Preserved Dinosaur Blood Vessels

The discovery of preserved soft tissues in dinosaur bone has a history stretching back over half a century. In 1966, Roman Pawlicki, A. Korbel, and H. Kubiak published a paper in Nature reporting cells, collagen fibrils, and vascular structures in dinosaur bone, marking the earliest scientific documentation of such preservation. However, these findings received limited attention at the time, and the prevailing assumption in paleontology remained that organic materials could not survive beyond roughly one million years.

The field was transformed in 2005 when Mary H. Schweitzer and colleagues at North Carolina State University published a landmark paper in Science (Schweitzer et al. 2005, DOI: 10.1126/science.1108397). Working with specimen MOR 1125, a 68-million-year-old Tyrannosaurus rex femur from the Hell Creek Formation of Montana, the team dissolved the mineral phase of the bone using EDTA and recovered transparent, flexible, hollow blood vessels. Some of these vessels contained small round microstructures that could be expressed from the vessel interiors, bearing morphological resemblance to red blood cells. This discovery challenged fundamental assumptions about the limits of organic preservation and ignited both intense scientific interest and significant controversy.

Research over the past two decades has revealed that dinosaur blood vessels can be preserved in fundamentally different ways, broadly categorized into two modes.

The first mode involves the recovery of pliable, semi-transparent vessels after chemical dissolution of the surrounding bone mineral. These structures, best exemplified by the work of Schweitzer and colleagues, retain flexibility and in some cases respond to antibodies against vertebrate-specific proteins such as collagen type I, elastin, laminin, and tropomyosin. Transmission electron microscopy (TEM) has revealed multi-layered wall architecture in these vessels, with features consistent with the endothelial lining (tunica intima), smooth muscle layer (tunica media), and external connective tissue (tunica adventitia) of living vertebrate blood vessels. Scanning electron microscopy (SEM) shows that the external surfaces of these vessels display a striated, fibrous texture consistent with remnant collagen of the tunica externa, which is virtually identical to the external texture observed in ostrich blood vessels used as comparative controls.

The second mode involves fully mineralized casts, where the original organic vessel has been replaced by or encased in iron-rich minerals. A prominent 2025 study by Mitchell et al. (DOI: 10.1038/s41598-025-06981-z) documented this type of preservation in a fractured rib from 'Scotty' (RSM P2523.8), the largest known Tyrannosaurus rex specimen, held at the Royal Saskatchewan Museum in Canada. Using synchrotron micro-computed tomography (SR-μCT) at the Canadian Light Source, the researchers visualized large, meandering, high-density structures within the bone that ranged from 100 μm to over 1 mm in diameter—far larger than typical Haversian canal vessels. These structures branched extensively and ran orthogonal to the osteons, from the cancellous interior toward the outer cortical bone and callus zone. Chemical analysis using synchrotron X-ray fluorescence (XRF) and X-ray absorption near-edge structure (XANES) revealed that the vessel casts were composed predominantly of iron(III) oxyhydroxide (goethite, FeOOH) with patches of pyrite (FeS₂), preserved in two distinct depositional layers.

Several interrelated chemical and taphonomic mechanisms have been proposed to explain how dinosaur blood vessels survive across tens of millions of years.

Iron-mediated Fenton chemistry and protein cross-linking: The most widely accepted mechanism, proposed and experimentally supported by Schweitzer et al. (2014, DOI: 10.1098/rspb.2013.2741), centers on the role of iron released from hemoglobin after death. In life, approximately 85% of iron in vertebrate bodies resides in hemoglobin within red blood cells. After death, erythrocyte lysis releases hemoglobin into the vascular lumen, where iron catalyzes Fenton-type reactions producing hydroxyl radicals. These radicals induce protein and lipid cross-linking in a manner analogous to chemical fixatives like formaldehyde, rendering the tissues resistant to enzymatic and microbial degradation. Experimental validation using an ostrich blood vessel model demonstrated that incubation in hemoglobin increased tissue stability more than 200-fold (from approximately 3 days to over 2 years at room temperature), with the greatest stabilization occurring in the presence of both hemoglobin and oxygen (HB + O₂ > HB − O₂ >> −O₂).

