originally described as a glass sponge, some scientists have proposed
a new and extinct Phylum Coeloscleritophora for these enigmatic
animals. This Class also includes Wiwaxiidae and other Cambrian
sclerite-bearing animals, that also occur in the famous Burgess
Shale of Canada and Chengjiang of China.
chancelloriid Allonnia coming from the Lower Cambrian Chengjiang
have done much to help science understand the body plan of these
animals now thought to have had a hollow skeleton made of sclerites
that manifest in a six-ray star shape.
sponges: When people think of fossils, they normally think first
of dinosaurs, of trilobites, of ammonites - but usually not sponges,
yet they should. The humble sponge during modern times is represented
by some 9000 species spread across the globe and occupying essentially
all aquatic environments. Despite their ubiquitous dispersion, they
are of truly ancient origin. Indeed, sponges may well be the "Lucy"
of all of the Kingdom Animalia, since they were likely to have been
the first animals on Earth with cooperative cells. More accurately,
based on phylogenetic data from sponges and other creatures of ancient
origin, we can conjecture that sponges occupy the oldest and lowest
branch on the animal family tree. Because the higher branches have
introduced additional innovations that account for animals' rich
diversity, the common ancestor of all animals likely resembled modern
sponges much more closely than any other living animals. Sponges'
ability to grow different cell types performing different and cooperative
functions was an innovation that underlies virtually all subsequent
advances in the animal kingdom. While their cell signally pathways
are simplistic compared to most modern animals, it obviously was
robust enough for them to survive the numerous mass extinctions
on Earth since the origin of sponges in Precambrian time.
See: Utah Cambrian Explosion