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I found a website about open fontenelles or moleras in toy breed dogs and the author stated the following:
The presence of a molera DOES NOT make the dog any more or less susceptible to brain injury, seizures or hydrocephalus.
Two other vets were quoted as follows:
Drs. Walters and Rivers, Veterinarians at the University of Minnesota, concluded that
there did not appear to be any relationship between the presence or size of a fontanelle and the condition of hydrocephalus.
* Dr. Alexander de Lahunta of Cornell University in New York,
one of the top neurologist in this country, stated that it would be wrong to conclude that any opening is abnormal.
Here is the link to the entire article:
http://www.prouxchihuahuas.com/id15.html
I would defintiely
not return the dog, this is very common and nothing to be overly concerned about, yes, take precautions to prevent injury but he should live a long and happy and healthy life.