Permineralization and mineral casting: In some specimens, the original organic material has been wholly or partially replaced by iron minerals. The Scotty rib study (Mitchell et al. 2025) documented a two-stage permineralization process: first, fine-grained botryoidal pyrite formed in anoxic conditions, coating the vessel walls; then, after an oxidation event, this pyrite was partially converted to goethite or hematite. A second generation of coarser crystalline pyrite subsequently formed during a second anoxic period, followed by partial oxidation to hematite. This complex depositional history reflects the fluctuating redox conditions of the burial environment over geological time.

Glycation and advanced glycation end-products (AGEs): A 2019 study (Anné et al., DOI: 10.1038/s41598-019-51680-1) identified evidence that glycation—a non-enzymatic reaction between sugars and proteins—may also contribute to vessel preservation by forming additional cross-links in structural proteins like collagen.

Bone mineral shielding: The dense hydroxyapatite matrix of bone provides a physical barrier against microbial access and environmental degradation, creating a protected microenvironment where cross-linked soft tissues can persist. This is supported by the observation that vessels are consistently recovered from within intact bone, not from exposed surfaces.

The year 2025 saw three major publications that significantly advanced understanding of dinosaur vascular preservation.

Schweitzer et al. (February 2025): Published in Scientific Reports (DOI: 10.1038/s41598-025-85497-y), this comprehensive study examined vessel-like material from six non-avian dinosaurs: four Tyrannosaurus rex specimens (MOR 555/USNM 555000, MOR 1125, MOR 1126, MOR 1128), one Brachylophosaurus canadensis (MOR 2598), and one indeterminate ceratopsid (MOR 10857). The fossils ranged in age from approximately 66 to 85 million years and came from different depositional environments. Using transmitted light microscopy, SEM, TEM, nano-CT, immunofluorescence, immunogold labeling, lactophenol cotton blue staining, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), the team demonstrated that vessels could be recovered from all six specimens regardless of taxon, geological age, or burial environment. Antibody tests confirmed the presence of elastin, tropomyosin, laminin, and hemoglobin proteins, and ToF-SIMS detected protein-related secondary ion fragments consistent with peptides rather than free amino acids. The study also acknowledged the presence of invasive microorganisms (fungal hyphae) in some samples, highlighting the complex taphonomic cocktail of endogenous and exogenous components.

Mitchell et al. (July 2025): Published in Scientific Reports (DOI: 10.1038/s41598-025-06981-z), this study focused on a fractured dorsal rib from 'Scotty' (RSM P2523.8), a T. rex discovered in 1991 near Eastend, Saskatchewan, Canada, in the Late Cretaceous Frenchman Formation (~67 Ma). Scotty is estimated to be the largest T. rex ever recovered, with approximately 65% of the skeleton preserved. The rib displayed an incompletely healed fracture with a large callus, suggesting the animal had sustained the injury months before death. Using synchrotron radiation at the Canadian Light Source, the team produced high-resolution 3D models revealing a network of large vessel-like structures that were interpreted as angiogenic blood vessels formed during fracture healing. These structures were absent in healthy sections of the same rib, supporting their angiogenic origin. Critically, this represented the first in situ, high-resolution, 3D characterization of dinosaur angiogenesis and demonstrated that pathological bones may be particularly promising targets for future soft tissue searches.

Long et al. (September 2025): Published in Proceedings of the Royal Society A (DOI: 10.1098/rspa.2025.0175), this study used Resonance Raman (RR) spectroscopy to confirm the presence of hemoglobin remnants in bone extracts from T. rex and Brachylophosaurus canadensis. The technique's double selectivity—tuning the laser to resonate specifically with heme-globin bonds—allowed the researchers to detect hemoglobin signal even within the complex chemical background of fossilized bone. The study also revealed that heme degrades by forming bonds with the mineral goethite, providing a direct chemical link between the biological iron source and the iron minerals consistently found associated with preserved dinosaur soft tissues.

