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Evolution of Archaeid and Mecysmaucheniid Spiders (Arachnida, Araneae)

  • Author(s): Wood, Hannah Marie
  • Advisor(s): Gillespie, Rosemary
  • et al.

The limits of the superfamily Palpimanoidea, as well as the phylogenetic placement of archaeid and mecysmaucheniid spiders within the Araneomorphae, are unresolved. Furthermore, the relationships between the extant and extinct archaeid taxa is also debated and unresolved. This study focuses on these issues by creating a phylogeny from molecular and morphological data and addresses three features of archaeid and mecysmaucheniid evolution: (1) inclusion of several lineages of fossil archaeids clarifies relationships between extant and extinct archaeids and helps explain the disjunct distribution whereby fossils are known only from the northern hemisphere while extant taxa are restricted to the southern hemisphere; (2) the placement of archaeids and mecysmaucheniids within the Araneomorphae; (3) the limits of the superfamily Palpimanoidea and its placement within the Araneomorphae. In addition, the timing of deep diversification within the Araneomorphae is estimated by enforcing a molecular clock that includes the archaeid fossil taxa as noncontemporaneous tips. These temporal estimations are used to examine biogeographic patterns of congruence with continental break-up. Total evidence analysis supports the monophyly of a redefined Palpimanoidea, which includes the archaeids and mecysmaucheniids. This study finds Palpimanoidea to be sister to the Entelegynae and to be an ancient group, with diversification occurring in the Permian. Furthermore, the split between the northern and southern archaeid fauna and the diversification of the southern archaeid clades was likely due to the vicariant events caused by the break-up of Pangaea and Gondwana.

Further study of archaeid spiders offers the possibility to better understand speciation patterns in a group of taxa that have low dispersal abilities and that likely have been on Madagascar since Pangean times. To examine speciation patterns in a lineage that likely did not disperse to Madagascar, the current study sets out to (1) create a phylogeny of archaeid spiders that thoroughly samples Madagascan lineages; (2) calibrate this phylogeny using fossil and geological data and determine the timing of splitting events between the different Gondwana fragments; (3) examine lineage through time plots to determine diversification patterns. In addition, the "neck" trait is treated as a continuous character and its evolution is examined. This study shows that archaeid biogeography patterns are likely explained by vicariance due to Gondwanan break up. The lineage through time plots reveal that the Madagascan archaeids have not experienced increases or decreases in the rate of diversification, meaning a constant rate of lineage accumulation cannot be rejected. Furthermore, the evolution of the "neck" best fits the Brownian motion model, implying that evolution of "neck" length is a product of genetic drift. For archaeid lineages that have been on Madagascar since pre-isolation times gradual accumulation appears to be the rule.

Next, the current study focuses on the trap-jaw in mecysmaucheniid spiders in order to address how it has become modified over the evolutionary history of the lineage. Evolution, function, and morphology of the trap-jaw are examined among different lineages of mecysmaucheniids to assess the extent of conservatism versus plasticity in the trait. This study involves molecular phylogenetic analyses, detailed morphological analysis of jaw structure, and high-speed video recording to assess the variability and evolution of the trap-jaw. Results indicate that there is a large degree of variation in jaw function spanning two orders of magnitude. Within mecysmaucheniids, rapid-inertia-based mechanisms have evolved in parallel 3-4 times. Examination of trap-jaw morphology reveals that each rapid-inertia-based mechanism is unique with different morphologies. The trap-jaw movements in some mecysmaucheniid lineages may be among the fastest animal movements known, with the fastest species attaining jaw-closing speeds of greater than 25 meters/second in less than one tenth of a millisecond.

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