Guppy,
The Atlas section of Dr. Pez is "closed for maintenance", and the Fishbase page "cannot be found"... But the Parnaíba River doesn't get close to Suriname. The Parnaíba River gets to the Sea, in the state of Piauí. The red circle shows where it is, in the image below:
Here's a pic of the Parnaíba River Delta:
And there are more pics of the Parnaíba River in
THIS LINK. This fish was (sorry, lost it...

) 40-45 mm long. I kept it for two months, it was eating VERY well, and it grew from 40 to 45 mm in this time, suggesting it's not
D. maculatus, as you thought. I'm pretty sure it's
Dormitator lophocephalus or anything undescribed close to it. Its body structure suggests it's not a fry, being compatible with a half-grown 100 mm fish. Hope we are right...
BTW, does the description of
D. lophocephalus have any drawing? Do you have the original paper, or the reference? Anyways, thank you very much for your help.
Best regards.
Daniel.