Genus Markuelia

Abstract

This page dicusses the Cambrian soft-bodied fossil embryo, Markuelia. The primary source references are Bengtson & Yue 1997, and Dong et al. 2004. Any errors, however, are mine.

Keywords: Cambrian, fossil embryo, Markuelia, Scalidophora, Cephalorhyncha

Introduction

Soft body preservation is the exception rather than the rule in the fossil record. Nevertheless, there is an abundance of small globular structures in the fossil record, including that of the Cambrian. One example is the genus Markuelia ...

early interpretations

A closer investigation of a collection of 15 specimens reveals that the chambers are segments of a wormlike animal that is tightly looped into a sphere ...

Geological Setting

Stratigraphy

Two species of this genus have been described to date. One, Markuelia hunanensis, is reported from the Middle Upper Cambrian Bitiao Formation of Wangcun, Hunan, south China. The other species, Markuelia secunda, derives from the basal Pestrotsvet Formation (Lower Cambrian, Lower Tommotian) in the classical Dvortsy section on the Aldan River in southern Yakutia, Siberia.

Preservation

The fossilized Early Cambrian embryos described here are preserved as diagenetically phosphatized replacements and encrustations in marine limestones. Early diagenetic phosphatization has tremendous potential for replicating delicate biological tissues but appears to take place under local conditions where pH and activities of P and HCO₃- turn phosphatization on and off, and the conditions conducive to phosphatization are enhanced by bacterial activity. Metazoan eggs are readily phosphatized under experimental conditions, and the occurrences reported by Zhang and Pratt 1994 and by Bengtson & Yue 1997, from beds where other fossilized nonstructural tissues are absent, suggest that eggs are particularly susceptible to such phosphatization.

Systematics

??? references ???

Superphylum ?Introverta Nielsen 1995

Markuelia is not unequivocally identifiable with any of the Cambrian segmented bilaterians known from soft-body preservation, although the regular pattern of processes should be recognizable if it persists into the adult stage. The development appears to be direct, with no evidence of a primary larva. The large number of equal segments and the wormlike body point to annelid affinity. If so, the direct development exhibited by Markuelia is anomalous.

The tendency of the rod-shaped organs to cut across segment boundaries in Markuelia secunda suggests that no transverse mesenteries were present, which would make the animal more comparable to arthropods or lobopods. Dong et al. (2004) thought that the lack of paired appendages argued against such an affinity, however, and speculated that Markuelia may instead be related to stem group Scalidophora (= Cephalorhyncha). The terminal position of the mouth and the circum-oral spines in Markuelia hunanensis are cited as supporting evidence.

The Inroverta is a grouping of five phyla - Nematoda, Nematomorpha, Priapula, Kinorhyncha and Loricifera - which share a number of features, most notably an eversible proboscis. "The structure of the mouth in Markuelia [hunanensis], although not definitive, is compatible with the presence of proboscis" (Dong et al. 2004, p. 239). If Markuelia is indeed a stem group scalidophoran, it implies that the indirect development (larvae) of priapulids and loriciferans is a derived, secondary condition. This view is also consistent with other evolutionary embryological studies.

Phylum Unknown

Genus Markuelia Val'kov 1983

Development: A notable feature of Markuelia is direct development, which "contrasts both with the primitive condition inferred for metazoan phyla and with many proposed hypotheses of affinity [Bengtson & Yue 1997; Conway Morris 1998], all of which prescribe indirect development" (Dong et al. 2004, p. 237).

If Markuelia is an annelid, it lacks the trochophore larva of polychaetes and would appear more similar to oligochaetes or leeches in mode of development. Likewise, terrestrial representatives of arthropods and lobopods favor direct development. Indirect development involving primary larvae is generally regarded as primitive among metazoans, direct development evolving independently when conditions do not favor the release of large numbers of larvae into the plankton. This may be the case in a harsh or unpredictable environment (reducing the survival chances for unprotected larvae), or where adult body size is small (preventing the production of large numbers of eggs).

Markuelia, however, may represent a biased sample of early metazoan development, because eggs of direct developers are generally larger (and yolkier) than those of indirect developers and thus may be more readily preserved as fossils and easier to identify.

