taxa between it and birds also had feathers. Excellent preservation of Triassic theropod footprints from Greenland provided Stephen Gatesy of Brown University and colleagues with one of the most direct means of reconstructing foot movement in this long-extinct dinosaur clade.

       Color reconstruction of Jeholodens jenkinsi on its fossil slab as 
       preserved in the stone. Watercolor image and fossil assembled 
       on computer.
       Mark A. Klinger, Carnegie Museum of Natural History.

The most complete skeleton ever found of a triconodont — a clade of mammals from the Mesozoic that is so primitive that it represents a stem taxon outside of monotremes, marsupials and placentals — emerged from work by Ji Qiang of the National Geological Museum of China and colleagues (see above illustration of Jeholodens jenkinsi). Despite its basal position on the mammalian tree (dictated by many parts of its skeleton), it has features of the shoulder that resemble some living mammals but appear to have evolved convergently. Each of these discoveries records the experimentation characterizing the early radiation of various living amniote clades.
 
 

3-D computer reconstruction of  theropod foot movements through sloppy mud. Penetration through the ground surface is shown in red. The first toe, which is not reversed as in modern birds, creates a rearward-pointing furrow (a, b) as it plunges down and forward. The sole of the foot leaves an impression at the back of the track (c) beacause it is not lifted as the foot sinks. All toes converge below the surface and emerge together from the front of the track (d). From work published by Steven Gatesy in the May 13, 1999 Nature. See also Geotimes, July 1999. Stephen Gatesy, Brown University.
Cladogram construction

Using simulations with a known phylogeny, David Fox and colleagues of the University of Michigan found that they more frequently recovered the correct phylogeny by applying stratocladistics rather than cladistics. In stratocladistics, one reconstructs a phylogeny using stratigraphic (temporal) data in parsimony analysis as cladistic character, like nucleotides, bony landmarks or other features of a taxon. These authors argue that this method explains more observations about life in the past. Despite this important result, others continue to object to the inclusion of stratigraphic data in phylogeny reconstruction and may not immediately embrace simulations as complex enough to generalize to empirical situations.

If Quaternary extinctions of large terrestrial animals resulted from extreme global climate change, rather than human hunting, then one would predict that taxa that go extinct do so at times of dramatic climate change. Intensified study with excellent temporal control of the fossil record of a 100-kilogram flightless bird from the Late Pleistocene of Australia by Gifford Miller of the University of Colorado at Boulder and colleagues showed that the bird’s extinction occurred around 50,000 years ago throughout its Australian range. Because this date coincides with the arrival of humans in Australia but not with any intense climate fluctuations, it becomes relatively unambiguous evidence against a significant causal role for climate in this Quaternary extinction.

Extinctions

If Quaternary extinctions of large terrestrial animals resulted from extreme global climate change, rather than human hunting, then one would predict that taxa that go extinct do so at times of dramatic climate change. Intensified study with excellent temporal control of the fossil record of a 100-kilogram flightless bird from the Late Pleistocene of Australia by Gifford Miller of the University of Colorado at Boulder and colleagues showed that the bird’s extinction occurred around 50,000 years ago throughout its Australian range. Because this date coincides with the arrival of humans in Australia but not with any intense climate fluctuations, it becomes relatively unambiguous evidence against a significant causal role for climate in this Quaternary extinction.

This litany of discoveries from the spectrum of vertebrate life should hearten those distressed by such setbacks to evolutionary study as the decision last fall by the Kansas Board of Education to eliminate evolution from the state’s school curriculum. Many Kansas universities openly opposed the decision, and the Society of Vertebrate Paleontology maintains publicly that the theory of evolution is highly corroborated by our observations of the fossil record (see www.museum.state.il.us/svp//). Continued discovery, study and popularization of material as extraordinary as that described in 1999 will undoubtedly generate great interest in the vertebrate stratigraphic record and may serve to teach about evolution, even if not in school.
 

O'Leary is an assistant professor in the Department of Anatomical Sciences at the State University of New York, Stony Brook. She studies mammalian evolution and conducts fieldwork in the Late Mesozoic and Early Cenozoic rocks of sub-Saharan Africa, attempting to recover poorly known vertebrate fossils.



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