Report of bioerosions and cells in Cainotheriidae (Mammalia, Artiodactyla) from the phosphorites of Quercy (SW France)

Abstract

The phosphorites of the Quercy from SouthWest France are well known for fossils preserved in 3D with phosphatized soft-tissues. Given that phosphatization is known to favor fine cellular preservation, the present study delves into the histological analysis of white and brown bones of Cainotheriidae (Artiodactyla) recently excavated from the DAM1 site near Caylus. Microscopy revealed that the white bones were completely filled with bacterial erosions, while the brown bones showed a pristine histology and intralacunar content resembling fossilized osteocytes in some areas. After decalcification, a brown bone revealed an abundance of blood vessel-like structures, innumerable osteocyte-like structures with canaliculi and a few chondrocyte-like structures, while a white bone revealed only blood vessel-like structures that looked eaten away. All the data combined suggest the brown bones were shielded from bacterial attacks and were filled with fossilized organic matter and original biological structures. The data taken all together do not support that these structures are casts, but indeed original and endogenous cells. This study encourages further histochemical and mineralogical analyses on Quercy fossils and the unique taphonomy of DAM1 to better understand fossilization processes and their impact on the color of bones, the chemistry of skeletal tissues, soft tissues, and cells.

Keywords: Bacterial erosions; Bone color; Cells; Phosphatization; Quercy.

Fig. 1

The eight Cainotheriidae fossils bones from DAM1 analyzed here and showing different colors. The colors range from white to brown. A, white patella UM-DAM1-278 (QU-1) and decalcified white patella UN-DAM1-285 (QU-7); B, brown patella UM-DAM1-279 (QU-2) and decalcified brown patella UN-DAM1-286 (QU-8); C, three unfused distal femoral epiphyses UM-DAM1-280, UM-DAM1-281 and UM-DAM1-282 (QU-3, QU-4 and QU-5) ranging from white to brown; D, distal femur UM-DAM1-283 (QU-6) with white epiphysis and a brown diaphysis.

Fig. 2

Histology of the white patella QU-1. A, photograph of the patella QU-1 with the red line indicating the direction of the section; B, histological cross section of QU-1 under transmitted light and its close-up (C) and (D), showing the bacterial colonies; E, SEM image of the section slice indicated by the box in B and its close-up (F), showing the bacterial colonies and decrease of bone matrix density. The white patella is heavily attacked by bacterial invasions. Slide is 94 micron thickness. bc, bacterial colony; cl, clay; oc, osteocyte; ol, osteocyte lacuna; tb, trabecula; ub, unaltered bone.

Fig. 3

SEM image of pores and channels made by a bacterial colony in QU-1, with fibrous material within the pores. The white arrows indicate the fibrous material in the pores.

Fig. 4

Histology of the brown patella QU-2. A, photograph of the patella QU-2 with the red line indicating the direction of the section; B, histological of slice one of QU-2 under transmitted light and its close-up (C), showing the unaltered bone tissue and the intralacunar preservation; D, histological of slice two of QU-2 under transmitted light and its close-up (E-G); H, corresponding SEM image of (E) and its close-up (I), showing the infilled osteocyte lacuna; These brown bone shows pristine histological preservation and no bacterial invasion. cl, chondrocyte lacuna; oc, osteocyte; ol, osteocyte lacuna; vc, vascular canal.

Fig. 5

Histology of three distal femoral heads (unfused) ranging from white (QU-3), to beige (QU-4) and to brown (QU-5). A, photograph of QU-3 to 5 with red line indicate the direction of section; B, histological of ground section slice of QU-3 under transmitted light and its close-up (C), showing the bacterial colonies as in the white patella QU-1; D, histological of ground section slice of QU-4 under transmitted light and its close-up (E-F); G, histological of ground section slice of QU-5 under transmitted light and its close-up (H-I); D-I show the unaltered bone and well preserved bone matrix as in the in the brown patella QU-2. Thickness of slide B, D and G is 89 microns, 98 microns and 95 microns respectively. bc, bacterial colony; ch, chondrocyte lacuna; cl, clay; ol, osteocyte lacuna; vc, vascular canal.

Fig. 6

Histology of a distal femur (QU-6) with brown diaphysis and white epiphysis. A, photograph of QU-6 with red line indicate the direction of section; B, histological of ground section slice of QU-6 under transmitted light and its close-up of epiphysis (C), metaphysis (D) and diaphysis (E), showing the bacterial colonies in the epiphysis as in the white patella QU-1, and unaltered bone in the metaphysis and diaphysis as in the brown patella QU-2; F, EDS of (E) showing the iron deposition in the bone matrix. Slide thickness is 120 microns. bc, bacterial colony; md, mineral deposition; ub, unaltered bone.

Fig. 7

EDS analysis of the intralacunar content in brown QU-2. A, close up SEM image of the osteocyte lacunae in Fig. 4I and EDS analysis on the same image (B), showing the different chemical element content of the different lacunae and different location in the same lacuna; Alumino-silicified (Abundant Si and Al, probably component of clay) (C), calcified (Abundant Ca and P, probably components of hydroxyapatite or fluoroapatite) (C-D), silicified (Abundant Si, probably a component of Silica) (D). Another analysis in a brown part of QU-6 (in the diaphysis) shows ironized intralacunar content (E).

Fig. 8

Microphotographs of decalcified contents of white patella QU-7 and brown patella QU-8. Decalcified contents of QU-8 after 4 days in EDTA (A-D). Decalcified contents of white patella QU-7 after 11 days in EDTA in (E) and (G). Decalcified content of brown QU-8 after 11 days in EDTA in (F) and (H). Images E and F have the same scale. Images G and H have the same scale. In Qu-7, the ‘broken blood vessel’ (bbv) is clearly eaten away (G); the red arrows in (H) are pointing at osteocytes in QU-8. bv, blood vessel; cc, chondrocyte; oc, osteocyte.