Markuelia hunanensis Dong et al. 2004

Description: The material described by Dong et al. (2004) comprises a collection of fossil embryos varying in size and the presumed developmental stage represented. The smallest and earliest stage specimens had a diameter of ~230 µm and represent ~480 cells. Larger specimens range up to ~410 µm representing later developmental stages. Differentiated cells do not fully occupy the volume of the embryo; the remaining undifferentiated tissue is presumed to be yolk.

The latest developmental stages possess a worm-like anatomy coiled into a sphere with the head and tail side by side. The trunk is approximately 180-190 µm wide and annulated into segments ~20-25 µm long, suggesting ~130 annulae in total. In fractured specimens, divisions between the annulae are seen to extend internally, supporting the notion that they are true "segments" and not simply surface ornamentation. Annulae are less obviously developed near the head and tail. The trunk possesses an ornament of fine ribbing, with no indication of the broadly conical processes typical of M. secunda (see below).

The head possesses a terminal mouth surrounded by at least three partly overlapping, flat, broadly triangular, rows of posteriorly directed spines which were presumably radially arranged in life. The tail region bears six curved spines, roughly equal in length, possibly arranged radially around a terminal depression or opening. The spines at both ends of the organism are smooth.

The reconstructed length of the whole organism is just over 3 mm.

Markuelia secunda who???

The anterior end is not well preserved in the available specimens; it tends to expand laterally and be quite featureless externally. In fig. 1, the anterior region has a more complex type of preservation, with irregular phosphatization visible internally. This specimen shows a boss-like structure in the middle of the expanded anterior region (fig. 1, middle left).

The posterior end has two modified segments, each with two symmetrically placed processes that join into a kind of posterior comb (fig. 1, bottom right).

The segments carry conical processes that vary somewhat in shape but where most clearly exposed (fig. 1, right) are divided into an apical, narrow part and a basal, more flaring part. Typically, a process recurs in a similar position on every third segment. There seems to be no morphological difference between the processes occurring on the flat, exposed side and those that are laterally placed, except that the former tend to be flatter, presumably because they have been squeezed against an outer egg membrane. Judging from their distribution on the body, the processes cannot be interpreted as paired appendages or parapodia.

Discussion: M. secunda appears to be more flattened than M. hunanensis, possibly exhibiting a bilateral rather than radial construction, particularly of the tail.

Broken specimens reveal internal rodlike structures (fig. 2, upper part), corresponding in number and position to the segments as visible on the body surface. The rods are about 10 µm in diameter, but as they consist of a diagenetic apatite crust, this dimension may not correspond to that of the original tissue. In fig. 2, some of the rods are at an angle to the segment boundaries, and elsewhere they have been observed to be detached from the body wall. Thus they seem to represent an organ system that is not part of body wall or mesenteries. There are several possible interpretations of these structures, including gut diverticules, blood vessels, muscles, nerves, nephridia, or gonads. On the basis of their regular distribution, thin rod shape, and presence in the embryonic state, however, we regard the structures most likely to be lateral nerve branches from a medial longitudinal nerve chord.

References

Bengtson, Stefan; Yue, Zhao 1997: Fossilized metazoan embryos from the earliest Cambrian. Science 277: 1645-1648.

Conway Morris, Simon 1998: Eggs and embryos from the Cambrian. Bioessays 20: 676-682.

Dong, Xi-ping; Donoghue, Philip C.J.; Cheng, Hong; Liu, Jian-bo 2004: Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. Nature 427: 237-240.

Nielsen, C. 1995: Animal evolution. First edition. Oxford University Press.

Val'kov, A.K. 1983 in Khomentovsky, V.V.; Yakshin, M.S.; Karlova, G.A. (eds.) 1983: Pozdnij Dokembrij Rannij Peleozoj Sibiri, Vendskie Othozheniya. Inst. Geol. Geofiz. SO AN SSSR, Novosibirsk, pp. 37-48.

Zhang, Xi-guang; Pratt, Brian R. 1994: Middle Cambrian arthropod embryos with blastomeres. Science 266: 637-639