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Author Hundaa, Brenda R. ♦ Hughesb, Nigel C.
Source CiteSeerX
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Subject Domain (in DDC) Computer science, information & general works ♦ Data processing & computer science
Subject Keyword Retrorsa Minuens ♦ Retrorsa Retrorsa ♦ Diminutive Trilobite Flexicalymene Retrorsa Minuens ♦ Paedomorphic Heterochrony ♦ Ancestral Ontogeny ♦ Growth Cessation ♦ Global Hetero-chronic Paedomorphosis ♦ Strong Similarity ♦ Maximum Size Relative ♦ Localized Change ♦ Complex Pattern ♦ Phylogenetic Analysis ♦ Non-uniform Change ♦ Neotonic Decrease ♦ Trilobite Specie ♦ Overall Form ♦ Marked Difference ♦ Co-occurring Taxon ♦ Size Reduction ♦ Global Hetero-chronic Modification ♦ Simple Progenetic Descendant ♦ Small Size ♦ Global Paedomorphic Evolution ♦ Upper Ordovician ♦ North America ♦ Common Ontogenetic Trajectory ♦ Plausible Example ♦ Character Development ♦ Novel Structure ♦ Sequential Progenesis ♦ Ontogenetic Trajectory ♦ Mean Size ♦ Subjacent Sister Subspecies
Abstract Series (Upper Ordovician) of North America lived approximately 445Ma and exhibited marked reduction in maximum size relative to its stratigraphically subjacent sister subspecies, Flexicalymene retrorsa retrorsa. Phylogenetic analysis is consistent with the notion that F. retrorsa retrorsa was the ancestor of F. retrorsa minuens. F. retrorsa minuens has been claimed to differ from F. retrorsa retrorsa ‘‘in size alone,’ ’ and thus presents a plausible example of global paedomorphic evolution in trilobites. Despite strong similarity in the overall form of the two subspecies, F. retrorsa minuens is neither a dwarf nor a simple progenetic descendant of F. retrorsa retrorsa. More complex patterns of global hetero-chronic paedomorphosis, such as a neotonic decrease in the rate of progress along a common ontogenetic trajectory with respect to size, coupled with growth cessation at a small size, ‘‘sequential’ ’ progenesis, or non-uniform changes in the rate of progress along a shared ontogenetic trajectory with respect to size, can also be rejected. Rather, differences between these subspecies are more consistent with localized changes in rates of character development than with a global hetero-chronic modification of the ancestral ontogeny. The evolution of F. retrorsa minuens from F. retrorsa retrorsa was largely dominated by modifications of the development of characters already evident in the ancestral ontogeny, not by the origin of novel structures. Factors promoting size reduction in F. retrorsa minuens appear to have been specific to this subspecies, because other co-occurring taxa, including other trilobite species, do not show marked differences in mean size.
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