Those animals died in the Noahic Flood, and were buried by the earthquakes and swift rushing flood waters. IMHO.
(Let’s not believe what the Scriptures say. Let’s spin a yarn about how it happened some other way).
Also, these animals and people came off the ark and their numbers increased exponentially. After the flood for a long period of time due to drastic weather change there was lots of ice left over. This ice melted and broke off burying much in it's wake as it melted and receded. Water also was left in large inland seas and rivers that eventually dried up. Large bodies of water would eventually break through natural dams and pour out into new areas burying people and animals in it's wake. Even for a long time after the flood due to drastic situations left behind from the flood system many fossils were created. Considered one of the oldest books in The Bible, Job references this time period.