70s YA novel about two boys travelling millions of years into the past, meeting a "cynogasaurus" and rival animal-themed caveman tribes

I read this in English in the late 70s in primary school.

What I remember:

• Two boys are somehow caught up in a time travel scenario. I think their father may have been the inventor of the time machine.
• Their initial journey takes them back about 200 million years.
• They meet an animal they identify as a cynogasaurus. At least, that’s what my memory told me. Google tells me my memory may be playing tricks and they met a cynognathus instead. It was definitely cynogsomething.
• They somehow jump forward to caveman times with the cynogasaurus/cynognathus in tow.
• They befriend a similarly-aged kid who’s a member of a particular tribe, possibly the leopard tribe.
• The leopard tribe are enemies of the zebra tribe. It’s possible I’ve got both animals wrong.
• Each tribe has a sacred animal. Disputes between the tribes are settled by their sacred animals fighting.
• Somehow the kids have caused the death of "their" tribe’s sacred animal so they offer the cynogawhatsit, which the cave-people call Nog, in its place. Nog wins the fight and stays on as the sacred animal when the kids leave.

Edit: I thought it may have been Lester Del Rey’s "Tunnel Through Time", the subject of a few queries here over the years. The cover looks familiar and some details match. However two comments have verified that it doesn’t feature the cynog* creature, sacred animals or the fight, which are what I recall about the book.