First Vertebrate Fossil From the Nummulites obtusus Bed — A Crocodilian Skull Replaced by Iron

Introduction

A crocodilian skull was recovered from a carbonate horizon in the Kutch Basin of western India that had never before yielded a vertebrate fossil. The skull — from the *Nummulites obtusus* bed of the Harudi Formation, middle Eocene, approximately 41–42 million years old — shows something remarkable: the bone tissue has been entirely replaced by iron-bearing minerals that have precisely replicated the original bone architecture at macroscopic scale. The teeth, the sutural contacts, the three-dimensional form — all preserved in iron where bone once was.

The surrounding matrix is packed with *Nummulites obtusus* — large foraminifera whose calcite shells show no ferruginous alteration whatsoever. Same horizon, same burial environment, completely different preservation. The iron replacement is confined entirely to the vertebrate material.

This short report, a collaboration between Blue Blocks Micro Research Institute and the Indian Statistical Institute, documents this selective preservation and proposes a mechanism: the organic matter in vertebrate bone — collagen and associated compounds — created a locally reducing microenvironment during early diagenesis that mobilised iron from sediment pore waters. The *Nummulites*, composed of essentially pure low-magnesium calcite with negligible organic content, generated no such environment. The differential organic content drove differential preservation.

Repository & Access

• Taphonomic Crocodilian Skull Findings (DOI)

Abstract

This work reports selective taphonomic alterations in a crocodilian skull from the *Nummulites obtusus* bed of the Harudi Formation (middle Eocene, Bartonian), Kutch Basin, western India, which is also the first vertebrate fossil reported from this horizon. This horizon is a bar/shoal setting with densely packed *N. obtusus* fossils, and the reported vertebrate fossil shows pervasive ferruginous mineral replacement where the iron-bearing minerals have precisely replicated the original bone architecture. This replacement is only localised to the vertebrate fossils, and the *Nummulites* tests do not show any such alteration. This selectivity is interpreted as a product of organic matter-driven early diagenesis. The decay of bone-hosted collagen and associated organic compounds created a locally reducing microenvironment that promoted iron mobilisation and precipitation within and around the bone framework, templated by the organic substrate. The *Nummulites* tests are composed of low magnesium calcite. This study thus documents a distinct taphonomic preservation mode in the Middle Eocene Harudi Formation, highlighting the role of differential organic content in driving selective authigenic mineralisation within carbonate-dominated depositional systems.

Geological Context

The Kutch Basin is a pericratonic rift basin on the western margin of India in present-day Gujarat. The Harudi Formation is of Bartonian age (middle Eocene, approximately 41–42.5 Ma) and represents a transgressive carbonate ramp succession. The Nummulites obtusus bed occurs towards the top of the formation and is a laterally persistent carbonate unit dominated by densely packed N. obtusus tests — the biostratigraphic index for the upper part of the formation.

Key Findings

1. First vertebrate fossil recovered from the *Nummulites obtusus* bed of the Harudi Formation. Despite intensive palaeontological sampling of the Harudi Formation over several decades, no vertebrate material had previously been reported from this specific carbonate horizon. Vertebrate fossils from the formation had predominantly been recovered from coquina units lower in the succession.

2. The crocodilian skull shows pervasive and morphologically faithful ferruginous replacement. Iron-bearing minerals — identified macroscopically as goethite, haematite, or limonite — have precisely replicated the external bone morphology, preserving surface textures, sutural contacts, and three-dimensional form. The fidelity of the replacement is remarkable at the macroscopic scale.

3. The iron replacement is entirely absent from co-occurring *Nummulites* tests. Foraminifera preserved in the same matrix retain their original whitish to pale cream carbonate colouration with no ferruginous staining or alteration. The spatial contrast between the intensely ferruginised skull and the unaltered foraminiferal matrix is sharp and consistent.

4. The selectivity is interpreted as a product of differential organic content. Vertebrate bone contains 20–30% collagen by weight. During early diagenesis, the decomposition of this organic matrix created a locally reducing microenvironment that mobilised dissolved iron from sediment pore waters. The *Nummulites* tests, composed of essentially pure low-magnesium calcite with negligible organic content, generated no such reducing environment.

