82
Journal of Paleontology 91(1):73–85
Table 1. Module diameter of Amsassia argentina n. sp. For seven specimens from La Silla Formation, two distinct areas within a transverse thin section were sampled, characterized by polygonal and subpolygonal to rounded mod- ules. Polygonal module diameter = (long axis + short axis)/2. Abbreviations: Max., maximum diameter (mm); Min., minimum diameter (mm); Avg., aver- age diameter (mm); SD, standard deviation; n, number of modules.
Module shape Polygonal
Subpolygonal to rounded
Max. Min. 1.12
0.84
0.53 0.44
Avg. SD n 0.87
0.54
0.14 0.11
38 41
2014; M. Lee et al., 2016). The Argentinean material is assigned to a new species, A. argentina. Species of Amsassia reported from northern Kazakhstan
and western Siberia have module diameters that are less variable in size than A. argentina n. sp.: A. princeps (0.73–1.0mm; Sokolov and Mironova, 1959), A. chaetetoides (1.0–1.4mm; Sokolov and Mironova, 1959), and the type species A. raduguini (1.1–1.4mm; Sokolov and Mironova, 1959). Amsassia shaanxiensis from northern China (Sun et al.,
2014) has modules generally polygonal to subpolygonal and closely packed, rarely rounded, and partially to entirely separated, with module diameter ranging from 0.46 to 1.66mm, which is larger than in A. argentina (0.41–1.11mm). Amsassia koreanensis M. Lee et al., 2016 was described
from Korea and China. This form has an exceptionally small module diameter (0.06–0.28 mm), which is different from the size range observed in A. argentina. Other species reported from China and Mongolia,
include A. minima (0.2–0.4mm; Yang et al., 1978), A. tudiaoensis (0.3–0.6mm; Yang et al., 1978), A.(?) minima (0.3–0.5mm; Lin and Chow, 1980), and A. sheshanensis (Deng, 1984; Ye et al., 1995), all with smaller module diameters and narrower size ranges than A. argentina n. sp. Amsassia soluta and A. abnormis (Yu, 1962) have a range of variation (0.5–1.1mm; M. Lee et al., 2016) that almost overlaps the range observed in A. argentina n. sp. However, module diameters of 0.3–0.4mm are found in our material, measured in loosely packed areas. The dark median line separating adjacent modules,
visible in some sections, could be related with early diagenesis, and apparently not related with the presence of a true medial plate or fused epithecae of adjacent modules. Unfortu- nately, poor preservation prevents a complete analysis of this aspect.
Amsassia is a problematic modular organism with a coral-
like skeleton that recentlywas reevaluated by Sun et al. (2014) and M. Lee et al. (2016). After a comprehensive analysis, Amsassia was compared with different types of corals, tetradiids, chaetetid sponges, and algae, those authors suggested that Amsassia may represent an extinct group of calcareous algae. Lichenaria is a common Lower and Middle Ordovician
tabulate, considered the basal genus in the early radiation of tabulates (Scrutton, 1984, 1997; Pandolfi, 1989). In species of Lichenaria, the corallites exclusively show a cerioid structure and are separated by fibrous, continuously fused common walls. The septal structures are absent and it has abundant tabulae (Elias et al., 2008, and references therein). Lichenaria grew exclusively through lateral corallite increase, which is consid- ered to be the key feature for identifying the genus (Elias et al.,
2008). All these characteristics clearly separate our material from the lichenariid stock.
Paleogeographic distribution of Amsassia and its role in the early development of metazoan reefs
In the last few years, there have been several new contributions on upper Cambrian to Lower Ordovician reefs and reef-related organisms (Adachi et al., 2009, 2012a, b, 2015; Hong et al., 2012, 2015; Wang et al., 2012; Choh et al., 2013; Kruse and Reitner, 2014; Lee et al., 2014, and references therein). Those studies have begun to fill in the well-known gap in metazoan reef records after the demise of archaeocyaths in the late early Cambrian (Pratt et al., 2001; Webby, 2002; Rowland and Sha- piro, 2002). After collapse of the early Cambrian calcimicrobial-
archaeocyathan reef consortium, metazoans played little part in reef construction for ca. 30Myr (Rowland and Shapiro, 2002; Adachi et al., 2015). Many reef-mound findings in the post-extinction interval were completely constituted by microbes (Webby, 2002; Rowland and Shapiro, 2002; Lee et al., 2015).
Nevertheless, exceptional metazoan reefs have been docu-
mented from the interval commencing around the beginning of the Furongian Epoch, when spiculate sponges began to actively participate in reef construction. Some of these reefs are mainly microbial, but demosponges helped as secondary reef builders (Cañas and Carrera, 1993; Riding and Zhuravlev, 1995; Shapiro and Rigby, 2004; Kruse and Zhuravlev, 2008; Wang et al., 2012; Choh et al., 2013; Kruse and Reitner, 2014) (mainly summarized in Rowland and Shapiro, 2002; Hong et al., 2012; Adachi et al., 2015). Orchoclad demosponges diversified in the late Cambrian as
the main metazoan constituents of reef settings. Orchoclads, in particular the anthaspidellids, had tough, ladder-like, desma- based skeletal construction that allowed them to occupy high- energy reef environments (Carrera and Botting, 2008). Other mainly Early Ordovician reef-related metazoans are
the receptaculitid calathids, which commonly are associated with the orchoclad demosponges and recognized as accessory reef constructors (Carrera, 1991; Church, 1991; Cañas and Carrera, 1993; Liu et al., 2005; Adachi et al., 2012a; Hong et al., 2012; Wang et al., 2012; Choh et al., 2013; Li et al., 2015). Calathids are not as diverse as the orchocladine demosponges, but show a widespread distribution (Rowland and Shapiro, 2002; Webby, 2002; Wang et al., 2012). The stromatoporoids or stromatoporoid-like organisms,
commonly domical or encrusting forms, became frame builders or accessory frame builders in varying degrees by the early Tremadocian (Toomey and Nitecki, 1979; Keller and Flügel, 1996; Webby, 2002; Cañas and Carrera, 2003; Zhen and Picket, 2008). Pulchrilamina Toomey and Ham, 1967 from the Lower Ordovician of Utah and New Mexico (Toomey and Nitecki, 1979) was one of the oldest recorded forms. New records of Lower Ordovician reef-related stromatoporoids have been reported from China and Korea (J.-H. Lee et al., 2016; Li et al., 2016). Stromatoporoids and stromatoporoid-like organisms were classically recognized as frame builders because of their
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