Collagen-Eating Microbes May Explain Gaps in the Fossil Record

The fossil record often contains puzzling "gaps": in certain geological strata, hard-tissue fossils such as bones and teeth suddenly become scarce or even disappear entirely. Traditional explanations mostly attribute this to poor burial conditions, later geological alteration, or sampling bias, but a new study proposes a rather disruptive idea—that blooms of microbes specialized in "eating" collagen may have accelerated the decomposition of bone organic matter, thereby leaving blanks in the fossil record.

The "Missing Bones" in the Fossil Record

The formation of bone fossils is essentially the result of a contest between minerals (mainly hydroxyapatite) and organic matter (predominantly collagen) during burial. Collagen accounts for the vast majority of bone organic components, and its degradation rate directly affects whether bone can maintain structural integrity over vast geological timescales.

When microbial communities can efficiently decompose collagen, the organic scaffold of bone is rapidly hollowed out, and the remaining mineral portion becomes more susceptible to compaction, dissolution, or alteration during diagenesis. The end result is not "no organisms died," but rather "nothing was left after death." This offers a biological explanation—not merely a geological one—for discontinuities in the fossil record.

Hydroxyapatite is the main component of the inorganic phase of bones and teeth, with the chemical formula Ca₁₀(PO₄)₆(OH)₂, belonging to the hexagonal crystal system. In bone, it exists as nanoscale needle-like or plate-like crystals oriented along the long axis of collagen fibers, together with collagen forming the basic structural unit known as "mineralized collagen fibrils." This organic-inorganic composite structure gives bone high strength and toughness: the mineral phase provides compressive resistance, while collagen fibers bear tensile loads and dissipate fracture energy. Precisely because the two are tightly bound, the preservation of bone fossils requires that the mineral phase maintain its morphology after organic matter degradation, or be filled and replaced by secondary minerals such as calcite or quartz through diagenesis. Once the collagen scaffold is rapidly removed, the loosely accumulated hydroxyapatite crystallites are highly prone to recrystallization or dissolution under burial compaction, and bone morphology is lost. Understanding this mineralization mechanism is a key prerequisite for judging whether microbial degradation can truly "erase" fossils.

How Microbes "Eat" Collagen

Collagen is a highly structurally stable fibrous protein that ordinary microbes find difficult to degrade directly. However, specific groups of bacteria and fungi can secrete collagenases that cleave collagen into small peptides and amino acids, which are then further metabolized and utilized.

The core question researchers focus on is: could the abundance and activity of such "collagen-eating" microbes rise sharply under certain environmental conditions? If the answer is yes, then the early degradation window after bone burial may be shorter than previously thought, and the probability of fossil preservation correspondingly decreases. This means that "blanks" in the fossil record may not represent a decline in biodiversity, but may simply reflect preservation conditions being rewritten at the microscopic level by microbes.

Collagenase is a class of proteases capable of specifically cleaving the triple-helical regions of native collagen, belonging to the matrix metalloproteinase (MMP) family or the bacterial collagenase family. Because of its tight triple-helical structure formed by Gly-X-Y repeat sequences, collagen is resistant to most proteases; ordinary proteases can only act on its non-helical telopeptide regions. Collagenase, however, can hydrolyze peptide bonds at specific sites within the triple helix under physiological conditions, cutting collagen molecules into fragments that become denatured and susceptible to further degradation by gelatinases and peptidases. Microbial collagenases (such as Clostridium histolyticum collagenase) play important roles in natural organic matter cycling and are also common tools in tissue engineering and cell isolation experiments. For paleontologists, the presence of collagenase activity or related genes in sediments can serve as a potential biomarker of past "collagen-eating microbe" activity, providing measurable clues for identifying fossil preservation bias.

Implications for Paleontological Methodology

The value of this idea lies in pulling the interpretive authority of the fossil record back from pure geology to ecology and microbiology. When analyzing fossil preservation conditions in the past, researchers focused more on sedimentation rates, redox conditions, and diagenetic temperatures, while the role of microbial communities was often overlooked.

If collagen-eating microbial activity can indeed significantly affect bone preservation, then future fossil surveys may need to simultaneously collect microbial markers from sediments to determine whether a given stratum experienced "microbial clearing." This can also help distinguish "true extinction" from "false gaps," with direct significance for understanding the pace of biological evolution before and after mass extinction events.

Microbial biomarkers refer to molecular traces such as lipids, pigments, or gene fragments derived from specific microbial groups, commonly used to reconstruct microbial community composition and activity in geological records. Common types include membrane lipids of bacteria and archaea (such as glycerol dialkyl glycerol tetraethers, GDGTs), fungal ergosterol, and microbial DNA fragments preserved in sediments. In fossil preservation research, if microbial lipids or functional genes related to collagen degradation (such as collagenase genes) can be detected in sediments surrounding bone fossils, it can be inferred that the stratum experienced strong microbial degradation pressure. Combined with sedimentological and diagenetic indicators, such biomarkers hold promise for establishing identification criteria for "microbial preservation bias," transforming gaps in the fossil record from inexplicable "noise" into attributable ecological signals.

A Hypothesis Yet to Be Tested

It must be emphasized that this is an explanatory hypothesis, not an established mechanism. The causal relationship between microbial activity and fossil preservation needs to be verified through controlled experiments and more field sections. Microbial communities differ enormously across depositional environments, and collagen degradation rates may also vary significantly with temperature, pH, and oxygen conditions.

Moreover, gaps in the fossil record are often the result of multiple superimposed factors; microbial action may be one link in the chain but can hardly explain all discontinuity phenomena alone. Incorporating this hypothesis into the existing framework, rather than replacing original explanations, is the more prudent attitude.

Summary

"Collagen-eating microbes" offer a microscopic perspective: the incompleteness of the fossil record may not only be the result of geological processes but also a byproduct of microbial metabolism. It reminds the paleontological community that when interpreting the presence or absence of bone fossils, it is worth including the decomposers under the microscope in the consideration.

One-line Summary

Collagen-eating microbes accelerate bone organic matter decomposition and may explain gaps in the fossil record.

Paragraph Summary

The article introduces an explanatory hypothesis proposed by a new study: blooms of microbes specialized in decomposing collagen may accelerate bone organic matter degradation, causing bones to be "hollowed out" before diagenesis and thereby leaving gaps in the fossil record. Traditionally such discontinuities are mostly attributed to poor burial conditions or sampling bias, but this hypothesis pulls interpretive authority from pure geology back to ecology and microbiology. The article outlines the contest between bone mineralization and collagen degradation, the mechanism of collagenase action, and points out that future fossil surveys should simultaneously collect microbial biomarkers to distinguish "true extinction" from "false gaps." The author also emphasizes that this remains a hypothesis awaiting verification through controlled experiments and field sections, and should be incorporated into rather than replace existing explanatory frameworks.