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sclerotized legs compound eyes lateral lobes complex segmentation jointed appendages

Fig. 11.1. Extreme interpretations for fossil specimens from the Burgess Shale. The phylogenetic tree contains seven taxa. Among these, five are extinct; these are indicated with tombstones. In the graphic at the top, fossil specimens representing species in the genera Aysheaia, Hallucigenia, Kerygmachela, Opabinia, and Anomalocaris (right to left) compose a valid evolutionary group that shared a most-recent common ancestor with, but is distinct from, another group comprising velvet worms (second from the right) and arthropods (represented by a dragonfly, at the far right). In the graphic at the bottom, fossil specimens representing species in the genera Aysheaia, Hallucigenia, Kerygmachela, Opabinia, and Anomalocaris (left to right) originated between velvet worms and arthropods; the traits that originated during that period, which are represented as rectangles, were steps en route to arthropod origins. Rectangles represent jointed appendages, complex segmentation, lateral lobes, compound eyes, and sclerotized legs (lower to upper). What might be interpreted as a sudden Arthropod origin and subsequent radiation (top graphic) actually might have comprised a gradual process (bottom graphic).

sclerotized legs compound eyes lateral lobes complex segmentation jointed appendages

Fig. 11.1. Extreme interpretations for fossil specimens from the Burgess Shale. The phylogenetic tree contains seven taxa. Among these, five are extinct; these are indicated with tombstones. In the graphic at the top, fossil specimens representing species in the genera Aysheaia, Hallucigenia, Kerygmachela, Opabinia, and Anomalocaris (right to left) compose a valid evolutionary group that shared a most-recent common ancestor with, but is distinct from, another group comprising velvet worms (second from the right) and arthropods (represented by a dragonfly, at the far right). In the graphic at the bottom, fossil specimens representing species in the genera Aysheaia, Hallucigenia, Kerygmachela, Opabinia, and Anomalocaris (left to right) originated between velvet worms and arthropods; the traits that originated during that period, which are represented as rectangles, were steps en route to arthropod origins. Rectangles represent jointed appendages, complex segmentation, lateral lobes, compound eyes, and sclerotized legs (lower to upper). What might be interpreted as a sudden Arthropod origin and subsequent radiation (top graphic) actually might have comprised a gradual process (bottom graphic).

from excessive (crown group) extinctions (Budd, personal communication). From this perspective, upper-level categories like phyla are old because, in classifying organisms, taxonomists must identify hierarchically nesting, general traits, and this necessitates delving far into the past (Fitch and Sudhaus, 2002); although researchers have yet to reconcile origin dates from fossil-based and molecular-based phylogenies for metazoans (e.g., Wray et al., 1996), consensus about relationships is emerging (see Chapter 8). Researchers must strive to test whether fossil specimens that lack many phylum-level traits might be members of extant phyla, nevertheless.

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