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Thecostraca Look for this name in NCBI Wikipedia Animal Diversity Web
Wolfe et al. 2016
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This fossil is preserved as a carbonate concretion from the volcaniclastic Herefordshire Lagerstätte of Herefordshire, England, at the Sheinwoodian-Homerian stage boundary, within the Early Silurian Wenlock Series (Siveter, 2008). As the Homerian begins at 430.5 Ma ± 0.7 Myr, a minimum age constraint for the Herefordshire can thus be placed at 429.8 Ma.
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A soft maximum age is obtained from the oldest mandibulate, Y. dianensis, which was recovered from the Yu'anshan Formation at Xiaotan section, Yongshan, Yunnan Province, attributed to the Eoredlichia–Wutingaspis Biozone (Zhang et al., 2007). Chinese Cambrian stratigraphy has been revised substantially and the Eoredlichia –Wutingaspis Biozone is no longer recognized (Peng, 2003, 2009). However, Eoredlichia is known to co-occur with Hupeidiscus, which is diagnostic of the Hupeidiscus-Sinodiscus Biozone, which is formally recognized as the second biozone of the Nangaoan Stage of the Qiandongian Series of the Cambrian of China (Peng and Babcock,2008). The Nangaoan is the proposed third stage of the Cambrian System for the International Geologic Timescale (Peng et al., 2012a).Thus, a soft maximum constraint can be established on the age of the lower boundary of the Nangaoan, which has been dated to 521 Ma (Peng et al., 2012a; Peng and Babcock, 2008).
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In a phylogenetic analysis of morphology, R. reduncus is the sister group of two species of Thoracica (both are members of Balanomorpha)(Legg et al., 2013). This is a position likely within the crown group of Thecostraca, however, no other thecostracans (such as Facetotecta and Ascothoracida) were included. Generally, the cyprid larval morphotype (with an elongated head shield, six swimming thoracopods, and robustmodified antennules) is considered a synapomorphy of Thecostraca (Høeg et al., 2004). R. reduncus differs from cirripede cyprids as it lacks attachment discs, and its abdomen extends past the head shield; a differentiated abdomen is a condition of Ascothoracida (Briggs et al.,2005; Høeg et al., 2009a). Based on the presence of five shell plates in a juvenile specimen, Høeg et al. (2009b) suggested that R. reduncus may be placed on the cirripede stem lineage. In combination with the phylogenetic analysis of Legg et al. (2013), these apomorphies indicate that R. reduncus is very likely a member of crown group Thecostraca, and likely on the stem of Cirripedia.
Legg, D.A., Sutton, M.D., Edgecombe, G.D. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4, 2485.
Høeg, J.T., Lagersson, N.C., and Glenner, H. 2004. The complete cypris larva and its significance in thecostracan phylogeny. In: Scholtz, G. (Ed.), Evolutionary and Developmental Biology of Crustacea, Crustacean Issues. AA Balkema Publishers, Lisse, Netherlands, pp. 197–215.
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