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The early Middle Eocene locality of Grube Messel, near Darmstadt (Germany), is famous for its complete vertebrate skeletons. The degree of preservation of soft tissues, such as body silhouettes, internal organs and gut contents, is frequently remarkable. The present specimen was analyzed for remnants of the reproductive system. Classic anatomy and osteology and high-resolution micro-x-ray were applied to describe the fetus of the European Eocene equoid Eurohippus messelensis. Scanning electronic microscopy (SEM) was used for determination of soft tissue remnants. The fetus is the earliest and best-preserved fossil specimen of its kind. The postcranial fetal skeleton is almost complete and largely articulated, allowing the conclusion that the pregnant mare was in late gestation. The apparent intrauterine position of the fetus is normal for the phase of pregnancy. Death of mare and fetus were probably not related to problems associated with parturition. Soft tissue interpreted as the uteroplacenta and a broad uterine ligament are preserved due to bacterial activity and allow considerations on the evolutionary development of the structures.
Conflict of interest statement
Competing Interests: The authors have declared that no competing interests exist.
Fig 1. Map of Germany showing the…
Fig 1. Map of Germany showing the geographic location of Grube Messel and Eckfeld.1
–Drawing:…Fig 1. Map of Germany showing the geographic location of Grube Messel and Eckfeld. –Drawing: Senckenberg Forschungsinstitut Frankfurt, Mascha Siemund.
Fig 2. Skeleton of a mare of…
Fig 2. Skeleton of a mare of Eurohippus messelensis with fetus (white ellipse).2
The specimen…Fig 2. Skeleton of a mare of Eurohippus messelensis with fetus (white ellipse). The specimen was discovered and excavated by a team of the Senckenberg Research Institute Frankfurt at the Grube Messel (Germany; inv. no. SMF-ME-11034), shoulder height ca. 30 cm, scale = 10 cm.–Photo: Senckenberg Forschungsinstitut Frankfurt, Sven Tränkner.
Fig 35 a) Line drawing of the…
Fig 313 a) Line drawing of the exposed side of the fetus of Eurohippus messelensis… Fig 3 a) Line drawing of the exposed side of the fetus of Eurohippus messelensis based on a reduced-contrast photo as background. Bones of the mare (black) are shown for orientation. Notice the presence of the uteroplacenta as indicated by a fine wrinkling lateral of the right femur, covering a large part of the fetus. The white line distinguishes the uteroplacenta on the right side from the exposed bones on the left. dP1 = first lower deciduous premolar of the right side, dP4 = last upper deciduous premolar of the right side.—Photo: Senckenberg Forschungsinstitut Frankfurt, Sven Tränkner; line drawing: Jens Lorenz Franzen. b) Line drawing of the skeleton of the fetus and adjacent bones of the mare based on a micro-x-ray (Fig 4). The outline of the uteroplacenta is taken from Fig 3a. Dark grey are the bones of the mares, white those of the fetus. Light grey is the uteroplacenta. Lettering is in Fig 3a except for a few bones and the broad ligament, which are only identifiable on the micro-x-ray (Fig 3b). Scale of a and b = 10 cm.–Micro-x-ray: Senckenberg Forschungsinstitut Frankfurt, Jörg Habersetzer; line drawing: Jens Lorenz Franzen.
Fig 4. The fetus analyzed in detail…
Fig 4. The fetus analyzed in detail by high-resolution micro-x-ray.5
Bones of the mare are…Fig 4. The fetus analyzed in detail by high-resolution micro-x-ray. Bones of the mare are indicated by black lettering, bones and teeth of the fetus by white lettering. L2-7 = lumbar vertebrae 2–7 of the mare. Scale = 10 cm.–Anatomical interpretation: Jens Lorenz Franzen; micro-x-ray: Senckenberg Forschungsinstitut Frankfurt, Jörg Habersetzer.
Fig 5. a) Occlusal surface of the…
Fig 5. a) Occlusal surface of the last upper deciduous premolar (dP 4 ) of…Fig 5. a) Occlusal surface of the last upper deciduous premolar (dP4) of the right side seen from its mesiolingual corner. b) Buccal aspect of the right dP1. Scale of a and b = 5 mm.—Photo: Senckenberg Forschungsinstitut Frankfurt, Sven Tränkner.
Fig 6. Identification of the broad ligament…
Fig 6. Identification of the broad ligament (ligamentum latum uteri).5
a) The broad ligament in…Fig 6. Identification of the broad ligament (ligamentum latum uteri). a) The broad ligament in the fossil mare from the Grube Messel. Sacrum and lumbar vertebrae (L6-7) belong to the mare. Not to scale.—Micro-x-ray: Senckenberg Forschungsinstitut Frankfurt, Jörg Habersetzer.–b) Position and morphology of the broad ligament (ligamentum latum uteri) attaching the uterine horn containing the fetus to the lumbar vertebrae and the pelvis of a modern horse (from Benesch 1957).
Fig 7. Identification of the uteroplacenta.1
Fig 7. Identification of the uteroplacenta.9
The photograph of the original upper side of the…Fig 7. Identification of the uteroplacenta. The photograph of the original upper side of the fossil horse was taken before transfer into epoxy resin during preparation. It displays a black shadow (edged in white) covering the fetus. Arrows point to wrinkling structures, which appeared on the external wall of the uterine horn after accidental rupture. Scale = 5 cm. Right upper corner: for orientation, the outline of the uteroplacenta is shown on a micro-x-ray of the whole skeleton of the mare.—Photo: Senckenberg Forschungsinstitut Frankfurt, Sven Tränkner; micro-x-ray: Senckenberg Forschungsinstitut Frankfurt, Jörg Habersetzer.
Fig 8. Photograph with Scanning Electronic Microscope…
Fig 8. Photograph with Scanning Electronic Microscope (SEM) of the yellow structure on the exposed…Fig 8. Photograph with Scanning Electronic Microscope (SEM) of the yellow structure on the exposed side of the fetus. Notice the tiny rod-shaped bodies, which correspond in size and morphology with bacteria. Insert: arrow points to the sampling site. Scale = 1 μ.–SEM: Senckenberg Forschungsinstitut Frankfurt, Renate Rabenstein.
Fig 9. Close-up of the dorsal edge…
Fig 9. Close-up of the dorsal edge of the uteroplacenta displaying a characteristic folding known…Fig 9. Close-up of the dorsal edge of the uteroplacenta displaying a characteristic folding known as “major curvature” in extant horses. For orientation see Fig 2. Scale = 5 cm.—Photo: Senckenberg Forschungsinstitut Frankfurt, Sven Tränkner.
Fig 10. Reconstruction of the original position…
Fig 10. Reconstruction of the original position of the fetus.5
The reconstruction refers to the…Fig 10. Reconstruction of the original position of the fetus. The reconstruction refers to the almost completely preserved and articulated fetal skeleton as indicated by high-resolution micro-x-ray. Scale = 10 cm.–Reconstruction: Jens Lorenz Franzen; drawing: Mascha Siemund. See this image and copyright information in PMC
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