According to Wikipedia article--http://en.wikipedia.org/wiki/Platelet,
platelets exist only in mammals. The following micrographs show
mammalian platelet remains, which were found in samples of red
rainfalls that occurred in 2001 in Kerala, India.

Fig. 1: shows a non-activated platelet found in red rain
http://www.wretch.cc/album/show.php?i=lin440315&b=29&f=1782738622&p=31
Source and credit for Fig.1: Fig. 13 at http://arxiv.org/PS_cache/astro-ph/pdf/0601/0601022v1.pdf

Fig. 2: human platelets for comparison
http://www.wretch.cc/album/show.php?i=lin440315&b=29&f=1782738625&p=32
Source and credit for Fig.2: Image 11 at
http://www.optics.rochester.edu/workgroups/cml/opt307/spr06/john/lesoineTEMSEM.htm

Fig. 3: shows two types of human platelets for comparison
http://www.wretch.cc/album/show.php?i=lin440315&b=29&f=1782738624&p=33
Source and credit for Fig.3: http://www.visualsunlimited.com/image/I00004pkCAJnGNCE

Fig. 4: shows activated platelets found in red rain
http://www.wretch.cc/album/show.php?i=lin440315&b=29&f=1782738623&p=34
Source and credit for Fig. 4: Fig. 12 at http://arxiv.org/PS_cache/astro-ph/pdf/0601/0601022v1.pdf

Fig. 5: shows activated human platelets for comparison
http://www.wretch.cc/album/show.php?i=lin440315&b=29&f=1782738626&p=35
Source and credit for Fig. 5: http://www.visualsunlimited.com/image/I00000K.Kh2n2LlU

Related findings:
http://wretchfossil.blogspot.com/2011/07/red-rain-in-india-did-contain-mammalian.html