Complex sentence: Because the shield consisted of one solid piece it probably did not grow during the animals life; paleontologists presume "Tremataspis" larvae lacked the shield, which would appear later in life.
Split into simpler sentences: Because the shield consisted of one solid piece, it probably did not grow during the animal's life. Paleontologists presume "Tremataspis" larvae lacked the shield, which would appear later in life.