How to find early sponge animal fossils?
Where and how to find the oldest sponge fossils? This is a question that has frustrated paleontologists and biologists for many years. In a Nature Communications paper, we answer this question by examining an early Cambrian sponge fossil.
Many scientists believe that, among all living animals, sponges were the first to diverge. Thus their earliest fossil record is crucial for understanding when and how animals evolved and diversified. Most scientists believe that sponges diverged in the Precambrian time, before ~540 million years ago. This is because molecular clocks say that sponges diverged in the Precambrian, molecular fossils – albeit controversial – say that sponges existed in the Precambrian, and many other animals – including bilaterally symmetric animals or bilaterians – left a rich record in the Precambrian as much as 550 million year ago. However, there is so far no convincing sponge fossil from the Precambrian, and this represents an annoying problem for paleontologists, particularly considering that most living sponges have biomineralized skeletons or spicules that should be easily preserved in the fossil record. Several hypotheses have been put forward to explain the lack of Precambrian sponge fossils. One posits that Precambrian environments did not favor the preservation of sponge spicules in the fossil record. Another hypothesis is that, although sponges diverged in the Precambrian, they did not evolve spicules until later in the Cambrian Period. Thus, Precambrian sponge animals may have been small and lacked spicules, hence very low preservation potential. However, it is difficult to test these hypotheses without sponge fossils showing how spicules were acquired. In other words, we cannot effectively test these hypotheses without transitional forms bridging sponges with fully mineralized spicules and those without spicules at all.
Fig. 1. An early Cambrian quarry in South China (a) where abundant sponge spicules (b) have been found.
As part of my PhD project at Virginia Tech, I have been working with collaborators at the Nanjing Institute and Geology and Paleontology in China, to explore early Cambrian sponges in South China. In particular, we have been excavating for fossils, not only sponges but all kinds of other fossils, at a quarry in Xiuning County of southern Anhui Province, South China (Fig. 1a). From my advisor’s previous work, this quarry is particularly rich in large sponge fossils with exceptionally well preserved spicules (Fig. 1b). When I was digging in the quarry, I discovered a kind of small spheroidal fossils (Fig. 2a) that initially did not appear to be sponge fossils because, unlike other sponges in the quarry, these fossils are full of organic matter but few if any spicules discernable to naked eyes. These fossils turned out to be a big suprise when I took a closer look at them under the microscopes back in the lab. I discovered they are actually tiny sponges with many well-preserved spicules concealed in the organic mass (Fig. 2b). An even greater is that these spicules are composed mostly of organic material and very little biomineralic material. This is very different from spicules of living sponges, which are composed mostly of biomineralic material and very little organic material. We suddenly realized that we have discovered the transitional animal with weakly biomineralized spicules that bridge sponges with spicules and those with none.
Fig. 2. Small sponge fossils (a) with weakly biomineralized spicules (b). The spicule shown here is demineralized by geological processes so that what is remaining now is organic material and what used to be biomineralic material is represented by a narrow gap.
Our discovery provides strong evidence supporting the hypothesis that Precambrian sponges may have had only weakly biomineralized spicules or even completely lacked biomineralized spicules (Fig. 3). It thus provides a new search image for Precambrian sponge body fossils. Perhaps Precambrian sponges are not preserved as biomineralized spicules, but as organic remains. This is to say that we should not exclude Precambrian fossils from sponges simply because they lack biomineralized spicules, and we should revisit Precambrian carbonaceous macrofossils that had been previously disregarded as sponges because of the lack of biomineralized spicules. In the end, molecular clocks, molecular fossils, and body fossils should tell a coherent story about early animal evolution.
Fig. 3. Morphological reconstruction of early sponges with transitional, weakly biomineralized spicules.
Click to download the article paper Spiculogenesis and biomineralization in early sponge animals