Placentation in the pronghorn (Antilocapra americana)

Pregnant uterus of the pronghorn. Note the fused amnions in the corpus uteri.
From Wislocki and Fawcett Bull Museum Comp Zool Harvard 1949; 101: 545-559.

The pronghorn is the sole survivor of a North American lineage of ruminants. (Antilocapridae). Its placenta was described by Wislocki and Fawcett (full text available at Biodiversity Heritage Library). It is polycotyledonary and epitheliochorial. The chorionic villi show a pattern of branching distinct from that of other ruminants. Interestingly, Wislocki and Fawcett described and illustrated binucleate giant cells. The amnion is larger than the allantoic sac. There is a fetus in each horn of the bicornuate uterus and the two amnions fuse back to back in the region of the uterine corpus.

Female pronghorn in Wyoming.
Photo by Yathin S Krishnappa Wikimedia Commons CC BY-SA 3.0

The pronghorn bears a superficial resemblance to an antelope, but this is the result of convergent evolution. Pronghorns (Antilocapridae) share a common ancestor with the giraffe and okapi (Giraffidae) whereas Bovidae (including antelopes) is a distinct lineage. The evolution of placentation in even-toed ungulates has been traced by Andrea Mess and Karl Klisch (here).

Given the taxonomic position of the pronghorn, a recent study has examined glycosylation at the fetal maternal interface and compared it with the giraffe, okapi and various bovids (here). The expression of pregnancy-associated glycoproteins (PAGs) in binucleate trophoblast cells was also examined.

Embryo competition in the pronghorn: penetration of the membranes
of a distal embryo by the necrotic tip of a proximal embryo.
From O'Gara Amer J Anat 1969; 125: 217-232. 

Several authors have noted that the number of corpora lutea exceeds the number of fetuses (usually twins). O'Gara (reference here) found that some reduction occurred during the phase of blastocyst elongation. Often two blastocysts managed to implant in the same horn. However, the membranes of the proximal embryo (nearest the uterine body) formed a necrotic tip. As the conceptus grew, this tip pierced the chorion and allantois of the distal embryo, resulting in its death. As O'Gara wrote, "The phenomenom of the necrotic tip acting as a lethal weapon is apparently unique to the pronghorn."

A downside to long gestation periods

Giraffe with calves - John Storr (Wikimedia Commons)

Giraffes (Giraffa camelopardalis) and the forest-dwelling okapi (Okapia johnstoni) are the survivors of a much broader radiation. Recently Clauss and Rössner (here) used the New and Old Worlds (NOW) data base of fossil mammals to examine the prevalence of various types of ruminant over time.

Rise and fall of tragulids 

The most basal group is Tragulidae, represented by present day chevrotains or mouse deer. In the Early Miocene they were the most prevalent ruminants but were subsequently displaced by the pecorans. The principal difference was the evolution in pecorans of an additional forestomach, the omasum, and this seems to have given them the edge over tragulids. 

Both tragulids and pecorans have synepitheliochorial placentation where binucleate trophoblast cells fuse with uterine epithelial cells at the fetal-maternal interface. However, tragulids lack the cotyledons typical of the placenta in pecorans.

Rise and fall of giraffoids

Among pecorans, Giraffidae is basal to both Bovidae (cattle, sheep, antelopes) and Cervidae (deer). Looking at the equivalent fossil clades, Clauss and Rössner found that Giraffoidea were much more  abundant in the later Early to Middle Miocene but then steadily declined to be supplanted by Bovoidea in Africa and by Bovoidea and Cervoidea in Eurasia. Why?

Based on extant species, the most striking difference between giraffids and other pecoran ruminants is their extremely long gestation - in excess of a year. Assuming this also was the case in fossil species, could that explain their decline? Clauss and Rössner argue that they were unable to develop a seasonal breeding pattern. This put them at a disadvantage compared to other pecorans with shorter gestation times such as bovids and cervids.  

Perhaps in support of this narrative, fossil giraffoids occupied a much greater range of ecological niches and many were grazers. The giraffes grew a long neck and survived as browsers in a special niche. The okapi is highly adapted to its restricted forest habitat.

Placentation in the giraffe has recently been described (see previous post).