The preservation of dinosaur blood vessels and associated soft tissues has been one of the most debated topics in modern paleontology. Several alternative hypotheses have been proposed to explain the recovered structures.

In 2008, Kaye, Gaugler, and Sawlowicz proposed that the vessel-like structures were actually bacterial biofilms rather than endogenous dinosaur tissue. However, multiple lines of evidence have since countered this hypothesis: immunological reactivity to vertebrate-specific proteins not found in microbes; peptide sequence data matching vertebrate rather than microbial proteins; identification of histones (eukaryote-specific nuclear proteins); and distinct morphological features including luminal-exterior surface asymmetry and tight junctions inconsistent with biofilm architecture.

Another concern relates to contamination—the possibility that modern proteins or microorganisms have infiltrated the fossil bone. The 2025 Schweitzer et al. study addressed this by employing multiple complementary analytical techniques and by demonstrating that protein signals localized specifically to sheet-like structures consistent with vessel walls, rather than being randomly distributed. The study also explicitly acknowledged microbial components but demonstrated these were distinguishable from endogenous vascular remains.

A more fundamental challenge comes from theoretical models of biomolecule degradation, which predict that proteins should not survive beyond approximately 1 million years and DNA beyond roughly 100,000 years under standard diagenetic conditions. The iron-mediated preservation hypothesis provides a chemical mechanism to explain how these theoretical limits can be exceeded under specific conditions, but some researchers remain unconvinced that any biological macromolecule can truly persist across the 66–195 million year timescales involved.

The preservation of dinosaur blood vessels carries implications across multiple scientific disciplines.

Paleophysiology: Analysis of preserved vascular structures can provide information about dinosaur healing responses, metabolic rates, and circulatory systems. The Scotty rib study (Mitchell et al. 2025) demonstrated that examining fracture-induced angiogenesis in dinosaurs could reveal how these animals healed from injuries, potentially offering comparisons with healing in modern birds and crocodilians.

Molecular paleontology: The recovery of proteins (collagen, elastin, hemoglobin) from dinosaur vessels opens the possibility of using molecular data to independently test phylogenetic relationships determined by morphological characters. Protein sequences recovered from T. rex and B. canadensis have already been used to demonstrate molecular phylogenetic consistency with morphology-based trees.

Taphonomy: Understanding the mechanisms that preserve blood vessels informs broader questions about exceptional preservation in the fossil record. The 2025 findings suggest that vessel preservation may be more common than previously thought and that injury-related pathologies in bone create favorable conditions for vascular preservation due to increased vessel density and larger vessel size.

Evolutionary biology: Preserved hemoglobin and structural proteins from deep time could potentially reveal physiological adaptations not discernible from skeletal morphology alone, such as oxygen-transport adaptations or tissue-repair strategies in extinct lineages.

The Scotty blood vessel discovery was recognized by the Smithsonian Magazine as one of the top ten dinosaur discoveries of 2025. The recognition highlighted both the novelty of the in situ synchrotron-based approach and its potential to reshape future research strategies, particularly by directing attention toward pathological bones as targets for soft tissue preservation studies.

Ongoing and planned research directions include applying the synchrotron-based in situ approach to additional pathological dinosaur bones from different species and time periods; using iron chelation techniques to unmask proteins hidden beneath mineral crusts; comparing angiogenic vessel morphology across dinosaur taxa and their extant relatives (birds and crocodilians); investigating whether DNA or DNA-protein cross-links can be detected in association with preserved vascular tissues; and refining chemical models of the diagenetic pathways that lead to long-term molecular preservation. The convergence of advanced physics instrumentation (synchrotron radiation, Resonance Raman spectroscopy) with molecular biology and paleontology promises to continue yielding discoveries that fundamentally alter understanding of what the fossil record can reveal.