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Maiacetus: displaced fetus or last meal?
Posted by thewissen on 01 May 2009 at 13:46 GMT
Maiacetus: displaced fetus or last meal?
J. G. M. Thewissen1*, and William A. McLellan2
1 Department of Anatomy and Neurobiology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, United States of America
2 Department of Biology and Marine Biology and Marine Mammal Stranding Program, University of North Carolina, Wilmington, North Carolina, United States of America
In a recent paper in PLOS One, Gingerich et al.  describe a remarkable fossil skeleton of an Eocene whale with a smaller individual inside its body cavity. The authors infer that this represents a mother with her near-term fetus, and use position and relative development of the fetus to make far reaching inferences about the mode of parturition and level of development of early whale calves at birth. We argue that the position and preservational status of the smaller individual suggest that it does not represent a fetus in its in-vivo position.
In general in mammals, near-term fetuses are located in the pelvis and lower abdomen of the mother. In cattle, a member of the sister clade to cetaceans, fetuses are born head-first and the head of the calf is near the sacrum, with the root of the tail reaching cranial to the first lumbar vertebra . In contrast, modern cetacean near-term fetuses are curved inside the uterus [3, 4, 5], and they only stretch out at birth, when the tail fluke emerges from the mother’s body [3, 6]. However, near-term fetuses of modern cetaceans too are located near the caudal end of the lumbar vertebral series.
In the specimen of Maiacetus, the head of the smaller individual is near lumbar vertebrae 2 to 4, which is the approximate level of the stomach. The neck and thorax of the body of the smaller individual extend cranial into the thorax of the larger individual, where its lungs and heart would have been located in life. Only the anterior half of the body of the smaller individual is present, but it reaches far anterior into the larger individual’s thorax. This position, in fact, is found commonly during necropsies of pregnant modern baleen whales that have stranded. In these moderately decomposed baleen whale carcasses, rapid putrification of the lower gastrointestinal tract has taken place and the fetus is displaced into the thoracic cavity of the mother. Apparently the lower GI tract, bloating with putrification gasses, pushes the fetus cranially where it passes through the diaphragm and comes to rest in the thoracic cavity.
The smaller Maiacetus specimen lacks lumbar and caudal regions of the skeleton. Observations on stranded and putrified mysticetes during necropsies also suggest an explanation for this. In stranded cetaceans where the fetus is forced cranially into the maternal thorax, the peduncle and fluke of the fetus sometimes remain in the abdominal cavity. There they are subject to scavenging from, for instance, sharks which devour the soft-walled abdomen. If the smaller Maiacetus is indeed a fetus, it was almost certainly displaced post-mortem.
The preservation of the smaller specimen also deserves notice. As a fetus inside its mother’s intact body, the smaller individual would have been somewhat protected from post-mortem damage by its mother’s thoracic cage and abdominal wall. However, the caudal half of the smaller individual’s body is missing altogether and the tip of its snout is not preserved either.
A possible explanation of this preservational status of the smaller individual is that it was a calf and was cut into pieces by the larger individual and then partly ingested. Gingerich et al.  briefly do consider the option that the specimen represents evidence of predation instead of pregnancy, but reject it by observing the ‘absence of any damage to the skull.’ However, they do cite such evidence lateron, mentioning the ‘missing anterior end of the rostrum.’ Moreover, the entire skull of the smaller individual is crushed and poorly preserved (as is not surprising with juvenile or fetal bone), and it is difficult to assess whether such damage is the result of predation or postburial damage during fossilization. Modern crocodilians tear large prey items into pieces and swallow individual chunks without chewing . Such feeding modes could be similar to feeding modes in protocetid cetaceans.
It has been suggested that some protocetids eat fish , and Gingerich et al.  propose that Maiacetus too has a ‘piscivorous dentition.’ However, they  fail to document what features indicate piscivory and the statement stands in contrast to their comment that ‘protocetids had a shearing dentition used to slice and chew their prey’ since most piscivorous mammals, including most dolphins and seals, do not chew their prey at all, but swallow small prey whole or, occasionally, break larger prey into pieces that are swallowed without mastication .
The skeleton of Maiacetus is well-preserved and elucidates the anatomy of protocetid cetaceans considerably. However, it is highly unlikely that the smaller specimen is located in the uterus in its in-vivo position, and position and preservation of the smaller individual are more consistent with an interpretation of the smaller individual being a displaced fetus or a prey item located in the digestive tract of the larger individual. This suggests that the inferences regarding head-first birth, precocial development, mating systems, and sexual dimorphism of Maiacetus made by Gingerich et al.  are premature. We argue that detailed knowledge of taphonomic processes of modern marine mammals is needed before life history inferences about fossils such as Maiacetus can be made confidently.
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