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fossilized and have been used as paleoecological indicators from 

 the early Tertiary (Adey, 1979). Coralline growth is slow; yearly 

 bands in Clathromorphum in the warmer fringes of the Subarctic 

 and the Aleutian Islands are 300-400 pm in thickness. In the 

 colder Subarctic, the principal habitat of C. cornpactum^ they are 

 considerably thinner and drop below 100 pm yr ' in the Arctic. 

 Although the potential for climate archiving with corallines has 

 been known since the 1960s (Adey, 1965; Chave and Wheeler, 

 1965), its application has been frustrated until recently by a lack 

 of understanding of their anatomical complexity and its irregu- 

 larities, as well as by the lack of appropriate instrumental tools. 



Species of Clathromorphum have a unique anatomy that is 

 particularly useful in the development of climate archives in Sub- 

 arctic marine environments (Adey, 1965; Halfar et al., 2011a). 

 Unlike the progressive cell growth in other Subarctic corallines, 

 where maximum cell length is achieved several to a dozen cells 

 below the meristem, creating a diffuse, time-delayed band of cal- 

 cifying cells, Clathromorphimt produces all of its growth and 

 calcification in the meristem cell layer (a cambium analog; Adey 

 et al., 2005; Figure 2C). As shown here, in Clathromorphitm the 

 band of calcification and growth is even narrower than meristem 

 cell length, occurring on a plane a few microns thick passing lat- 

 erally through the meristem; this ensures the tight temporal link- 

 ing of ambient water climate and chemistry to a narrow plane 

 of calcite crystals. These species have an intricate anatomy that 

 includes calcite crystals that preserve, in their trace element and 

 isotopic structure, an array of environmental proxies. 



In addition to their carbonate skeleton, species of Clath- 

 romorphum have evolved partial protection against grazing by 

 invertebrates in both their anatomy (the sunken meristem) and 

 their primary asexual reproduction (conceptacles are sunken in 

 the crust and produced in winter). Working with Clathromor- 

 phum circumscriptum, Steneck (1982) demonstrated a "co- 

 evolved interdependency" with the limpet Acmaea testudmalis. 



Removal of epithallial surface cells by limpets during grazing 

 equaled the production rate of epithallial cells in the meristem 

 while freeing the surface of epiphytes. The same relationship 

 likely exists for C. compactum and C. uereostratuni, as fine graz- 

 ing marks are abundant on the surface of most collected speci- 

 mens (in most of the Subarctic, the chiton genera Toiucella and 

 hchnochitou dominate). Since routine grazing by chitons and 

 limpets is concentrated on the upper surface of the epithallium, 

 the perithallium, which builds cell-by-cell below the meristem. 



FIGURE 2C. SEM image of fractured C. compactum mound. The 

 lower right face (right arrow) shows calcified perithallium filaments. 

 The upper thin slab breaking off is epithallium. The exposed surface 

 to the left (left arrow) is meristem, separated through the middle of 

 the layer at the plane of growth and calcification. 



