LICHIDA Moore 1959 |
last revised 02 September 2011 by S. M. Gon III
Introduction: typically spiny with densely granulate or tuberculate exoskeletons and complex glabellar lobation. This is a concept of the order that does not include the families Odontopleuridae and Damesellidae, more recently considered members of a related, but distinct order Odontopleurida. Examine an alternate handling of
Introduction: medium to large trilobites, typically surface sculpturing involves two size classes of granules or tubercles.
| ||ADDITIONAL CLASSIFICATION NOTES FOR LICHIDA: |
This treatment distinguishes an Order Lichida (Lichakephalidae+Lichidae) separate from Order Odontopleurida (Odontopleuridae+Damesellidae), although some treatments treat the two as sister clades; both groups bearing tubercles and/or spiny prosopon, complex lobation of the glabella, and certain protaspid similarities suggest the two are ultimately related (e.g., Thomas & Holloway 1988, Fortey 1997). Tripp & Evitt (1981) point to major differences in origin of lobation (lichid lobes arising from the axial furrow, while odontopleurid lobes arise via standard glabella development). Whittington (2002) notes that the Odontopleurida and Lichida differ in several respects, including ornamentation (primarily granules and tubercles in lichids, spines in odontopleurids), extent of ventral calcification (extended doublure with terrace ridges in lichids, not developed in odontopleurids), and hypostome size (larger in lichids), in addition to the glabellar lobation differences. Even if both lichids and odontopleurids are kept in one order (Lichida), it is acknowledged that despite their common origin, they have diverged significantly in the course of their evolution.
The primitive family Lichakephalidae includes genera that have affinities to Lichidae and others to Odontopleuridae, and some workers (e.g., Shergold et al 2000) do not accept the synonymy of Eoacidaspididae and Lichakephalidae. Examination of Cambrian lichakephalid/eoacidaspidid genera Acidaspides and Acidaspidella (Bruton 1983) suggest they should be assigned to Odontopleuridae. This suggests that the Lichakephalidae as listed above is paraphyletic, and not all members should be included in Lichoidea. Primitive Ordovician lichakephalids (e.g., Lichakephalus) bear pygidia that are unlike typical lichids, some being almost styginid-like, however Whittington (2002) suggests that a close relationship between lichids and styginids appears unlikely. In 2005, Pollit et al applied cladistic analysis to the Superfamily Lichoidea (Lichidae+Lichakephalidae) and proposed subfamilial and tribal subdivisions for the family Lichidae. Click here for a cladogram of Lichoid relationships from the Pollit et al 2005 paper (warning, large graphic file).
Expanded and complete genera listings for the families above are based on Jell & Adrain (2003).
Jell, P.A. & J.M. Adrain. 2003. Available generic names for trilobites. Memoirs of the Queensland Museum 48(2):331-553.
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