5. The taphonomic mode documents a previously undescribed preservation style in the Harudi Formation. The combination of selectivity (vertebrate-only) and fidelity (morphologically faithful pseudomorphic replacement) within a carbonate ramp setting with co-occurring unaltered foraminifera is particularly instructive for understanding local controls on differential preservation in Eocene marine settings.

6. The specimen likely drifted from the lagoonal environment and was rapidly buried in the bar/shoal setting. The skull was preserved ventral side up, suggesting overturning prior to or during early burial, followed by compaction that contributed to its fragmented condition.

Study Parameters

Specimen & Setting

Specimen: Partial crocodilian skull (PG/KR/Cr) — Crocodilia indet. (formal assignment pending). Horizon: Nummulites obtusus bed, Harudi Formation. Age: Middle Eocene (Bartonian), ~41–42.5 Ma. Basin: Kutch Basin, western India (Gujarat). Depositional Environment: Bar/shoal setting within carbonate ramp succession. Preservation: Pervasive ferruginous replacement (goethite/haematite/limonite). Selectivity: Iron replacement confined to vertebrate material; Nummulites tests unaltered. Mechanism: Organic matter-driven early diagenesis. Repository: Presidency University, Kolkata, India.

Collaboration

This research is a collaboration between:

- Blue Blocks Micro Research Institute (Hyderabad, India) — Sreemoyee Chakraborty, Research Head and STEM Lead
- Indian Statistical Institute (Kolkata, India) — Prof. Dhurjati Prasad Sengupta

The specimen (PG/KR/Cr) is housed at Presidency University, Kolkata, India, and is available to researchers upon reasonable request.

This collaboration represents the extension of BBMRI's research programme into domain-specialist palaeontological work — the same geological formations (Kutch Basin, Harudi Formation, Fulra Limestone) that form the basis of the CS-2026-005 case study in which adolescents were taught to identify foraminifera from these horizons.

References (Selected)

Bajpai, Thewissen & Conley (2011); Behrensmeyer (1978); Chakraborty & Sengupta (2023); Dauphin (2022); Fernández-Jalvo et al. (2014); Pfretzschner (2004); Saraswati et al. (2016); Trueman, Benton & Palmer (2003).

Study Parameters

References (Selected)

Bajpai, S., Thewissen, J. G. M., Conley, R. W. (2011). Cranial anatomy of middle Eocene *Remingtonocetus* from Kutch, India. *Journal of Paleontology*, 85, 703–718.

Behrensmeyer, A. K. (1978). Taphonomic and ecologic information from bone weathering. *Paleobiology*, 4, 150–162.

Chakraborty, S., & Sengupta, D. P. (2023). A new skull of early cetacean *Remingtonocetus harudiensis* from the Eocene of Kutch Basin, India. *Palaeoworld*, 32(3), 509–522.

Dauphin, Y. (2022). Vertebrate taphonomy and diagenesis: implications of structural and compositional alterations of phosphate biominerals. *Minerals*, 12, 180.

Fernández-Jalvo, Y., et al. (2014). Bacterial origin of iron-rich microspheres in Miocene mammalian fossils. *Palaeogeography, Palaeoclimatology, Palaeoecology*, 414, 6–18.

Pfretzschner, H.-U. (2004). Fossilization of Haversian bone in aquatic environments. *CR Palevol*, 3, 605–616.

Saraswati, P. K., et al. (2016). Facies characteristics and depositional environments of the middle Eocene Harudi Formation, Kachchh, western India. *Facies*, 63, 1–20.

Trueman, C. N., Benton, M. J., & Palmer, M. R. (2003). Geochemical taphonomy of shallow marine vertebrate assemblages. *Palaeogeography, Palaeoclimatology, Palaeoecology*, 197, 151–169.

Citation Note

Selected references foundational to the taphonomic interpretation. Full reference list available in the Research Square preprint.

How to Cite (APA)

Chakraborty, S., & Sengupta, D. P. (2026). Taphonomic significance of the selective Iron replacement in a crocodilian skull from the Nummulites obtusus bed, Harudi Formation, Kutch Basin, Western India [Short report, preprint]. Research Square. https://doi.org/10.21203/rs.3.rs-9531614/v1

Cross-References

• Deep Time Case Study (CS-2026-005)
• Publications Index

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• Deep Time Palaeontology (Case 5)
• Cross-Study Dataset
• Publications Index