Mammals that cannot see in the light
The sloths, anteaters and armadillos are a puzzlesome group. Genomic studies have finally revealed their intricate relationships but what about their ancestors? One thing stands out in their very ancient past. They all have monochromatic vision, now confirmed as due to the lack of cones in their retina.
With several genomes to work on, a common ancestor with at most, an limited-cone monochrome type of vision was most likely. The group, the Xenarthra, therefore had lived in dim-light conditions such as those found in subterranean habitats.
The nine-banded armadillo, Dasypus novemcinctus, the 2-toed sloth, Choloepus hoffmanni, and the extinct ground sloth Mylodon darwini, were used for sequencing analysis. Five more armadillos, two more sloths and two more anteaters were also compared. The cone and phototransduction genes were generally inactivated, with a large deletion in Dasypus and Mylodon. This implies that 95mya, a stem ancestor was similarly vision deficient. A mutation in the 2-toed sloth is similar to that in pygmy sperm whales and a false vampire bat, as well as in 3-toed sloths and some armadillos. That means that 80 mya, monochromatic vision was likely in the early ancestral forms. Whether they were nocturnal or not is a valid question, as all mammals could have adopted that habit at the time, because of reptilian competition.
The Xenarthrans (sloths, armadillos and anteaters) are the only mammal group to be monochromatic at 65.5 mya. The modern examples must have poor vision in dim light and total blindness in bright light. The low light of rainforest possibly provides limited vision for them. With the number of traffic accidents for armadillos and ant eaters, conservation now needs to take account of their degenerate vision, or careless drivers!
Christopher A. Emerling and Mark S. Springer of the University of California, Riverside, propose that a subterranean origin explains most, if not all, of this group being rod monochromatic. The paper can be found in Proc.Roy.Soc.B as, Genomic evidence for rod monochromacy in sloths and armadillos suggests early subterranean history for Xenarthra. A genetic bottleneck would result from their underground existence at around the infamous K-Pg boundary (the mass extinction event after the Chicxulub impact.) Their likely burrowing habit would also ensure some survived the terrible repercussions and infra-red radiation Subsequent evolution would produce their terrestrial and arboreal descendants, which all have borrowing features, even when they dont use them. Other mammals possibly survived in a similar fashion, and there is one fossil to indicate this. What happened to these unique mammals helped them through a crisis but has limited their evolution in many ways since
To investigate one of the evolutionary strands, in a tiny island off South America, have a look at the pygmy 3-toed sloth.