Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan

Abstract

ABSTRACT. A low diversity fossil assemblage dominated by Mongolitubulus spines, from the Early Cambrian Geres Member of the Shabakhty Formation at Koksu, Malyi Karatau, Kazakhstan, enables identi®cation of its sclerite variability within the scleritome. No other sclerotised structures could be matched with the spines, indicating that these were the only sclerotised part of the body and were periodically shed. The prominent spines of Mongolitubulus have a scale-like external ornamentation and ®brous internal structure. Possibly they armed the body of a lobopodian similar to Xenusion. The Mongolitubulus spines belong to a series of Cambrian phosphatic fossils, the other end members of which are sclerites of Microdictyon, the second most abundant component of the Koksu assemblage. The latter super®cially resemble schizochroal trilobite eyes, but in life covered the dorso-lateral sides of the body segments, as is evident from complete specimens from the Chinese Chengjiang locality. A pair of sclerites of Microdictyon, representing uncompleted exuviation, from Koksu show that each hole originally contained a non-phosphatised lenticular body. This makes a compound eye nature of the Microdictyon sclerites likely, and even their homology with arthropod eyes cannot be excluded.KEY WORDS: lobopodians, sclerites, Early Cambrian, Kazakstan.A F T E R more than 30 years of extensive research on Cambrian phosphatic and secondarily phosphatised skeletal microfossils, initiated by the classic works of Rozanov and Missarzhevsky (1966) and Rozanov et al. (1969), a lot of assembled information on isolated sclerites awaits anatomical interpretation. How risky it is to use such data to restore soft anatomy is strikingly exempli®ed by discoveries of complete fossil organisms (or recognition of earlier ®nds) which were the bearers of sclerites of the HadimopanellaMilaculum type (Kraft and Mergl 1989; Hinz et al. 1990;Conway Morris 1997), Microdictyon (Chen et al. 1989, 1995 or Halkieria (Conway Peel 1990, 1995). In all of these cases earlier attempts to guess the relationships of the fossils failed. The real anatomy has appeared too bizarre to be inferred from the limited morphological evidence provided by sclerites alone. Even now, for only a fraction of the Cambrian sclerites can zoological af®liations be offered. To wait, however, for the discovery of complete soft body fossils for many other kinds of sclerites would not be practical. Working hypotheses on af®nities and function of isolated sclerites are of use simply to guide the search for more complete fossils. In fact, sclerites are usually poorly preserved in otherwise exquisitely preserved soft bodied fossils. One approach to interpretation, which has a long tradition, is to arrange the data into a morphology transition series with forms of known relationships included as end-members. Its reliability depends on the speci®city of the morphology, how much of the series of morphologic transformations is complete, and how much time and space separate particular links of the cha...

Early Cambrian eodiscoid trilobite Hupeidiscus orientalis from South China: ontogeny and implications for affinities of Mongolitubulus-like sclerites

“…This kind of spines has been widely reported from the lower Cambrian, e.g. in Mongolia (Missarzhevsky 1977), Kazakhstan (Dzik 2003), Antarctica (Gazdzicki & Wrona 1986), Greenland (Peel & Blaker 1988, Skovsted & Peel 2001, and Australia (Topper et al 2007). Since Mongolitubulus was described mainly on the basis of isolated sclerites, its scleritome and affinities remained unresolved, being assigned to protoconodonts (Missarzhevsky 1977, Esakova & Zhegallo 1996, or interpreted as carapace spines of bivalved arthropods (Melnikova 2000, Skovsted & Peel 2001 or as lobopodian spines (Dzik 2003).…”

“…Although the scleritomes and zoological affinities of some isolated sclerites, such as Hadimopanella-like sclerites (Hinz et al 1990, Conway Morris 1997, Microdictyon (Chen et al 1989), Halkieria (Conway Morris & Peel 1990), and Paterimitra (Skovsted et al 2009), have been well documented by discovery of extraordinarily preserved material, overall configurations of the scleritomes of many groups still remain enigmatic. In most of the above cases, previous scleritome reconstructions based on disarticulated sclerites have been shown to be inappropriate, though some analyses may provide general guidelines for studying assemblages of disarticulated sclerites or searching for more complete fossils (Bengtson & Conway Morris 1984, Conway Morris & Chen 1997, Dzik 2003. Discoveries of exceptionally preserved specimens have been hitherto the most convincing approach for documenting scleritomes from isolated sclerites, and the search for articulated or well phosphatized fossils continues to be an essential impetus in better understanding the biology of isolated sclerites.…”

“…in Mongolia (Missarzhevsky 1977), Kazakhstan (Dzik 2003), Antarctica (Gazdzicki & Wrona 1986), Greenland (Peel & Blaker 1988, Skovsted & Peel 2001, and Australia (Topper et al 2007). Since Mongolitubulus was described mainly on the basis of isolated sclerites, its scleritome and affinities remained unresolved, being assigned to protoconodonts (Missarzhevsky 1977, Esakova & Zhegallo 1996, or interpreted as carapace spines of bivalved arthropods (Melnikova 2000, Skovsted & Peel 2001 or as lobopodian spines (Dzik 2003). The discovery of bradoriid specimens with scaly ornamented spines from Australia provides definitive evidence for a bradoriid affinity, at least for some Mongolitubulus-like spines (Topper et al 2007), though it could not conclude that all scaly ornamented spines are related to bradoriids.…”

A bradoriid and brachiopod dominated shelly fauna from the Furongian (Cambrian) of Västergötland, Sweden

“…The specimens described herein are also very similar to Nikolarites spasskyi Vasil'eva, 1994 from the upper Atdabanian Stage of the Lena-Aldan region in Siberia (Vasil'eva 1994(Vasil'eva , 1998. These fossils probably represent parts of arthropod, probably bradoriid carapaces (Dzik 2003;. Low tuberculate cones are twice as abundant as tuberculate spines in the collection (62 cones and 32 spines can be recognised).…”

“…Ranges of morphological variability in both form species are, however, broad and overlap, so that it is not possible to associate them with a single or several natural species (Barragán et al 2014a, b). Topper et al 2007), lobopodians (Dzik 2003;Caron et al 2013), or trilobites (Li et al 2012 Mongolitubulus spinosus (Hinz, 1987) Description.-Gently curved hollow calcium phosphatic spines nearly circular in cross-section. The wall consists of two layers.…”