Gingerich Philip D, Ul-Haq Munir, von Koenigswald Wighart, Sanders William J, Smith B Holly, Zalmout Iyad S
Department of Geological Sciences, Museum of Paleontology, University of Michigan, Ann Arbor, Michigan, United States of America.
PLoS One. 2009;4(2):e4366. doi: 10.1371/journal.pone.0004366. Epub 2009 Feb 4.
Protocetidae are middle Eocene (49-37 Ma) archaeocete predators ancestral to later whales. They are found in marine sedimentary rocks, but retain four legs and were not yet fully aquatic. Protocetids have been interpreted as amphibious, feeding in the sea but returning to land to rest.
METHODOLOGY/PRINCIPAL FINDINGS: Two adult skeletons of a new 2.6 meter long protocetid, Maiacetus inuus, are described from the early middle Eocene Habib Rahi Formation of Pakistan. M. inuus differs from contemporary archaic whales in having a fused mandibular symphysis, distinctive astragalus bones in the ankle, and a less hind-limb dominated postcranial skeleton. One adult skeleton is female and bears the skull and partial skeleton of a single large near-term fetus. The fetal skeleton is positioned for head-first delivery, which typifies land mammals but not extant whales, evidence that birth took place on land. The fetal skeleton has permanent first molars well mineralized, which indicates precocial development at birth. Precocial development, with attendant size and mobility, were as critical for survival of a neonate at the land-sea interface in the Eocene as they are today. The second adult skeleton is the most complete known for a protocetid. The vertebral column, preserved in articulation, has 7 cervicals, 13 thoracics, 6 lumbars, 4 sacrals, and 21 caudals. All four limbs are preserved with hands and feet. This adult is 12% larger in linear dimensions than the female skeleton, on average, has canine teeth that are 20% larger, and is interpreted as male. Moderate sexual dimorphism indicates limited male-male competition during breeding, which in turn suggests little aggregation of food or shelter in the environment inhabited by protocetids.
CONCLUSIONS/SIGNIFICANCE: Discovery of a near-term fetus positioned for head-first delivery provides important evidence that early protocetid whales gave birth on land. This is consistent with skeletal morphology enabling Maiacetus to support its weight on land and corroborates previous ideas that protocetids were amphibious. Specimens this complete are virtual 'Rosetta stones' providing insight into functional capabilities and life history of extinct animals that cannot be gained any other way.
原鲸科动物是始新世中期（4900万至3700万年前）的古鲸类食肉动物，是后来鲸鱼的祖先。它们出现在海洋沉积岩中，但仍保留着四条腿，尚未完全水生。原鲸科动物被认为是两栖动物，在海中觅食，但回到陆地休息。
方法/主要发现：从巴基斯坦早始新世中期的哈比卜·拉希组中描述了一种新的体长2.6米的原鲸科动物——印度 Maiacetus inuus 的两具成年骨骼。印度 Maiacetus inuus 与同时期的古代鲸鱼不同，它的下颌联合处融合，脚踝处有独特的距骨，且后肢在颅后骨骼中所占比例较小。一具成年骨骼为雌性，体内有一个接近足月的单一大胎儿的头骨和部分骨骼。胎儿骨骼的位置表明是头先娩出，这是陆地哺乳动物的典型特征，而现存鲸鱼并非如此，这证明分娩发生在陆地上。胎儿骨骼的第一恒磨牙矿化良好，这表明出生时发育早熟。早熟的发育以及随之而来的体型和活动能力，对始新世陆地与海洋交界处的新生儿生存至关重要，如今依然如此。第二具成年骨骼是已知最完整的原鲸科动物骨骼。保存有关节的脊柱有7个颈椎、13个胸椎、6个腰椎、4个荐椎和21个尾椎。所有四肢以及手和脚都保存了下来。这具成年骨骼的线性尺寸平均比雌性骨骼大12%，犬齿大20%，被认为是雄性。适度的两性异形表明繁殖期间雄性之间的竞争有限，这反过来表明原鲸科动物栖息的环境中食物或庇护所的聚集程度较低。
结论/意义：发现一个处于头先娩出位置的接近足月胎儿，为早期原鲸科鲸鱼在陆地产仔提供了重要证据。这与印度 Maiacetus 的骨骼形态使其能够在陆地上支撑体重相一致，并证实了之前关于原鲸科动物是两栖动物的观点。如此完整的标本实际上是“罗塞塔石碑”，能让我们深入了解已灭绝动物的功能能力和生活史，而这是通过其他方式无法获得的。
