Page 251 - Introduction to Paleobiology and The Fossil Record

238 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a)
(b) (c)

(e)
(d) (f)

(h)

(g) (i)

Figure 10.3 Animal embryos from the Doushantou Formation, China. (a) Surface of embryo
based on tomographic scans together with (b) an orthoslice revealing subcellular structures
analogous to modern lipids and (c) an orthoslice at the boundary between two cells. (c, f) Two-
cell embryo of the sea urchin Heliocidaris showing lipid vesicles for comparison. (e) Orthoslice
rendering of a possible embryo revealing internal structures. (g–i) Models of tetrahedrally
arranged cells. Relative scale bar (see top left): 170 μm (a–d, f), 270 μm (e), 150 μm (g–i).
(Courtesy of Philip Donoghue.)

Biomarker evidence strong indicators of life forms. More exciting

is the fact that specific biomarkers may be
Biomarkers, essentially the biochemical fi n- related to particular groups of organisms. Sig-
gerprints of life, have become increasingly nificantly, biomarkers associated with meta-

important in astrobiology, where they have zoan demosponges (see p. 262) have now
been sought in the quest for extraterrestrial been reported from rocks older than the Edia-

life. But they are also of considerable impor- caran, confirming the presence of basal meta-
tance in the investigation of Precambrian life zoans at this time. But since the sponges are
(see p. 188), where other lines of evidence are paraphyletic, biomarkers from the homoscle-
lacking. Thus amino acids, hopanes, some romorph sponges (see p. 262) would also
types of hydrocarbons, evidence of isotopic have to be present to prove the presence of
12
fractionation in carbon ( C) and biofi lms are the eumetazoans.